[PATCH v1 0/2] drivers: don't select DMA_CMA or CMA

[PATCH v1 0/2] drivers: don't select DMA_CMA or CMA

[David Hildenbrand]

Trying to set CONFIG_CMA=y with CONFIG_DMA_CMA=n revealed that we have
three drivers that select these options. Random drivers should not
override user settings of such core knobs. Let's use "imply DMA_CMA"
instead, such that user configuration and dependencies are respected.

Cc: Joel Stanley <joel@jms.id.au>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Andrew Jeffery <andrew@aj.id.au>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Russell King <linux+etnaviv@armlinux.org.uk>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Peter Collingbourne <pcc@google.com>
Cc: linux-aspeed at lists.ozlabs.org
Cc: dri-devel at lists.freedesktop.org
Cc: linux-arm-kernel at lists.infradead.org
Cc: etnaviv at lists.freedesktop.org
Cc: linux-fbdev at vger.kernel.org

David Hildenbrand (2):
  drivers/video/fbdev: don't select DMA_CMA
  drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

 drivers/gpu/drm/aspeed/Kconfig  | 3 +--
 drivers/gpu/drm/etnaviv/Kconfig | 3 +--
 drivers/video/fbdev/Kconfig     | 2 +-
 3 files changed, 3 insertions(+), 5 deletions(-)

-- 
2.30.2

[PATCH v1 1/2] drivers/video/fbdev: don't select DMA_CMA

[David Hildenbrand]

Random drivers should not override a user configuration of core knobs
(e.g., CONFIG_DMA_CMA=n). Use "imply" instead, to still respect
dependencies and manual overrides.

"This is similar to "select" as it enforces a lower limit on another
 symbol except that the "implied" symbol's value may still be set to n
 from a direct dependency or with a visible prompt."

Implying DRM_CMA should be sufficient, as that depends on CMA.

Note: If this is a real dependency, we should use "depends on DMA_CMA"
instead -  but I assume the driver can work without CMA just fine --
esp. when we wouldn't have HAVE_DMA_CONTIGUOUS or CMA right now.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 drivers/video/fbdev/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/Kconfig b/drivers/video/fbdev/Kconfig
index 4f02db65dede..d37cd5341e1b 100644
--- a/drivers/video/fbdev/Kconfig
+++ b/drivers/video/fbdev/Kconfig
@@ -2186,7 +2186,7 @@ config FB_HYPERV
 	select FB_CFB_COPYAREA
 	select FB_CFB_IMAGEBLIT
 	select FB_DEFERRED_IO
-	select DMA_CMA if HAVE_DMA_CONTIGUOUS && CMA
+	imply DMA_CMA
 	help
 	  This framebuffer driver supports Microsoft Hyper-V Synthetic Video.
 
-- 
2.30.2

[PATCH v1 1/2] drivers/video/fbdev: don't select DMA_CMA

[Mike Rapoport]

On Thu, Apr 08, 2021 at 11:20:10AM +0200, David Hildenbrand wrote:
> Random drivers should not override a user configuration of core knobs
> (e.g., CONFIG_DMA_CMA=n). Use "imply" instead, to still respect
> dependencies and manual overrides.
> 
> "This is similar to "select" as it enforces a lower limit on another
>  symbol except that the "implied" symbol's value may still be set to n
>  from a direct dependency or with a visible prompt."
> 
> Implying DRM_CMA should be sufficient, as that depends on CMA.

	  ^ DMA_CMA ?

> Note: If this is a real dependency, we should use "depends on DMA_CMA"
> instead -  but I assume the driver can work without CMA just fine --
> esp. when we wouldn't have HAVE_DMA_CONTIGUOUS or CMA right now.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  drivers/video/fbdev/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/video/fbdev/Kconfig b/drivers/video/fbdev/Kconfig
> index 4f02db65dede..d37cd5341e1b 100644
> --- a/drivers/video/fbdev/Kconfig
> +++ b/drivers/video/fbdev/Kconfig
> @@ -2186,7 +2186,7 @@ config FB_HYPERV
>  	select FB_CFB_COPYAREA
>  	select FB_CFB_IMAGEBLIT
>  	select FB_DEFERRED_IO
> -	select DMA_CMA if HAVE_DMA_CONTIGUOUS && CMA
> +	imply DMA_CMA
>  	help
>  	  This framebuffer driver supports Microsoft Hyper-V Synthetic Video.
>  
> -- 
> 2.30.2
> 

-- 
Sincerely yours,
Mike.

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[David Hildenbrand]

Random drivers should not override a user configuration of core knobs
(e.g., CONFIG_DMA_CMA=n). Use "imply" instead, to still respect
dependencies and manual overrides.

"This is similar to "select" as it enforces a lower limit on another
 symbol except that the "implied" symbol's value may still be set to n
 from a direct dependency or with a visible prompt."

Implying DRM_CMA should be sufficient, as that depends on CMA.

Note: If this is a real dependency, we should use "depends on DMA_CMA"
instead -  but I assume the driver can work without CMA just fine --
esp. when we wouldn't have HAVE_DMA_CONTIGUOUS right now.

Signed-off-by: David Hildenbrand <david@redhat.com>
---
 drivers/gpu/drm/aspeed/Kconfig  | 3 +--
 drivers/gpu/drm/etnaviv/Kconfig | 3 +--
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/aspeed/Kconfig b/drivers/gpu/drm/aspeed/Kconfig
index 5e95bcea43e9..a055f763d230 100644
--- a/drivers/gpu/drm/aspeed/Kconfig
+++ b/drivers/gpu/drm/aspeed/Kconfig
@@ -6,9 +6,8 @@ config DRM_ASPEED_GFX
 	depends on MMU
 	select DRM_KMS_HELPER
 	select DRM_KMS_CMA_HELPER
-	select DMA_CMA if HAVE_DMA_CONTIGUOUS
-	select CMA if HAVE_DMA_CONTIGUOUS
 	select MFD_SYSCON
+	imply DRM_CMA
 	help
 	  Chose this option if you have an ASPEED AST2500 SOC Display
 	  Controller (aka GFX).
diff --git a/drivers/gpu/drm/etnaviv/Kconfig b/drivers/gpu/drm/etnaviv/Kconfig
index faa7fc68b009..5f5576b7221a 100644
--- a/drivers/gpu/drm/etnaviv/Kconfig
+++ b/drivers/gpu/drm/etnaviv/Kconfig
@@ -9,9 +9,8 @@ config DRM_ETNAVIV
 	select THERMAL if DRM_ETNAVIV_THERMAL
 	select TMPFS
 	select WANT_DEV_COREDUMP
-	select CMA if HAVE_DMA_CONTIGUOUS
-	select DMA_CMA if HAVE_DMA_CONTIGUOUS
 	select DRM_SCHED
+	imply DMA_CMA
 	help
 	  DRM driver for Vivante GPUs.
 
-- 
2.30.2

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[Mike Rapoport]

On Thu, Apr 08, 2021 at 11:20:11AM +0200, David Hildenbrand wrote:
> Random drivers should not override a user configuration of core knobs
> (e.g., CONFIG_DMA_CMA=n). Use "imply" instead, to still respect
> dependencies and manual overrides.
> 
> "This is similar to "select" as it enforces a lower limit on another
>  symbol except that the "implied" symbol's value may still be set to n
>  from a direct dependency or with a visible prompt."
> 
> Implying DRM_CMA should be sufficient, as that depends on CMA.

          ^ DMA_CMA
> 
> Note: If this is a real dependency, we should use "depends on DMA_CMA"
> instead -  but I assume the driver can work without CMA just fine --
> esp. when we wouldn't have HAVE_DMA_CONTIGUOUS right now.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  drivers/gpu/drm/aspeed/Kconfig  | 3 +--
>  drivers/gpu/drm/etnaviv/Kconfig | 3 +--
>  2 files changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/aspeed/Kconfig b/drivers/gpu/drm/aspeed/Kconfig
> index 5e95bcea43e9..a055f763d230 100644
> --- a/drivers/gpu/drm/aspeed/Kconfig
> +++ b/drivers/gpu/drm/aspeed/Kconfig
> @@ -6,9 +6,8 @@ config DRM_ASPEED_GFX
>  	depends on MMU
>  	select DRM_KMS_HELPER
>  	select DRM_KMS_CMA_HELPER
> -	select DMA_CMA if HAVE_DMA_CONTIGUOUS
> -	select CMA if HAVE_DMA_CONTIGUOUS
>  	select MFD_SYSCON
> +	imply DRM_CMA

Ditto

>  	help
>  	  Chose this option if you have an ASPEED AST2500 SOC Display
>  	  Controller (aka GFX).
> diff --git a/drivers/gpu/drm/etnaviv/Kconfig b/drivers/gpu/drm/etnaviv/Kconfig
> index faa7fc68b009..5f5576b7221a 100644
> --- a/drivers/gpu/drm/etnaviv/Kconfig
> +++ b/drivers/gpu/drm/etnaviv/Kconfig
> @@ -9,9 +9,8 @@ config DRM_ETNAVIV
>  	select THERMAL if DRM_ETNAVIV_THERMAL
>  	select TMPFS
>  	select WANT_DEV_COREDUMP
> -	select CMA if HAVE_DMA_CONTIGUOUS
> -	select DMA_CMA if HAVE_DMA_CONTIGUOUS
>  	select DRM_SCHED
> +	imply DMA_CMA
>  	help
>  	  DRM driver for Vivante GPUs.
>  
> -- 
> 2.30.2
> 

-- 
Sincerely yours,
Mike.

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[David Hildenbrand]

On 08.04.21 11:56, Mike Rapoport wrote:
> On Thu, Apr 08, 2021 at 11:20:11AM +0200, David Hildenbrand wrote:
>> Random drivers should not override a user configuration of core knobs
>> (e.g., CONFIG_DMA_CMA=n). Use "imply" instead, to still respect
>> dependencies and manual overrides.
>>
>> "This is similar to "select" as it enforces a lower limit on another
>>   symbol except that the "implied" symbol's value may still be set to n
>>   from a direct dependency or with a visible prompt."
>>
>> Implying DRM_CMA should be sufficient, as that depends on CMA.
> 
>            ^ DMA_CMA
>>
>> Note: If this is a real dependency, we should use "depends on DMA_CMA"
>> instead -  but I assume the driver can work without CMA just fine --
>> esp. when we wouldn't have HAVE_DMA_CONTIGUOUS right now.
>>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> ---
>>   drivers/gpu/drm/aspeed/Kconfig  | 3 +--
>>   drivers/gpu/drm/etnaviv/Kconfig | 3 +--
>>   2 files changed, 2 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/aspeed/Kconfig b/drivers/gpu/drm/aspeed/Kconfig
>> index 5e95bcea43e9..a055f763d230 100644
>> --- a/drivers/gpu/drm/aspeed/Kconfig
>> +++ b/drivers/gpu/drm/aspeed/Kconfig
>> @@ -6,9 +6,8 @@ config DRM_ASPEED_GFX
>>   	depends on MMU
>>   	select DRM_KMS_HELPER
>>   	select DRM_KMS_CMA_HELPER
>> -	select DMA_CMA if HAVE_DMA_CONTIGUOUS
>> -	select CMA if HAVE_DMA_CONTIGUOUS
>>   	select MFD_SYSCON
>> +	imply DRM_CMA
> 
> Ditto

Gah, thanks!


-- 
Thanks,

David / dhildenb

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[Arnd Bergmann]

On Thu, Apr 8, 2021 at 11:22 AM David Hildenbrand <david@redhat.com> wrote:
>
> Random drivers should not override a user configuration of core knobs
> (e.g., CONFIG_DMA_CMA=n). Use "imply" instead, to still respect
> dependencies and manual overrides.
>
> "This is similar to "select" as it enforces a lower limit on another
>  symbol except that the "implied" symbol's value may still be set to n
>  from a direct dependency or with a visible prompt."
>
> Implying DRM_CMA should be sufficient, as that depends on CMA.
>
> Note: If this is a real dependency, we should use "depends on DMA_CMA"
> instead -  but I assume the driver can work without CMA just fine --
> esp. when we wouldn't have HAVE_DMA_CONTIGUOUS right now.

'imply' is almost never the right solution, and it tends to cause more
problems than it solves.

In particular, it does not prevent a configuration with 'DRM_CMA=m'
and 'DRMA_ASPEED_GFX=y', or any build failures from such
a configuration.

If you want this kind of soft dependency, you need
'depends on DRM_CMA || !DRM_CMA'.

         Arnd

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[David Hildenbrand]

On 08.04.21 12:20, Arnd Bergmann wrote:
> On Thu, Apr 8, 2021 at 11:22 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> Random drivers should not override a user configuration of core knobs
>> (e.g., CONFIG_DMA_CMA=n). Use "imply" instead, to still respect
>> dependencies and manual overrides.
>>
>> "This is similar to "select" as it enforces a lower limit on another
>>   symbol except that the "implied" symbol's value may still be set to n
>>   from a direct dependency or with a visible prompt."
>>
>> Implying DRM_CMA should be sufficient, as that depends on CMA.
>>
>> Note: If this is a real dependency, we should use "depends on DMA_CMA"
>> instead -  but I assume the driver can work without CMA just fine --
>> esp. when we wouldn't have HAVE_DMA_CONTIGUOUS right now.
> 
> 'imply' is almost never the right solution, and it tends to cause more
> problems than it solves.

I thought that was the case with "select" :)

> 
> In particular, it does not prevent a configuration with 'DRM_CMA=m'

I assume you meant "DRM_CMA=n" ? DRM_CMA cannot be built as a module.

> and 'DRMA_ASPEED_GFX=y', or any build failures from such
> a configuration.

I don't follow. "DRM_CMA=n" and 'DRMA_ASPEED_GFX=y' is supposed to work 
just fine (e.g., without HAVE_DMA_CONTIGUOUS) or what am I missing?

> 
> If you want this kind of soft dependency, you need
> 'depends on DRM_CMA || !DRM_CMA'.

Seriously? I think the point of imply is "please enable if possible and 
not prevented by someone else". Your example looks more like a NOP - no? 
Or will it have the same effect?


-- 
Thanks,

David / dhildenb

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[David Hildenbrand]

On 08.04.21 12:27, David Hildenbrand wrote:
> On 08.04.21 12:20, Arnd Bergmann wrote:
>> On Thu, Apr 8, 2021 at 11:22 AM David Hildenbrand <david@redhat.com> wrote:
>>>
>>> Random drivers should not override a user configuration of core knobs
>>> (e.g., CONFIG_DMA_CMA=n). Use "imply" instead, to still respect
>>> dependencies and manual overrides.
>>>
>>> "This is similar to "select" as it enforces a lower limit on another
>>>    symbol except that the "implied" symbol's value may still be set to n
>>>    from a direct dependency or with a visible prompt."
>>>
>>> Implying DRM_CMA should be sufficient, as that depends on CMA.
>>>
>>> Note: If this is a real dependency, we should use "depends on DMA_CMA"
>>> instead -  but I assume the driver can work without CMA just fine --
>>> esp. when we wouldn't have HAVE_DMA_CONTIGUOUS right now.
>>
>> 'imply' is almost never the right solution, and it tends to cause more
>> problems than it solves.
> 
> I thought that was the case with "select" :)
> 
>>
>> In particular, it does not prevent a configuration with 'DRM_CMA=m'
> 
> I assume you meant "DRM_CMA=n" ? DRM_CMA cannot be built as a module.
> 
>> and 'DRMA_ASPEED_GFX=y', or any build failures from such
>> a configuration.
> 
> I don't follow. "DRM_CMA=n" and 'DRMA_ASPEED_GFX=y' is supposed to work
> just fine (e.g., without HAVE_DMA_CONTIGUOUS) or what am I missing?
> 
>>
>> If you want this kind of soft dependency, you need
>> 'depends on DRM_CMA || !DRM_CMA'.
> 
> Seriously? I think the point of imply is "please enable if possible and
> not prevented by someone else". Your example looks more like a NOP - no?
> Or will it have the same effect?

I just tried (remove CONFIG_DMA_CMA from .config followed by make) and 
the default will be set to "N" (when querying the user). So it indeed 
looks like a NOP - unless I am missing something.

-- 
Thanks,

David / dhildenb

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[Arnd Bergmann]

On Thu, Apr 8, 2021 at 12:29 PM David Hildenbrand <david@redhat.com> wrote:
> On 08.04.21 12:20, Arnd Bergmann wrote:
> > On Thu, Apr 8, 2021 at 11:22 AM David Hildenbrand <david@redhat.com> wrote:
> >>
> >> Random drivers should not override a user configuration of core knobs
> >> (e.g., CONFIG_DMA_CMA=n). Use "imply" instead, to still respect
> >> dependencies and manual overrides.
> >>
> >> "This is similar to "select" as it enforces a lower limit on another
> >>   symbol except that the "implied" symbol's value may still be set to n
> >>   from a direct dependency or with a visible prompt."
> >>
> >> Implying DRM_CMA should be sufficient, as that depends on CMA.
> >>
> >> Note: If this is a real dependency, we should use "depends on DMA_CMA"
> >> instead -  but I assume the driver can work without CMA just fine --
> >> esp. when we wouldn't have HAVE_DMA_CONTIGUOUS right now.
> >
> > 'imply' is almost never the right solution, and it tends to cause more
> > problems than it solves.
>
> I thought that was the case with "select" :)

Yes, but that's a different set of problems

> >
> > In particular, it does not prevent a configuration with 'DRM_CMA=m'
>
> I assume you meant "DRM_CMA=n" ? DRM_CMA cannot be built as a module.

Ok, at least that makes it easier.

> > and 'DRMA_ASPEED_GFX=y', or any build failures from such
> > a configuration.
>
> I don't follow. "DRM_CMA=n" and 'DRMA_ASPEED_GFX=y' is supposed to work
> just fine (e.g., without HAVE_DMA_CONTIGUOUS) or what am I missing?

I thought you were trying to solve the problem where DRMA_ASPEED_GFX
can optionally link against CMA but would fail to build when the CMA code
is in a loadable module.

If the problem you are trying to solve is a different one, you need a different
solution, not what I posted above.

> > If you want this kind of soft dependency, you need
> > 'depends on DRM_CMA || !DRM_CMA'.
>
> Seriously? I think the point of imply is "please enable if possible and
> not prevented by someone else".

That used to be the meaning, but it changed a few years ago. Now
it means "when a used manually turns on this symbol, turn on the
implied one as well, but let them turn it off again if they choose".

This is pretty much a NOP.

> Your example looks more like a NOP - no?
> Or will it have the same effect?

The example I gave is only meaningful if both are tristate, which is
not the case here as you explain.

It is a somewhat awkward way to say "prevent this symbol from
being =y if the dependency is =m".

      Arnd

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[David Hildenbrand]

>>> In particular, it does not prevent a configuration with 'DRM_CMA=m'
>>
>> I assume you meant "DRM_CMA=n" ? DRM_CMA cannot be built as a module.
> 
> Ok, at least that makes it easier.
> 
>>> and 'DRMA_ASPEED_GFX=y', or any build failures from such
>>> a configuration.
>>
>> I don't follow. "DRM_CMA=n" and 'DRMA_ASPEED_GFX=y' is supposed to work
>> just fine (e.g., without HAVE_DMA_CONTIGUOUS) or what am I missing?
> 
> I thought you were trying to solve the problem where DRMA_ASPEED_GFX
> can optionally link against CMA but would fail to build when the CMA code
> is in a loadable module.

Yes. I was trying to say: it works with this patch just fine. The issue 
you described does not seem to apply (DRM_CMA=m).

> 
>> Your example looks more like a NOP - no?
>> Or will it have the same effect?
> 
> The example I gave is only meaningful if both are tristate, which is
> not the case here as you explain.

Okay, thanks.

> 
> It is a somewhat awkward way to say "prevent this symbol from
> being =y if the dependency is =m".

What would be the right thing to do in the case here then to achieve the 
"if DRMA_ASPEED_GFX is enabled, also enable DMA_CMA id possible"?

One approach could be to have for DMA_CMA

default y if DRMA_ASPEED_GFX

but it feels like the wrong way to tackle this.

Thanks!

-- 
Thanks,

David / dhildenb

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[Arnd Bergmann]

On Thu, Apr 8, 2021 at 1:00 PM David Hildenbrand <david@redhat.com> wrote:
> >
> > It is a somewhat awkward way to say "prevent this symbol from
> > being =y if the dependency is =m".
>
> What would be the right thing to do in the case here then to achieve the
> "if DRMA_ASPEED_GFX is enabled, also enable DMA_CMA id possible"?
>
> One approach could be to have for DMA_CMA
>
> default y if DRMA_ASPEED_GFX
>
> but it feels like the wrong way to tackle this.

I'm still not sure what you are trying to achieve. Is the idea only to provide
a useful default for DMA_CMA depending on which drivers are enabled?

This is something you could do using a hidden helper symbol like

config DRMA_ASPEED_GFX
       bool "Aspeed display driver"
       select DRM_WANT_CMA

config DRM_WANT_CMA
       bool
       help
          Select this from any driver that benefits from CMA being enabled

config DMA_CMA
       bool "Use CMA helpers for DRM"
       default DRM_WANT_CMA

         Arnd

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[David Hildenbrand]

On 08.04.21 13:44, Arnd Bergmann wrote:
> On Thu, Apr 8, 2021 at 1:00 PM David Hildenbrand <david@redhat.com> wrote:
>>>
>>> It is a somewhat awkward way to say "prevent this symbol from
>>> being =y if the dependency is =m".
>>
>> What would be the right thing to do in the case here then to achieve the
>> "if DRMA_ASPEED_GFX is enabled, also enable DMA_CMA id possible"?
>>
>> One approach could be to have for DMA_CMA
>>
>> default y if DRMA_ASPEED_GFX
>>
>> but it feels like the wrong way to tackle this.
> 
> I'm still not sure what you are trying to achieve. Is the idea only to provide
> a useful default for DMA_CMA depending on which drivers are enabled?

"Random drivers should not override a user configuration of core knobs
(e.g., CONFIG_DMA_CMA=n)."

Let's assume I'm a distribution and want to set CONFIG_CMA=n or want to 
set CONFIG_DMA_CMA=n with CONFIG_CMA=y; there is no way to do that with 
e.g., DRMA_ASPEED_GFX=y because it will always override my (user!) 
setting -- even though it doesn't really always need it. Using "select" 
is the problem here.

> 
> This is something you could do using a hidden helper symbol like
> 
> config DRMA_ASPEED_GFX
>         bool "Aspeed display driver"
>         select DRM_WANT_CMA
> 
> config DRM_WANT_CMA
>         bool
>         help
>            Select this from any driver that benefits from CMA being enabled
> 
> config DMA_CMA
>         bool "Use CMA helpers for DRM"
>         default DRM_WANT_CMA
> 
>           Arnd
> 

That's precisely what I had first, with an additional "WANT_CMA" --  but 
looking at the number of such existing options (I was able to spot 1 !) 
I wondered if there is a better approach to achieve the same; "imply" 
sounded like a good candidate.

-- 
Thanks,

David / dhildenb

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[Arnd Bergmann]

On Thu, Apr 8, 2021 at 2:00 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 08.04.21 13:44, Arnd Bergmann wrote:
> > On Thu, Apr 8, 2021 at 1:00 PM David Hildenbrand <david@redhat.com> wrote:
> >>>
> >>> It is a somewhat awkward way to say "prevent this symbol from
> >>> being =y if the dependency is =m".
> >>
> >> What would be the right thing to do in the case here then to achieve the
> >> "if DRMA_ASPEED_GFX is enabled, also enable DMA_CMA id possible"?
> >>
> >> One approach could be to have for DMA_CMA
> >>
> >> default y if DRMA_ASPEED_GFX
> >>
> >> but it feels like the wrong way to tackle this.
> >
> > I'm still not sure what you are trying to achieve. Is the idea only to provide
> > a useful default for DMA_CMA depending on which drivers are enabled?
>
> "Random drivers should not override a user configuration of core knobs
> (e.g., CONFIG_DMA_CMA=n)."
>
> Let's assume I'm a distribution and want to set CONFIG_CMA=n or want to
> set CONFIG_DMA_CMA=n with CONFIG_CMA=y; there is no way to do that with
> e.g., DRMA_ASPEED_GFX=y because it will always override my (user!)
> setting -- even though it doesn't really always need it. Using "select"
> is the problem here.

I agree on the part of removing the 'select' if we don't need it. The
part I couldn't figure out was what the 'imply' is supposed to help with.
Most other users that added imply tried (and failed) to fix a build problem.

> > This is something you could do using a hidden helper symbol like
> >
> > config DRMA_ASPEED_GFX
> >         bool "Aspeed display driver"
> >         select DRM_WANT_CMA
> >
> > config DRM_WANT_CMA
> >         bool
> >         help
> >            Select this from any driver that benefits from CMA being enabled
> >
> > config DMA_CMA
> >         bool "Use CMA helpers for DRM"
> >         default DRM_WANT_CMA
> >
> >           Arnd
> >
>
> That's precisely what I had first, with an additional "WANT_CMA" --  but
> looking at the number of such existing options (I was able to spot 1 !)
> I wondered if there is a better approach to achieve the same; "imply"
> sounded like a good candidate.

I can probably find a couple more, but regardless of how many others
exist, this would be a much clearer way of doing it than 'imply' since it
has none of the ambiguity and misuse problems.

I think the reason we don't see more is that generally speaking, those
defaults are widely ignored anyway. You almost always start out with
a defconfig file that contains everything you know you need, and then
you add bits to that. Having the default in any form only helps to
make that defconfig file one line shorter, while requiring other users
to add another line to turn it off when they do not want it.

         Arnd

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[Linus Walleij]

On Thu, Apr 8, 2021 at 2:01 PM David Hildenbrand <david@redhat.com> wrote:

> > This is something you could do using a hidden helper symbol like
> >
> > config DRMA_ASPEED_GFX
> >         bool "Aspeed display driver"
> >         select DRM_WANT_CMA
> >
> > config DRM_WANT_CMA
> >         bool
> >         help
> >            Select this from any driver that benefits from CMA being enabled
> >
> > config DMA_CMA
> >         bool "Use CMA helpers for DRM"
> >         default DRM_WANT_CMA
> >
> >           Arnd
> >
>
> That's precisely what I had first, with an additional "WANT_CMA" --  but
> looking at the number of such existing options (I was able to spot 1 !)

If you do this it probably makes sense to fix a few other drivers
Kconfig in the process. It's not just a problem with your driver.
"my" drivers:

drivers/gpu/drm/mcde/Kconfig
drivers/gpu/drm/pl111/Kconfig
drivers/gpu/drm/tve200/Kconfig

certainly needs this as well, and pretty much anything that is
selecting DRM_KMS_CMA_HELPER or
DRM_GEM_CMA_HELPER "wants" DMA_CMA.

Yours,
Linus Walleij

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[Arnd Bergmann]

On Thu, Apr 8, 2021 at 2:50 PM Linus Walleij <linus.walleij@linaro.org> wrote:
>
> On Thu, Apr 8, 2021 at 2:01 PM David Hildenbrand <david@redhat.com> wrote:
>
> > > This is something you could do using a hidden helper symbol like
> > >
> > > config DRMA_ASPEED_GFX
> > >         bool "Aspeed display driver"
> > >         select DRM_WANT_CMA
> > >
> > > config DRM_WANT_CMA
> > >         bool
> > >         help
> > >            Select this from any driver that benefits from CMA being enabled
> > >
> > > config DMA_CMA
> > >         bool "Use CMA helpers for DRM"
> > >         default DRM_WANT_CMA
> > >
> > >           Arnd
> > >
> >
> > That's precisely what I had first, with an additional "WANT_CMA" --  but
> > looking at the number of such existing options (I was able to spot 1 !)
>
> If you do this it probably makes sense to fix a few other drivers
> Kconfig in the process. It's not just a problem with your driver.
> "my" drivers:
>
> drivers/gpu/drm/mcde/Kconfig
> drivers/gpu/drm/pl111/Kconfig
> drivers/gpu/drm/tve200/Kconfig
>
> certainly needs this as well, and pretty much anything that is
> selecting DRM_KMS_CMA_HELPER or
> DRM_GEM_CMA_HELPER "wants" DMA_CMA.

Are there any that don't select either of the helpers and
still want CMA? If not, it would be easy to just add

   default  DRM_KMS_CMA_HELPER || DRM_GEM_CMA_HELPER

and skipt the extra symbol.

        Arnd

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[David Hildenbrand]

On 08.04.21 15:19, Arnd Bergmann wrote:
> On Thu, Apr 8, 2021 at 2:50 PM Linus Walleij <linus.walleij@linaro.org> wrote:
>>
>> On Thu, Apr 8, 2021 at 2:01 PM David Hildenbrand <david@redhat.com> wrote:
>>
>>>> This is something you could do using a hidden helper symbol like
>>>>
>>>> config DRMA_ASPEED_GFX
>>>>          bool "Aspeed display driver"
>>>>          select DRM_WANT_CMA
>>>>
>>>> config DRM_WANT_CMA
>>>>          bool
>>>>          help
>>>>             Select this from any driver that benefits from CMA being enabled
>>>>
>>>> config DMA_CMA
>>>>          bool "Use CMA helpers for DRM"
>>>>          default DRM_WANT_CMA
>>>>
>>>>            Arnd
>>>>
>>>
>>> That's precisely what I had first, with an additional "WANT_CMA" --  but
>>> looking at the number of such existing options (I was able to spot 1 !)
>>
>> If you do this it probably makes sense to fix a few other drivers
>> Kconfig in the process. It's not just a problem with your driver.
>> "my" drivers:
>>
>> drivers/gpu/drm/mcde/Kconfig
>> drivers/gpu/drm/pl111/Kconfig
>> drivers/gpu/drm/tve200/Kconfig
>>
>> certainly needs this as well, and pretty much anything that is
>> selecting DRM_KMS_CMA_HELPER or
>> DRM_GEM_CMA_HELPER "wants" DMA_CMA.
> 
> Are there any that don't select either of the helpers and
> still want CMA? If not, it would be easy to just add
> 
>     default  DRM_KMS_CMA_HELPER || DRM_GEM_CMA_HELPER
> 
> and skipt the extra symbol.

That sounds like a reasonable thing to do. I'll look into that.

-- 
Thanks,

David / dhildenb

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[David Hildenbrand]

On 08.04.21 14:49, Linus Walleij wrote:
> On Thu, Apr 8, 2021 at 2:01 PM David Hildenbrand <david@redhat.com> wrote:
> 
>>> This is something you could do using a hidden helper symbol like
>>>
>>> config DRMA_ASPEED_GFX
>>>          bool "Aspeed display driver"
>>>          select DRM_WANT_CMA
>>>
>>> config DRM_WANT_CMA
>>>          bool
>>>          help
>>>             Select this from any driver that benefits from CMA being enabled
>>>
>>> config DMA_CMA
>>>          bool "Use CMA helpers for DRM"
>>>          default DRM_WANT_CMA
>>>
>>>            Arnd
>>>
>>
>> That's precisely what I had first, with an additional "WANT_CMA" --  but
>> looking at the number of such existing options (I was able to spot 1 !)
> 
> If you do this it probably makes sense to fix a few other drivers
> Kconfig in the process. It's not just a problem with your driver.
> "my" drivers:
> 

:) I actually wanted to convert them to "depends on DMA_CMA" but ran 
into recursive dependencies ...

> drivers/gpu/drm/mcde/Kconfig
> drivers/gpu/drm/pl111/Kconfig
> drivers/gpu/drm/tve200/Kconfig

I was assuming these are "real" dependencies. Will it also work without 
DMA_CMA?

> 
> certainly needs this as well, and pretty much anything that is
> selecting DRM_KMS_CMA_HELPER or
> DRM_GEM_CMA_HELPER "wants" DMA_CMA.

"wants" as in "desires to use but can life without" or "wants" as in 
"really needs it". ?

-- 
Thanks,

David / dhildenb

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[Arnd Bergmann]

On Thu, Apr 8, 2021 at 6:45 PM David Hildenbrand <david@redhat.com> wrote:
> On 08.04.21 14:49, Linus Walleij wrote:
> > On Thu, Apr 8, 2021 at 2:01 PM David Hildenbrand <david@redhat.com> wrote:
> >
> >>> This is something you could do using a hidden helper symbol like
> >>>
> >>> config DRMA_ASPEED_GFX
> >>>          bool "Aspeed display driver"
> >>>          select DRM_WANT_CMA
> >>>
> >>> config DRM_WANT_CMA
> >>>          bool
> >>>          help
> >>>             Select this from any driver that benefits from CMA being enabled
> >>>
> >>> config DMA_CMA
> >>>          bool "Use CMA helpers for DRM"
> >>>          default DRM_WANT_CMA
> >>>
> >>>            Arnd
> >>>
> >>
> >> That's precisely what I had first, with an additional "WANT_CMA" --  but
> >> looking at the number of such existing options (I was able to spot 1 !)
> >
> > If you do this it probably makes sense to fix a few other drivers
> > Kconfig in the process. It's not just a problem with your driver.
> > "my" drivers:
> >
>
> :) I actually wanted to convert them to "depends on DMA_CMA" but ran
> into recursive dependencies ...
>
> > drivers/gpu/drm/mcde/Kconfig
> > drivers/gpu/drm/pl111/Kconfig
> > drivers/gpu/drm/tve200/Kconfig

Right, this is the main problem caused by using 'select' to
force-enable symbols that other drivers depend on.

Usually, the answer is to be consistent about the use of 'select'
and 'depends on', using the former only to enable symbols that
are hidden, while using 'depends on' for anything that is an
actual build time dependency.

> I was assuming these are "real" dependencies. Will it also work without
> DMA_CMA?

I think in this case, it is fairly likely to work without DMA_CMA when the
probe function gets called during a fresh boot, but fairly likely to fail if
it gets called after the system has run for long enough to fragment the
free memory.

The point of DMA_CMA is to make it work reliably.

      Arnd

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[David Hildenbrand]

On 08.04.21 22:29, Arnd Bergmann wrote:
> On Thu, Apr 8, 2021 at 6:45 PM David Hildenbrand <david@redhat.com> wrote:
>> On 08.04.21 14:49, Linus Walleij wrote:
>>> On Thu, Apr 8, 2021 at 2:01 PM David Hildenbrand <david@redhat.com> wrote:
>>>
>>>>> This is something you could do using a hidden helper symbol like
>>>>>
>>>>> config DRMA_ASPEED_GFX
>>>>>           bool "Aspeed display driver"
>>>>>           select DRM_WANT_CMA
>>>>>
>>>>> config DRM_WANT_CMA
>>>>>           bool
>>>>>           help
>>>>>              Select this from any driver that benefits from CMA being enabled
>>>>>
>>>>> config DMA_CMA
>>>>>           bool "Use CMA helpers for DRM"
>>>>>           default DRM_WANT_CMA
>>>>>
>>>>>             Arnd
>>>>>
>>>>
>>>> That's precisely what I had first, with an additional "WANT_CMA" --  but
>>>> looking at the number of such existing options (I was able to spot 1 !)
>>>
>>> If you do this it probably makes sense to fix a few other drivers
>>> Kconfig in the process. It's not just a problem with your driver.
>>> "my" drivers:
>>>
>>
>> :) I actually wanted to convert them to "depends on DMA_CMA" but ran
>> into recursive dependencies ...
>>
>>> drivers/gpu/drm/mcde/Kconfig
>>> drivers/gpu/drm/pl111/Kconfig
>>> drivers/gpu/drm/tve200/Kconfig
> 
> Right, this is the main problem caused by using 'select' to
> force-enable symbols that other drivers depend on.
> 
> Usually, the answer is to be consistent about the use of 'select'
> and 'depends on', using the former only to enable symbols that
> are hidden, while using 'depends on' for anything that is an
> actual build time dependency.
> 
>> I was assuming these are "real" dependencies. Will it also work without
>> DMA_CMA?
> 
> I think in this case, it is fairly likely to work without DMA_CMA when the
> probe function gets called during a fresh boot, but fairly likely to fail if
> it gets called after the system has run for long enough to fragment the
> free memory.
> 
> The point of DMA_CMA is to make it work reliably.

Right, and even at runtime there is no guarantee that DMA_CMA will do 
anything -- especially if we don't reserve a CMA region (e.g., "cma=X").

So this really sounds like a

"desires DMA_CMA"

and achieving that via an additional symbol or via "default y if ..." 
for DMA_CMA sounds reasonable.

Thanks!

-- 
Thanks,

David / dhildenb

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[Linus Walleij]

On Thu, Apr 8, 2021 at 6:44 PM David Hildenbrand <david@redhat.com> wrote:

> > drivers/gpu/drm/mcde/Kconfig
> > drivers/gpu/drm/pl111/Kconfig
> > drivers/gpu/drm/tve200/Kconfig
>
> I was assuming these are "real" dependencies. Will it also work without
> DMA_CMA?

It will mostly work but that is only because the reservations are
mostly contiguous anyway because they are done early and
are small. The hardware requires contiguous buffers in all
three cases. I'm not sure I always got it right.

> > certainly needs this as well, and pretty much anything that is
> > selecting DRM_KMS_CMA_HELPER or
> > DRM_GEM_CMA_HELPER "wants" DMA_CMA.
>
> "wants" as in "desires to use but can life without" or "wants" as in
> "really needs it". ?

I don't know the exact semantics of using DRM_KMS_CMA*
without actually using DMA_CMA. I suspect small allocations
will be contiguous and big allocations will start to fragment?
but it's just my guess. I guess "really need it"?

Yours,
Linus Walleij

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[Daniel Vetter]

On Thu, Apr 08, 2021 at 12:20:50PM +0200, Arnd Bergmann wrote:
> On Thu, Apr 8, 2021 at 11:22 AM David Hildenbrand <david@redhat.com> wrote:
> >
> > Random drivers should not override a user configuration of core knobs
> > (e.g., CONFIG_DMA_CMA=n). Use "imply" instead, to still respect
> > dependencies and manual overrides.
> >
> > "This is similar to "select" as it enforces a lower limit on another
> >  symbol except that the "implied" symbol's value may still be set to n
> >  from a direct dependency or with a visible prompt."
> >
> > Implying DRM_CMA should be sufficient, as that depends on CMA.
> >
> > Note: If this is a real dependency, we should use "depends on DMA_CMA"
> > instead -  but I assume the driver can work without CMA just fine --
> > esp. when we wouldn't have HAVE_DMA_CONTIGUOUS right now.
> 
> 'imply' is almost never the right solution, and it tends to cause more
> problems than it solves.
> 
> In particular, it does not prevent a configuration with 'DRM_CMA=m'
> and 'DRMA_ASPEED_GFX=y', or any build failures from such
> a configuration.
> 
> If you want this kind of soft dependency, you need
> 'depends on DRM_CMA || !DRM_CMA'.

The problem is that depends on is a real pain for users to find their
drivers. This is why we have a ton of select, because the kconfig ui tends
to suck.

If you want to change this, we need automatic conflict resolution like apt
and other package managers have, with suggestions how to fix the config if
you want to enable a driver, but some of its requirements are missing. The
current approach of hiding driver symbols complete if any of their
dependencies are off is really not great.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

[PATCH v1 2/2] drivers/gpu/drm: don't select DMA_CMA or CMA from aspeed or etnaviv

[Jason Gunthorpe]

On Thu, Apr 08, 2021 at 01:38:17PM +0200, Daniel Vetter wrote:

> If you want to change this, we need automatic conflict resolution like apt
> and other package managers have, with suggestions how to fix the config if
> you want to enable a driver, but some of its requirements are missing. The
> current approach of hiding driver symbols complete if any of their
> dependencies are off is really not great.

+1 to this.. 

Though Kconfig is basically already unusuable unless you have hours to
carefully craft the kconfig you need to get out.

I'm not sure trying to optimize this by abusing the existing language
rules is such a good idea.

I gave a very half hearted go at a simple heuristic solution solve for
kconfig a while ago. It is good enough to sometimes automate a kconfig
task, but it is not so nice.

I use it to do things like "turn on all RDMA drivers" which is quite
a hard to do by hand.

It looks liked heursitics need a lot of fine tuning as the
conditionals are complex enough that it is hard to guess which branch
is going to yield a success.

Jason