[PATCH] powerpc: Kconfig.cputype: Disallow TUNE_CELL on LE systems

[PATCH] powerpc: Kconfig.cputype: Disallow TUNE_CELL on LE systems

[Thomas Huth]

It looks somewhat weird that you can enable TUNE_CELL on little
endian systems, so let's disable this option with CPU_LITTLE_ENDIAN.

Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 I first thought that it might be better to make this option depend
 on PPC_CELL instead ... but I guess it's a bad idea to depend a
 CPU option on a platform option? Alternatively, would it make
 sense to make it depend on (GENERIC_CPU || CELL_CPU) instead?

 arch/powerpc/platforms/Kconfig.cputype | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/platforms/Kconfig.cputype b/arch/powerpc/platforms/Kconfig.cputype
index c140e94..d93e131 100644
--- a/arch/powerpc/platforms/Kconfig.cputype
+++ b/arch/powerpc/platforms/Kconfig.cputype
@@ -149,7 +149,7 @@ config 6xx
 
 config TUNE_CELL
 	bool "Optimize for Cell Broadband Engine"
-	depends on PPC64 && PPC_BOOK3S
+	depends on PPC64 && PPC_BOOK3S && !CPU_LITTLE_ENDIAN
 	help
 	  Cause the compiler to optimize for the PPE of the Cell Broadband
 	  Engine. This will make the code run considerably faster on Cell
-- 
1.8.3.1

Re: [PATCH] powerpc: Kconfig.cputype: Disallow TUNE_CELL on LE systems

[Michael Ellerman]

On Fri, 2015-09-18 at 16:17 +0200, Thomas Huth wrote:
> It looks somewhat weird that you can enable TUNE_CELL on little
> endian systems, so let's disable this option with CPU_LITTLE_ENDIAN.
> 
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  I first thought that it might be better to make this option depend
>  on PPC_CELL instead ... but I guess it's a bad idea to depend a
>  CPU option on a platform option? Alternatively, would it make
>  sense to make it depend on (GENERIC_CPU || CELL_CPU) instead?

Hmm, it's a little backward, but I think it would be fine, and less confusing
for users. Both PS3 and Cell select PPC_CELL, so it would work in both those
cases.

cheers

Re: [PATCH] powerpc: Kconfig.cputype: Disallow TUNE_CELL on LE systems

[Thomas Huth]

On 21/09/15 09:18, Michael Ellerman wrote:
> On Fri, 2015-09-18 at 16:17 +0200, Thomas Huth wrote:
>> It looks somewhat weird that you can enable TUNE_CELL on little
>> endian systems, so let's disable this option with CPU_LITTLE_ENDIAN.
>>
>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>> ---
>>  I first thought that it might be better to make this option depend
>>  on PPC_CELL instead ... but I guess it's a bad idea to depend a
>>  CPU option on a platform option? Alternatively, would it make
>>  sense to make it depend on (GENERIC_CPU || CELL_CPU) instead?
> 
> Hmm, it's a little backward, but I think it would be fine, and less confusing
> for users. Both PS3 and Cell select PPC_CELL, so it would work in both those
> cases.

It's just that when you step through the kernel config (e.g. with "make
menuconfig"), you normally step through the "Processor support" first,
and then later do the "Platform support". I think most users won't look
back into "Processor support" again once they already reached the
"Platform support" section, so this TUNE_CELL option then might appear
unnoticed when you enable a Cell platform under "Platform support".

That's why thought that CPU_LITTLE_ENDIAN or (GENERIC_CPU || CELL_CPU)
might be the better option here... but if you think PPC_CELL is the
better way to go, I'm also fine to change my patch to use this instead
(in the end, TUNE_CELL is likely hardly used anymore nowadays, so it
should not be that bad if it does not appear "immediately" to the user).

 Thomas

Re: [PATCH] powerpc: Kconfig.cputype: Disallow TUNE_CELL on LE systems

[Michael Ellerman]

On Mon, 2015-09-21 at 12:07 +0200, Thomas Huth wrote:
> On 21/09/15 09:18, Michael Ellerman wrote:
> > On Fri, 2015-09-18 at 16:17 +0200, Thomas Huth wrote:
> >> It looks somewhat weird that you can enable TUNE_CELL on little
> >> endian systems, so let's disable this option with CPU_LITTLE_ENDIAN.
> >>
> >> Signed-off-by: Thomas Huth <thuth@redhat.com>
> >> ---
> >>  I first thought that it might be better to make this option depend
> >>  on PPC_CELL instead ... but I guess it's a bad idea to depend a
> >>  CPU option on a platform option? Alternatively, would it make
> >>  sense to make it depend on (GENERIC_CPU || CELL_CPU) instead?
> > 
> > Hmm, it's a little backward, but I think it would be fine, and less confusing
> > for users. Both PS3 and Cell select PPC_CELL, so it would work in both those
> > cases.
> 
> It's just that when you step through the kernel config (e.g. with "make
> menuconfig"), you normally step through the "Processor support" first,
> and then later do the "Platform support". I think most users won't look
> back into "Processor support" again once they already reached the
> "Platform support" section, so this TUNE_CELL option then might appear
> unnoticed when you enable a Cell platform under "Platform support".

Ah OK. Personally I almost never use menuconfig, but I guess some folks do.

That actually seems like we should reorder those sections, ie. put platform
support first, and then processor support. After all there's not much point
agonising over whether to tune for CELL cpus if you then don't enable a Cell
platform.

I'm not sure if it's that simple in practice ... :)

cheers

Re: [PATCH] powerpc: Kconfig.cputype: Disallow TUNE_CELL on LE systems

[Thomas Huth]

On 06/10/15 12:05, Michael Ellerman wrote:
> On Mon, 2015-09-21 at 12:07 +0200, Thomas Huth wrote:
>> On 21/09/15 09:18, Michael Ellerman wrote:
>>> On Fri, 2015-09-18 at 16:17 +0200, Thomas Huth wrote:
>>>> It looks somewhat weird that you can enable TUNE_CELL on little
>>>> endian systems, so let's disable this option with CPU_LITTLE_ENDIAN.
>>>>
>>>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>>>> ---
>>>>  I first thought that it might be better to make this option depend
>>>>  on PPC_CELL instead ... but I guess it's a bad idea to depend a
>>>>  CPU option on a platform option? Alternatively, would it make
>>>>  sense to make it depend on (GENERIC_CPU || CELL_CPU) instead?
>>>
>>> Hmm, it's a little backward, but I think it would be fine, and less confusing
>>> for users. Both PS3 and Cell select PPC_CELL, so it would work in both those
>>> cases.
>>
>> It's just that when you step through the kernel config (e.g. with "make
>> menuconfig"), you normally step through the "Processor support" first,
>> and then later do the "Platform support". I think most users won't look
>> back into "Processor support" again once they already reached the
>> "Platform support" section, so this TUNE_CELL option then might appear
>> unnoticed when you enable a Cell platform under "Platform support".
> 
> Ah OK. Personally I almost never use menuconfig, but I guess some folks do.
> 
> That actually seems like we should reorder those sections, ie. put platform
> support first, and then processor support. After all there's not much point
> agonising over whether to tune for CELL cpus if you then don't enable a Cell
> platform.

Not sure whether reordering the sections make much sense - others might
think "I want to support Cell chips with my distro, so let's enable that
first, then let's see which platforms I can select next..." - so I'd
rather not do that.

> I'm not sure if it's that simple in practice ... :)

Maybe we could also simply remove the TUNE_CELL option nowadays? I think
this was used for building generic Linux distros, which are just
optimized for Cell ... but who is still doing that nowadays?

Alternatively, if that is not an option and if you don't like my patch
with CPU_LITTLE_ENDIAN, what about changing it to check "depends on
(GENERIC_CPU || CELL_CPU)" instead?

 Thomas