[PATCH] mmc: core Drop BLK_BOUNCE_HIGH

[PATCH] mmc: core Drop BLK_BOUNCE_HIGH

[Linus Walleij]

The MMC core sets BLK_BOUNCE_HIGH for devices where dma_mask
is unassigned.

For the majority of MMC hosts this path is never taken: the
OF core will unconditionally assign a 32-bit mask to any
OF device, and most MMC hosts are probed from device tree,
see drivers/of/platform.c:

of_platform_device_create_pdata()
        dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
        if (!dev->dev.dma_mask)
                dev->dev.dma_mask = &dev->dev.coherent_dma_mask;

of_amba_device_create()
        dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
        dev->dev.dma_mask = &dev->dev.coherent_dma_mask;

MMC devices that are probed from ACPI or PCI will likewise
have a proper dma_mask assigned.

The only remaining devices that could have a blank dma_mask
are platform devices instantiated from board files.

These are mostly used on systems without CONFIG_HIGHMEM
enabled which means the block layer will not bounce, and in
the few cases where it is enabled it is not used anyway:
for example some OMAP2 systems such as Nokia n800/n810 will
create a platform_device and not assign a dma_mask, however
they do not have any highmem, so no bouncing will happen
anyway: the block core checks if max_low_pfn >= max_pfn
and this will always be false.

Should it turn out there is a platform_device with blank
DMA mask actually using CONFIG_HIGHMEM somewhere out there
we should set dma_mask for it, not do this trickery.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
---
 drivers/mmc/core/queue.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/drivers/mmc/core/queue.c b/drivers/mmc/core/queue.c
index a0a2412f62a7..316415588a77 100644
--- a/drivers/mmc/core/queue.c
+++ b/drivers/mmc/core/queue.c
@@ -351,8 +351,6 @@ static void mmc_setup_queue(struct mmc_queue *mq, struct mmc_card *card)
 	if (mmc_can_erase(card))
 		mmc_queue_setup_discard(mq->queue, card);
 
-	if (!mmc_dev(host)->dma_mask || !*mmc_dev(host)->dma_mask)
-		blk_queue_bounce_limit(mq->queue, BLK_BOUNCE_HIGH);
 	blk_queue_max_hw_sectors(mq->queue,
 		min(host->max_blk_count, host->max_req_size / 512));
 	if (host->can_dma_map_merge)

---
base-commit: 6613476e225e090cc9aad49be7fa504e290dd33d
change-id: 20240124-mmc-no-blk-bounce-high-d84e8898c707

Best regards,
-- 
Linus Walleij <linus.walleij@linaro.org>

Re: [PATCH] mmc: core Drop BLK_BOUNCE_HIGH

[Arnd Bergmann]

On Thu, Jan 25, 2024, at 09:50, Linus Walleij wrote:
> The MMC core sets BLK_BOUNCE_HIGH for devices where dma_mask
> is unassigned.
>
> For the majority of MMC hosts this path is never taken: the
> OF core will unconditionally assign a 32-bit mask to any
> OF device, and most MMC hosts are probed from device tree,
> see drivers/of/platform.c:
>
> of_platform_device_create_pdata()
>         dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
>         if (!dev->dev.dma_mask)
>                 dev->dev.dma_mask = &dev->dev.coherent_dma_mask;
>
> of_amba_device_create()
>         dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
>         dev->dev.dma_mask = &dev->dev.coherent_dma_mask;
>
> MMC devices that are probed from ACPI or PCI will likewise
> have a proper dma_mask assigned.
>
> The only remaining devices that could have a blank dma_mask
> are platform devices instantiated from board files.
>
> These are mostly used on systems without CONFIG_HIGHMEM
> enabled which means the block layer will not bounce, and in
> the few cases where it is enabled it is not used anyway:
> for example some OMAP2 systems such as Nokia n800/n810 will
> create a platform_device and not assign a dma_mask, however
> they do not have any highmem, so no bouncing will happen
> anyway: the block core checks if max_low_pfn >= max_pfn
> and this will always be false.
>
> Should it turn out there is a platform_device with blank
> DMA mask actually using CONFIG_HIGHMEM somewhere out there
> we should set dma_mask for it, not do this trickery.
>
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Acked-by: Arnd Bergmann <arnd@arndb.de>

I think it's worth mentioning the cb710 example here, which
uses a platform device as a child of a PCI device and
does not assign a DMA mask nor use DMA.

This one will see a change in behavior, meaning that the
blockdev buffers are no longer bounced. As far as I can
tell, this is fine because the driver appears to correctly
use the sg_iter infrastructure for mapping data pages,
but it would be good to have this confirmed by
Michał Mirosław because this code path has probably never
been tested without BLK_BOUNCE_HIGH.

Adding Michał to Cc.

     Arnd

Re: [PATCH] mmc: core Drop BLK_BOUNCE_HIGH

[Michał Mirosław]

On Thu, Jan 25, 2024 at 10:47:26AM +0100, Arnd Bergmann wrote:
> On Thu, Jan 25, 2024, at 09:50, Linus Walleij wrote:
> > The MMC core sets BLK_BOUNCE_HIGH for devices where dma_mask
> > is unassigned.
> >
> > For the majority of MMC hosts this path is never taken: the
> > OF core will unconditionally assign a 32-bit mask to any
> > OF device, and most MMC hosts are probed from device tree,
> > see drivers/of/platform.c:
> >
> > of_platform_device_create_pdata()
> >         dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
> >         if (!dev->dev.dma_mask)
> >                 dev->dev.dma_mask = &dev->dev.coherent_dma_mask;
> >
> > of_amba_device_create()
> >         dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
> >         dev->dev.dma_mask = &dev->dev.coherent_dma_mask;
> >
> > MMC devices that are probed from ACPI or PCI will likewise
> > have a proper dma_mask assigned.
> >
> > The only remaining devices that could have a blank dma_mask
> > are platform devices instantiated from board files.
> >
> > These are mostly used on systems without CONFIG_HIGHMEM
> > enabled which means the block layer will not bounce, and in
> > the few cases where it is enabled it is not used anyway:
> > for example some OMAP2 systems such as Nokia n800/n810 will
> > create a platform_device and not assign a dma_mask, however
> > they do not have any highmem, so no bouncing will happen
> > anyway: the block core checks if max_low_pfn >= max_pfn
> > and this will always be false.
> >
> > Should it turn out there is a platform_device with blank
> > DMA mask actually using CONFIG_HIGHMEM somewhere out there
> > we should set dma_mask for it, not do this trickery.
> >
> > Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
> 
> Acked-by: Arnd Bergmann <arnd@arndb.de>
> 
> I think it's worth mentioning the cb710 example here, which
> uses a platform device as a child of a PCI device and
> does not assign a DMA mask nor use DMA.
> 
> This one will see a change in behavior, meaning that the
> blockdev buffers are no longer bounced. As far as I can
> tell, this is fine because the driver appears to correctly
> use the sg_iter infrastructure for mapping data pages,
> but it would be good to have this confirmed by
> Michał Mirosław because this code path has probably never
> been tested without BLK_BOUNCE_HIGH.

Hi, this driver doesn't do DMA at all, so having DMA mask set or not
it should be good as long as the CPU can read/write the buffers.

Best Regards
Michał Mirosław

Re: [PATCH] mmc: core Drop BLK_BOUNCE_HIGH

[Linus Walleij]

On Fri, Feb 9, 2024 at 11:35 PM Michał Mirosław <mirq-linux@rere.qmqm.pl> wrote:

> > I think it's worth mentioning the cb710 example here, which
> > uses a platform device as a child of a PCI device and
> > does not assign a DMA mask nor use DMA.
> >
> > This one will see a change in behavior, meaning that the
> > blockdev buffers are no longer bounced. As far as I can
> > tell, this is fine because the driver appears to correctly
> > use the sg_iter infrastructure for mapping data pages,
> > but it would be good to have this confirmed by
> > Michał Mirosław because this code path has probably never
> > been tested without BLK_BOUNCE_HIGH.
>
> Hi, this driver doesn't do DMA at all, so having DMA mask set or not
> it should be good as long as the CPU can read/write the buffers.

The only difference is where the CPU have to read/write the
buffers really, before the change those were all guaranteed to
be in lowmem (bounced there by the block core), now they can
also be in highmem, but sg_miter will deal with it for sure.

Yours,
Linus Wallej

Re: [PATCH] mmc: core Drop BLK_BOUNCE_HIGH

[Arnd Bergmann]

On Sat, Feb 10, 2024, at 00:41, Linus Walleij wrote:
> On Fri, Feb 9, 2024 at 11:35 PM Michał Mirosław <mirq-linux@rere.qmqm.pl> wrote:
>
>> > I think it's worth mentioning the cb710 example here, which
>> > uses a platform device as a child of a PCI device and
>> > does not assign a DMA mask nor use DMA.
>> >
>> > This one will see a change in behavior, meaning that the
>> > blockdev buffers are no longer bounced. As far as I can
>> > tell, this is fine because the driver appears to correctly
>> > use the sg_iter infrastructure for mapping data pages,
>> > but it would be good to have this confirmed by
>> > Michał Mirosław because this code path has probably never
>> > been tested without BLK_BOUNCE_HIGH.
>>
>> Hi, this driver doesn't do DMA at all, so having DMA mask set or not
>> it should be good as long as the CPU can read/write the buffers.
>
> The only difference is where the CPU have to read/write the
> buffers really, before the change those were all guaranteed to
> be in lowmem (bounced there by the block core), now they can
> also be in highmem, but sg_miter will deal with it for sure.

Yes, that was my point: The sg_miter() code is meant to
handle exactly this case with highmem data, but as far
as I can tell, that code path has never been tested on
32-bit systems with highmem but without BLK_BOUNCE_HIGH.

     Arnd

Re: [PATCH] mmc: core Drop BLK_BOUNCE_HIGH

[Linus Walleij]

On Sat, Feb 10, 2024 at 12:58 PM Arnd Bergmann <arnd@arndb.de> wrote:
> On Sat, Feb 10, 2024, at 00:41, Linus Walleij wrote:

> > The only difference is where the CPU have to read/write the
> > buffers really, before the change those were all guaranteed to
> > be in lowmem (bounced there by the block core), now they can
> > also be in highmem, but sg_miter will deal with it for sure.
>
> Yes, that was my point: The sg_miter() code is meant to
> handle exactly this case with highmem data, but as far
> as I can tell, that code path has never been tested on
> 32-bit systems with highmem but without BLK_BOUNCE_HIGH.

It's actually possible to enforce testing of highmem scatterlists
to an MMC card (one need to be careful as this is destructive
testing!)
drivers/mmc/core/mmc_test.c

...but the one relevant target I have is a Kirkwood and it only
has 128 MB of memory so highmem won't be exercised.

I'll put this into the cover letter on the other series (fixing a bunch
of drivers to use sg_miter) though.

Yours,
Linus Walleij

Re: [PATCH] mmc: core Drop BLK_BOUNCE_HIGH

[Arnd Bergmann]

On Sat, Feb 10, 2024, at 20:38, Linus Walleij wrote:
> On Sat, Feb 10, 2024 at 12:58 PM Arnd Bergmann <arnd@arndb.de> wrote:
>> On Sat, Feb 10, 2024, at 00:41, Linus Walleij wrote:
>
>> > The only difference is where the CPU have to read/write the
>> > buffers really, before the change those were all guaranteed to
>> > be in lowmem (bounced there by the block core), now they can
>> > also be in highmem, but sg_miter will deal with it for sure.
>>
>> Yes, that was my point: The sg_miter() code is meant to
>> handle exactly this case with highmem data, but as far
>> as I can tell, that code path has never been tested on
>> 32-bit systems with highmem but without BLK_BOUNCE_HIGH.
>
> It's actually possible to enforce testing of highmem scatterlists
> to an MMC card (one need to be careful as this is destructive
> testing!)
> drivers/mmc/core/mmc_test.c
>
> ...but the one relevant target I have is a Kirkwood and it only
> has 128 MB of memory so highmem won't be exercised.

I think you can pass a vmalloc= command line option to the
kernel that will increase the size of the vmalloc are at
the expense of lowmem and give you some highmem instead.

      Arnd

Re: [PATCH] mmc: core Drop BLK_BOUNCE_HIGH

[Christoph Hellwig]

From the block POV: awesome, 

Acked-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH] mmc: core Drop BLK_BOUNCE_HIGH

[Ulf Hansson]

On Thu, 25 Jan 2024 at 09:50, Linus Walleij <linus.walleij@linaro.org> wrote:
>
> The MMC core sets BLK_BOUNCE_HIGH for devices where dma_mask
> is unassigned.
>
> For the majority of MMC hosts this path is never taken: the
> OF core will unconditionally assign a 32-bit mask to any
> OF device, and most MMC hosts are probed from device tree,
> see drivers/of/platform.c:
>
> of_platform_device_create_pdata()
>         dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
>         if (!dev->dev.dma_mask)
>                 dev->dev.dma_mask = &dev->dev.coherent_dma_mask;
>
> of_amba_device_create()
>         dev->dev.coherent_dma_mask = DMA_BIT_MASK(32);
>         dev->dev.dma_mask = &dev->dev.coherent_dma_mask;
>
> MMC devices that are probed from ACPI or PCI will likewise
> have a proper dma_mask assigned.
>
> The only remaining devices that could have a blank dma_mask
> are platform devices instantiated from board files.
>
> These are mostly used on systems without CONFIG_HIGHMEM
> enabled which means the block layer will not bounce, and in
> the few cases where it is enabled it is not used anyway:
> for example some OMAP2 systems such as Nokia n800/n810 will
> create a platform_device and not assign a dma_mask, however
> they do not have any highmem, so no bouncing will happen
> anyway: the block core checks if max_low_pfn >= max_pfn
> and this will always be false.
>
> Should it turn out there is a platform_device with blank
> DMA mask actually using CONFIG_HIGHMEM somewhere out there
> we should set dma_mask for it, not do this trickery.
>
> Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Applied for next, thanks!

Kind regards
Uffe


> ---
>  drivers/mmc/core/queue.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/drivers/mmc/core/queue.c b/drivers/mmc/core/queue.c
> index a0a2412f62a7..316415588a77 100644
> --- a/drivers/mmc/core/queue.c
> +++ b/drivers/mmc/core/queue.c
> @@ -351,8 +351,6 @@ static void mmc_setup_queue(struct mmc_queue *mq, struct mmc_card *card)
>         if (mmc_can_erase(card))
>                 mmc_queue_setup_discard(mq->queue, card);
>
> -       if (!mmc_dev(host)->dma_mask || !*mmc_dev(host)->dma_mask)
> -               blk_queue_bounce_limit(mq->queue, BLK_BOUNCE_HIGH);
>         blk_queue_max_hw_sectors(mq->queue,
>                 min(host->max_blk_count, host->max_req_size / 512));
>         if (host->can_dma_map_merge)
>
> ---
> base-commit: 6613476e225e090cc9aad49be7fa504e290dd33d
> change-id: 20240124-mmc-no-blk-bounce-high-d84e8898c707
>
> Best regards,
> --
> Linus Walleij <linus.walleij@linaro.org>
>