linux-kernel - Re: [PATCH] mmc: core Drop BLK_BOUNCE

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Date: Fri, 9 Feb 2024 23:35:13 +0100
From: Michał Mirosław <mirq-linux@...e.qmqm.pl>
To: Arnd Bergmann <arnd@...db.de>
Cc: Linus Walleij <linus.walleij@...aro.org>,
	Christoph Hellwig <hch@....de>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	"linux-mmc @ vger . kernel . org" <linux-mmc@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mmc: core Drop BLK_BOUNCE_HIGH

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@...aro.org>
> 
> Acked-by: Arnd Bergmann <arnd@...db.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