linux-kernel - Re: [PATCH] spi: Ensure memory used for spi_write_then

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Message-ID: <db36bbf0-0ad5-4c37-bfcc-917508805eba@sirena.org.uk>
Date: Thu, 20 Mar 2025 14:42:05 +0000
From: Mark Brown <broonie@...nel.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: Petr Tesarik <ptesarik@...e.com>,
	Grant Likely <grant.likely@...retlab.ca>,
	linux-kernel@...r.kernel.org, linux-spi@...r.kernel.org
Subject: Re: [PATCH] spi: Ensure memory used for spi_write_then_read() is DMA
 safe

On Thu, Mar 20, 2025 at 02:35:41PM +0100, Arnd Bergmann wrote:
> On Thu, Mar 20, 2025, at 13:29, Mark Brown wrote:

> > On a lot of systems most transfers are short and won't be DMAed at all
> > since PIO ends up being more efficient, and most hardware is perfectly
> > happy to DMA to/from wherever so *shrug*.  SPI_BUFSIZ is a maximum of 32
> > bytes which is going to be under the copybreak limit for quite a few
> > controllers, though admittedly 16 is also a popular number, so a lot of
> > the time we don't actually DMA out of it at all.

> I saw the same thing looked at it the other day and got confused
> about why 'local_buf' is allocated with GFP_DMA and 'buf'
> uses plain GFP_KERNEL when they are both used in the same place.

Really we just don't expect the small buffer to be DMAed.

> It also seems that the copy happens in the regmap_bulk_read()
> path but not the regmap_bulk_write(), which just passes down
> the original buffer without copying, as far as I can tell.

Yes, writes don't need to do anything.

> From what I found, there are two scenarios that may depend on
> GFP_DMA today:
> 
>  a) a performance optimization where allocating from GFP_DMA avoids
>     the swiotlb bounce buffering. This would be the normal case on
>     any 64-bit machine with more than 4GB of RAM and an SPI
>     controller with a 32-bit DMA mask.
>  b) An SPI controller on a 32-bit machine without swiotlb and an
>     effective DMA mask that covers less than the lowmem area.
>     E.g. on Raspberry Pi 4, the brcm,bcm2835-spi lives on a
>     bus with an 1GB dma-ranges translation, but there may be more
>     than 1GB of lowmem with CONFIG_VMSPLIT_2G=y and CONFIG_SWIOTLB=n.
>     Without GFP_DMA that would just end up causing data corruption.

That sounds about right.

> I think we have some corner cases where a driver allocates
> a GFP_DMA buffer, calls spi_write_then_read through regmap,
> which copies the data to the non-GFP_DMA global buffer,
> and then the SPI controller driver calls dma_map_single()
> on that, ending up with a third bounce buffer from
> swiotlb.

I suspect that practically speaking almost everything will be under the
copybreak limit.  Probably we should just make regmap use spi_sync()
with the supplied buffers here and avoid spi_write_then_read().

> I don't know what a good replacement interface would be, but
> ideally there should never be more than one copy here,
> which means that any temporary buffer would need to be
> allocated according to the dma_mask of the underlying
> bus master (dmaengine, spi controller, ...).

Which is a pain because even if you've got the device for the SPI
controller there's no way to discover if it does it's own DMA.

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