Re: [RFC PATCH 2/2] spi: atmel: Fix DMA transfers data corruption

[Trent Piepho <tpiepho@impinj.com>]

On Wed, 2017-11-15 at 18:35 +0200, Radu Pirea wrote:
> If the cache model is VIVT, DMA data transfers may not be valid and to
> ensure the validity of the data cache must be flushed and invalidated.
> 
> Signed-off-by: Radu Pirea <radu.pirea@microchip.com>
>  
> +#ifdef CONFIG_SOC_SAM_V4_V5
> +	/*
> +	 * On Atmel SoCs based on ARM9 cores, the data cache follows the VIVT
> +	 * model, hence the cache aliases issue can occur when buffers are
> +	 * allocated from DMA-unsafe areas, by vmalloc() for instance, where
> +	 * cache coherency is not taken into account or at least not handled
> +	 * completely (cache lines of aliases are not flushed and invalidated).
> +	 * This is not a theorical issue: it was reproduced when trying to mount
> +	 * a UBI file-system on a at91sam9g35ek board.
> +	 */
> +	flush_kernel_vmap_range((void *)xfer->rx_buf, xfer->len);
> +#endif

Does this call need to be inside an ifdef for a specific SOC?  I
believe that flush_kernel_vmap_range should expand to a no-op if the
cache is not VIVT or aliasing VIPT.  So it should be safe to always
call it.

It also doesn't seem right that the SPI driver needs to know which SoCs
have what kind of cache.  If there are more socs with more cache
details, does the spi driver need to expand the list?  If another
driver does DMA, does it need the list too?


>  	/* Send both scatterlists */
>  	rxdesc = dmaengine_prep_slave_sg(rxchan,
>  					 xfer->rx_sg.sgl, xfer->rx_sg.nents,

Does this problem also possibly affect all other users of dmaengine on
these SoCs?  I2C, serial, crpto, etc. I couldn't find any other driver
that need to make flush_kernel_vmap_range().  Which seems odd, since
there are many drivers that use DMA and run on ARM9 cores.  E.g., I
believe spi-mxs runs on ARM9 based iMX.23 and iMX.28 which also have
VIVT caches.  It doesn't make flush_kernel_vmap_range calls.  Does it
have this same bug?