linux-kernel - Re: Applied "spi: atmel: fix corrupted data issue on SAM9 family SoCs" to the spi tree

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Message-ID: <f2eb4994-2cea-4ad4-58ee-24a174d53f93@microchip.com>
Date:   Tue, 27 Jun 2017 11:05:56 +0200
From:   Cyrille Pitchen <cyrille.pitchen@...rochip.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Mark Brown <broonie@...nel.org>
CC:     <nicolas.ferre@...el.com>, <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>, <linux-spi@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: Applied "spi: atmel: fix corrupted data issue on SAM9 family
 SoCs" to the spi tree

Hi Russell,

Le 23/06/2017 à 19:18, Russell King - ARM Linux a écrit :
> On Fri, Jun 23, 2017 at 05:15:58PM +0100, Mark Brown wrote:
>> +#ifdef CONFIG_SOC_SAM_V4_V5
>> +	/*
>> +	 * Atmel SoCs based on ARM9 (SAM9x) cores should not use spi_map_buf()
>> +	 * since this later function tries to map buffers with dma_map_sg()
>> +	 * even if they have not been allocated inside DMA-safe areas.
>> +	 * On SoCs based on Cortex A5 (SAMA5Dx), it works anyway because for
>> +	 * those ARM cores, the data cache follows the PIPT model.
>> +	 * Also the L2 cache controller of SAMA5D2 uses the PIPT model too.
>> +	 * In case of PIPT caches, there cannot be cache aliases.
>> +	 * However 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 invalidated).
> 
> There is a solution to this - code (iow, callers of functions that perform
> IO) are expected to use flush_kernel_vmap_range() and
> invalidate_kernel_vmap_range() as documented in Documentation/cachetlb.txt
> to ensure that their "special" views are properly handled.
> 
> These are no-ops for PIPT caches, but aliasing caches have to implement
> them.
> 

So if I understand, calling those two functions at the right places, we
could use DMA transfer again without the need of copying data into a
bounce buffer? It sounds great, I will study that.

Thanks!

Cyrille