Re: [PATCH v2 4/6] spi: davinci: flush caches when performing DMA

[Frode Isaksen <fisaksen@xxxxxxxxxxxx>]

On 20/02/2017 07:55, Vignesh R wrote:
>
> On Friday 17 February 2017 05:37 PM, Russell King - ARM Linux wrote:
> [...]
>> SPI is rather another special case - rather than SPI following the 
>> established mechanism of passing data references via scatterlists or 
>> similar, it also passes them via virtual addresses, which means SPI 
>> can directly access the vmalloc area when performing PIO.  This 
>> really makes the problem more complex, because it means that if you 
>> do have a SPI driver that does that, it's going to be
>> reading/writing direct from vmalloc space.
>>
>> That's not a problem as long as the data is only accessed via
>> vmalloc space, but it will definitely go totally wrong if the data
>> is subsequently mapped into userspace.
>>
>> The other important thing to realise is that the interfaces in 
>> cachetlb.txt assume that it's the lowmem mapping that will be
>> accessed, and the IO device will push that data out to physical
>> memory (either via the DMA API, or flush_kernel_dcache_page()).
>> That's not true of SPI, as it passes virtual addresses around.
>>
>> So... overall, I'm not sure that this problem is properly solvable
>> given SPIs insistance on passing virtual addresses and the
>> differences in this area between SPI and block.
>>
> I am debugging another issue with UBIFS wherein pages allocated by
> vmalloc are in highmem region that are not addressable using 32 bit
> addresses and is backed by LPAE. So, a 32 bit DMA cannot access these
> buffers at all.
> When dma_map_sg() is called to map these pages by spi_map_buf() the
> physical address is just truncated to 32 bit in pfn_to_dma() (as part of
> dma_map_sg() call). This results in random crashes as DMA starts
> accessing random memory during SPI read.
>
> Given, the above problem and also issue surrounding VIVT caches, I am
> thinking of may be using pre-allocated fixed size bounce buffer to
> handle buffers not in lowmem mapping.
> I have tried using 64KB pre-allocated buffer on TI DRA74 EVM with QSPI
> running at 76.8MHz and do not see any significant degradation in
> performance with UBIFS. Mainly because UBIFS seems to use vmalloc'd
> buffers only during initial preparing and mounting phase and not during
> file read/write.
I am seeing a bug caused by VIVT cache in 'read_ltab()' function. In this function, the vmalloc'ed buffer is of size 11. Isn't it better to use kmalloc in this case ?

Thanks,
Frode
>
> So, is it an acceptable solution to modify spi_map_buf() to use bounce
> buffer when buffer does not belong to lowmem region?
>

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