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

[Vignesh R <vigneshr@xxxxxx>]

On Monday 20 February 2017 04:04 PM, Frode Isaksen wrote:
> 
> 
> On 20/02/2017 10:47, Vignesh R wrote:
>>
>> On Monday 20 February 2017 02:56 PM, Frode Isaksen wrote:
>>>
>>> On 20/02/2017 07:55, Vignesh R wrote:
>>>> On Friday 17 February 2017 05:37 PM, Russell King - ARM Linux wrote:
[...]
>>>> 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 ?
>> read_ltab() isn't the only place where vmalloc() is used. A quick grep
>> for vmalloc on fs/ubifs/ shows about ~19 occurrence. I guess every
>> vmalloc() call can potentially allocate memory from highmem and might
>> potentially cause issue for VIVT and such aliasing caches.
>> Fixing just one such case isn't going to help IMHO.
> Of course fixing it only in one place is of course not going to help..
> For the moment there are 3 solutions to the UBIFS DMA problem:
> 1) Always use bounce buffer for vmalloc'ed buffers - impacts everyone.
> 2) Remove use of vmalloc'ed buffers in UBIFS - is it possible ?

Maybe. But what about mtdblock with JFFS2 on top of it, mtdblock still
uses vmalloc'd buffers?


-- 
Regards
Vignesh
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