Re: [RFC PATCH 2/2] mtd: devices: m25p80: Enable spi-nor bounce buffer support

[Vignesh R <vigneshr@ti.com>]

On Wednesday 01 March 2017 04:13 PM, Cyrille Pitchen wrote:
> Le 01/03/2017 à 05:54, Vignesh R a écrit :
>>
>>
>> On Wednesday 01 March 2017 03:11 AM, Richard Weinberger wrote:
>>> Vignesh,
>>>
>>> Am 27.02.2017 um 13:08 schrieb Vignesh R:
>>>> Many SPI controller drivers use DMA to read/write from m25p80 compatible
>>>> flashes. Therefore enable bounce buffers support provided by spi-nor
>>>> framework to take care of handling vmalloc'd buffers which may not be
>>>> DMA'able.
>>>>
>>>> Signed-off-by: Vignesh R <vigneshr@ti.com>
>>>> ---
>>>>  drivers/mtd/devices/m25p80.c | 1 +
>>>>  1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/drivers/mtd/devices/m25p80.c b/drivers/mtd/devices/m25p80.c
>>>> index c4df3b1bded0..d05acf22eadf 100644
>>>> --- a/drivers/mtd/devices/m25p80.c
>>>> +++ b/drivers/mtd/devices/m25p80.c
>>>> @@ -241,6 +241,7 @@ static int m25p_probe(struct spi_device *spi)
>>>>  	else
>>>>  		flash_name = spi->modalias;
>>>>  
>>>> +	nor->flags |= SNOR_F_USE_BOUNCE_BUFFER;
>>>
>>> Isn't there a better way to detect whether a bounce buffer is needed or not?
>>
> 
> I agree with Richard: the bounce buffer should be enabled only if needed
> by the SPI controller.
> 
>> Yes, I can poke the spi->master struct to see of dma channels are
>> populated and request SNOR_F_USE_BOUNCE_BUFFER accordingly:
>>
>> -       nor->flags |= SNOR_F_USE_BOUNCE_BUFFER;
>> +       if (spi->master->dma_tx || spi->master->dma_rx)
>> +               nor->flags |= SNOR_F_USE_BOUNCE_BUFFER;
>> +
>>
> 
> However I don't agree with this solution: master->dma_{tx|rx} can be set
> for SPI controllers which already rely on spi_map_msg() to handle
> vmalloc'ed memory during DMA transfers.
> Such SPI controllers don't need the spi-nor bounce buffer.
> 
> spi_map_msg() can build a scatter-gather list from vmalloc'ed buffer
> then map this sg list with dma_map_sg(). AFAIK, It is safe to do so for
> architectures using PIPT caches since the possible cache aliases issue
> present for VIPT or VIVT caches is always avoided for PIPT caches.
> 
> For instance, the drivers/spi/spi-atmel.c driver relies on spi_map_sg()
> to be called from the SPI sub-system to handle vmalloc'ed buffers and
> both master->dma_tx and master->dma_rx are set by the this driver.
>
> 
> By the way, Is there any case where the same physical page is actually
> mapped into two different virtual addresses for the buffers allocated by
> the MTD sub-system? Because for a long time now I wonder whether the
> cache aliases issue is a real or only theoretical issue but I have no
> answer to that question.
> 

I have atleast one evidence of VIVT aliasing causing problem. Please see
this thread on DMA issues with davinci-spi driver
https://www.spinics.net/lists/arm-kernel/msg563420.html
https://www.spinics.net/lists/arm-kernel/msg563445.html

> Then my next question: is spi_map_msg() enough in every case, even with
> VIPT or VIVT caches?
> 

Not really, I am debugging another issue with UBIFS on DRA74 EVM (ARM
cortex-a15) 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.

IMO, there may be more undiscovered caveat with using dma_map_sg() for
non kmalloc'd buffers and its better that spi-nor starts handling these
buffers instead of relying on spi_map_msg() and working around every
time something pops up.

-- 
Regards
Vignesh

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