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

[Boris Brezillon <boris.brezillon@free-electrons.com>]

On Wed, 1 Mar 2017 17:16:30 +0530
Vignesh R <vigneshr@ti.com> wrote:

> 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.
> 

Ok, I had a closer look at the SPI framework, and it seems there's a
way to tell to the core that a specific transfer cannot use DMA
(->can_dam()). The first thing you should do is fix the spi-davinci
driver:

1/ implement ->can_dma()
2/ patch davinci_spi_bufs() to take the decision to do DMA or not on a
   per-xfer basis and not on a per-device basis

Then we can start thinking about how to improve perfs by using a bounce
buffer for large transfers, but I'm still not sure this should be done
at the MTD level...

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