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

[Boris Brezillon <boris.brezillon@xxxxxxxxxxxxxxxxxx>]

On Wed, 1 Mar 2017 17:16:30 +0530
Vignesh R <vigneshr@xxxxxx> 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@xxxxxx>
> >>>> ---
> >>>>  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.
> 

I agree that using a bounce buffer when the address is not in the
kmalloc range is the safest solution we have. Now, if we are concerned
about the perf penalty brought by the extra copy, we should patch
UBI/UBIFS to allocate small chunks using kmalloc instead of allocating
PEB/LEB buffers using vmalloc. Of course this implies some rework, but
at least we'll get rid of all the cache invalidation problems.

Also note that not all DMA controllers are supporting SG transfers, so
having UBI allocate X buffers of ubi->max_write_size using kmalloc
instead of one buffer of (X * ubi->max_write_size) size using vmalloc
is probably the way to go if we want to avoid the extra copy in all
cases.

I started to work on the ubi_buffer concept a while ago (an
object containing an array of kmalloc-ed bufs, and some helpers to
read/write from/to these buffers), but I don't know if I still have this
branch somewhere.
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