Handling vmalloc'd buffers in spi-nor

[Marek Vasut]

On 02/21/2017 09:37 AM, Vignesh R wrote:
> Hi all,

Hi!

> I have come across couple of problems[1] when using m25p80 driver with
> SPI flash over SPI bus with DMA. Basically filesystems like UBIFS may
> pass vmalloc'd buffers to SPI NOR layer which will end up in SPI layer.
> SPI core does try to handle such buffers (see spi_map_buf()) by doing
> vmalloc_to_page() and creating scatterlist.
> But, its known that this does not work well with VIVT/aliasing cache
> architectures.
> This also fails when buffers are addressed using LPAE (buffers in region
> higher than 32 bit addressable region), if DMA is 32bit only.
> 
> IMO, it would be better if vmalloc'd buffers are handled in SPI NOR
> layer, just like nand, instead of passing them onto SPI layer.
> In order to deal with vmalloc'd buffers passed by higher layers (like
> UBIFS), I propose to use pre-allocated per flash bounce buffer equal to
> the sector size of the flash. This feature can be enabled using a flag
> similar to NAND_USE_BOUNCE_BUFFER. The changes will be mostly restricted
> to spi_nor_read/write functions. This will help SPI drivers to use DMA
> with NOR flashes with filesystems.
> 
> This would also enable other SPI NOR drivers to safely use DMA in their
> read/write callbacks.
> 
> Any thoughts/comments?

Code please :-)

Let's discuss this over a patch. With the recent spi-nor and spi-nand
controllers, I think this makes sense though.

> Thanks!
> 
> [1]https://www.spinics.net/lists/arm-kernel/msg563773.html
> 


-- 
Best regards,
Marek Vasut