Re: [Linaro-mm-sig] [PATCH 2/4] staging: android: ion: Restrict cache maintenance to dma mapped memory

["Ørjan Eide" <Orjan.Eide@arm.com>]

On Wed, Feb 06, 2019 at 11:31:04PM -0800, Christoph Hellwig wrote:
> The CPU may only access DMA mapped memory if ownership has been
> transferred back to the CPU using dma_sync_{single,sg}_to_cpu, and then
> before the device can access it again ownership needs to be transferred
> back to the device using dma_sync_{single,sg}_to_device.
> 
> > I've run some testing, and this patch does indeed fix the crash in
> > dma_sync_sg_for_cpu when it tried to use the 0 dma_address from the sg
> > list.
> > 
> > Tested-by: Ørjan Eide <orjan.eide@arm.com>
> > 
> > I tested this on an older kernel, v4.14, since the dma-mapping code
> > moved, in v4.19, to ignore the dma_address and instead use sg_phys() to
> > get a valid address from the page, which is always valid in the ion sg
> > lists. While this wouldn't crash on newer kernels, it's still good to
> > avoid the unnecessary work when no CMO is needed.
> 
> Can you also test is with CONFIG_DMA_API_DEBUG enabled, as that should
> catch all the usual mistakes in DMA API usage, including the one found?

I checked again with CONFIG_DMA_API_DEBUG=y, both with and without this
patch, and I didn't get any dma-mapping errors. 

The issue I hit, without this patch, is when a CPU access starts after a
device have attached, which caused ion to create a copy of the buffer's
sg list with dma_address zeroed, but before the device have mapped the
buffer.

-- 
Ørjan