[PATCH] ARM: dma-mapping: Fix dma_get_sgtable() for regions without struct page

On Wed, Mar 29, 2017 at 10:51:05AM -0600, Shuah Khan wrote:

Hi Russell,

On Wed, Mar 29, 2017 at 10:33 AM, Russell King - ARM Linux
[off-list ref] wrote:

quoted

Okay, I'm about to merge the following patch for -rc, since refusing
to create a scattertable for non-page backed memory is the only valid
solution for that case.  I'm intending to queue this for -rc this
evening.

I was bit sidetracked and getting back to this today. I am just about
to test this change on my system. I will test your patch and send you
results.

This might not help my use-case - I suspect it will trip this check.
That said I will send you results very soon.

I do hope that it _does_ trip this check - the result should be that you
get an error without the kernel oopsing.

I have been thinking about this some. I would like to see if we can
provide a way forward to address the cases where coherent memory is
not guaranteed to be backed by struct pages. We know the memory isn't
backed at alloc time in dma_alloc_coherent(). Can we key off of that
maybe add a new attr flag to avoid page lookups. I am willing to work
on the fixing it.

What we need is to revise the dma-buf API and DMA APIs to allow passing
coherent memory through it - there is no current provision in the DMA APIs
to have coherent memory mapped for two peripherals.

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