Re: [PATCH] staging: ion: ion_cma_heap: Don't directly use dma_common_get_sgtable

Jon Medhurst (Tixy)
2015-07-17T16:29:42Z

On Fri, 2015-07-17 at 16:21 +0100, Robin Murphy wrote:
> This also begs the question as to what happens if the memory region _is_
> > contiguous but is in highmem or an ioremapped region. Should a device
> > always provide dma_ops for that case? Because I believe the current
> > implementation of dma_common_get_sgtable won't work for those as it uses
> > virt_to_page.
> >
> > I see that this point has been raised before [1] by Zeng Tao, and I
> > myself have been given a different fix to apply to a Linaro kernel tree.
> > However, both solutions looked wrong to me as they treat a dma_addr_t as
> > a physical address, so should at least be using dma_to_phys.
> > So, should we fix dma_common_get_sgtable or mandate that the device
> > has dma_ops? The latter seems to be implied by the commit message which
> > introduced the function:
> >
> > This patch provides a generic implementation based on
> > virt_to_page() call. Architectures which require more
> > sophisticated translation might provide their own get_sgtable()
> > methods.
>
> Given that we're largely here due to having poked this on arm64 systems,
> I'm inclined to think that implementing our own get_sgtable as per
> arch/arm is the right course of action. Since a lot of architectures
> using dma_common_get_sgtable don't even implement dma_to_phys,

I had another check and that seems to be true.

> I don't
> think it would be right to try complicating the common code for a case
> that seems to be all but common.

I'm inclined to agree, however I'm rather new to this area.

> I can spin an arm64 patch if you like.

That would be good. Especially as from what I see on the arm kernel
lists you are already working in that area... And my inbox has just
pinged with that patch from you, so I'll add a reference here [2] so
people coming across this thread can find it easily.

For 32-bit arm my $subject patch should fix ION as that already has the
DMA ops.

--
Tixy

[1] https://lkml.org/lkml/2014/12/1/584
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2015-July/357561.html