[Linaro-mm-sig] [PATCH v3 1/2] habanalabs: define uAPI to export FD for DMA-BUF
christian.koenig at amd.com
Thu Jun 24 08:07:14 UTC 2021
Am 24.06.21 um 07:34 schrieb Christoph Hellwig:
> On Wed, Jun 23, 2021 at 10:00:29PM +0300, Oded Gabbay wrote:
>> I understand the argument and I agree that for the generic case, the
>> top of the stack can't assume anything.
>> Having said that, in this case the SGL is encapsulated inside a dma-buf object.
> But the scatterlist is defined to have a valid page. If in dma-bufs you
> can't do that dmabufs are completely broken. Apparently the gpu folks
> can somehow live with that and deal with the pitfals, but for dma-buf
> users outside of their little fiefdom were they arbitrarily break rules
> it simply is not acceptable.
The key point is that accessing the underlying pages even when DMA-bufs
are backed by system memory is illegal. Daniel even created a patch
which mangles the page pointers in sg_tables used by DMA-buf to make
sure that people don't try to use them.
So the conclusion is that using sg_table in the DMA-buf framework was
just the wrong data structure and we should have invented a new one.
But then people would have complained that we have a duplicated
infrastructure (which is essentially true).
My best plan to get out of this mess is that we change the DMA-buf
interface to use an array of dma_addresses instead of the sg_table
object and I have already been working on this actively the last few month.
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