Re: [Linaro-mm-sig] [PATCH v3 1/2] habanalabs: define uAPI to export FD for DMA-BUF

["Christian König" <christian.koenig@amd.com>]

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.

Regards,
Christian.