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

[Christian König]

Am 24.06.21 um 10:12 schrieb Christoph Hellwig:
> On Thu, Jun 24, 2021 at 10:07:14AM +0200, Christian König wrote:
> > 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.
> Which is another goddamn layering violation of a subsystem that has no
> business at all poking into the scatterlist structure, yes.

Completely agree, but it is also the easiest way to get away from the 
scatterlist as trasnport vehicle for the dma_addresses.

[SNIP]

> > 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.
> Awesome!  I have a bit of related work on the DMA mapping subsystems, so
> let's sync up as soon as you have some first sketches.

Don't start cheering to fast.

I've already converted a bunch of the GPU drivers, but there are at 
least 6 GPU still needing to be fixed and on top of that comes VA-API 
and a few others.

What are your plans for the DMA mapping subsystem?

> Btw, one thing I noticed when looking over the dma-buf instances is that
> there is a lot of duplicated code for creating a sg_table from pages,
> and then mapping it.  It would be good if we could move toward common
> helpers instead of duplicating that all over again.

Can you give an example?

Thanks,
Christian.