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

[Christoph Hellwig <hch@lst.de>]

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.

> 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.

I think so.

> But then people would have complained that we have a duplicated 
> infrastructure (which is essentially true).

I doubt it.  At least if you had actually talked to the relevant people.
Which seems to be a major issue with what is going on GPU land.

> 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.

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.