[Linaro-mm-sig] [PATCH v3 1/2] habanalabs: define uAPI to export FD for DMA-BUF
hch at lst.de
Thu Jun 24 08:12:37 UTC 2021
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.
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