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.