[PATCH 1/2] drm/prime: Iterate SG DMA addresses separately

[Lucas Stach]

Am Donnerstag, den 12.04.2018, 11:35 +0200 schrieb Christian König:
> Am 12.04.2018 um 11:11 schrieb Lucas Stach:
> > Am Mittwoch, den 11.04.2018, 20:26 +0200 schrieb Christian König:
> > > Am 11.04.2018 um 19:11 schrieb Robin Murphy:
> > > > For dma_map_sg(), DMA API implementations are free to merge consecutive
> > > > segments into a single DMA mapping if conditions are suitable, thus the
> > > > resulting DMA addresses may be packed into fewer entries than
> > > > ttm->sg->nents implies.
> > > > 
> > > > drm_prime_sg_to_page_addr_arrays() does not account for this, meaning
> > > > its callers either have to reject the 0 < count < nents case or risk
> > > > getting bogus addresses back later. Fortunately this is relatively easy
> > > > to deal with having to rejig structures to also store the mapped count,
> > > > since the total DMA length should still be equal to the total buffer
> > > > length. All we need is a separate scatterlist cursor to iterate the DMA
> > > > addresses separately from the CPU addresses.
> > > 
> > > Mhm, I think I like Sinas approach better.
> > > 
> > > See the hardware actually needs the dma_address on a page by page basis.
> > > 
> > > Joining multiple consecutive pages into one entry is just additional
> > > overhead which we don't need.
> > 
> > But setting MAX_SEGMENT_SIZE will probably prevent an IOMMU that might
> > be in front of your GPU to map large pages as such, causing additional
> > overhead by means of additional TLB misses and page walks on the IOMMU
> > side.
> > 
> > And wouldn't you like to use huge pages at the GPU side, if the IOMMU
> > already provides you the service of producing one large consecutive
> > address range, rather than mapping them via a number of small pages?
> 
> No, I wouldn't like to use that. We're already using that :)
> 
> But we use huge pages by allocating consecutive chunks of memory so that 
> both the CPU as well as the GPU can increase their TLB hit rate.

I'm not convinced that this is a universal win. Many GPU buffers aren't
accessed by the CPU and allocating huge pages puts much more strain on
the kernel MM subsystem.

> What currently happens is that the DMA subsystem tries to coalesce 
> multiple pages into on SG entry and we de-coalesce that inside the 
> driver again to create our random access array.

I guess the right thing would be to have a flag that tells the the DMA
implementation to not coalesce the entries. But (ab-)using max segment
size tells the DMA API to split the segments if they are larger than
the given size, which is probably not what you want either as you now
need to coalesce the segments again when you are mapping real huge
pages.

> That is a huge waste of memory and CPU cycles and I actually wanted to 
> get rid of it for quite some time now. Sinas approach seems to be a good 
> step into the right direction to me to actually clean that up.
> 
> > Detecting such a condition is much easier if the DMA map implementation
> > gives you the coalesced scatter entries.
> 
> A way which preserves both path would be indeed nice to have, but that 
> only allows for the TLB optimization for the GPU and not the CPU any 
> more. So I actually see that as really minor use case.

Some of the Tegras have much larger TLBs and better page-walk
performance on the system IOMMU compared to the GPU MMU, so they would
probably benefit a good deal even if the hugepage optimization only
targets the GPU side.

Regards,
Lucas