Re: [PATCH] TTM DMA pool v2.2 or [GIT PULL](stable/ttm.dma_pool.v2.3) for 3.3

[Konrad Rzeszutek Wilk]

. snip..
> > Some AMD boxes with the AMD-Vi chipset enable the SWIOTLB b/c not all of
> > the PCI devices are on the IOMMU chipset path. The Intel VT-d does the same
> > thing.
> > 
> > Hmm, I think the reason for those devices to turn SWIOTLB on is that they
> > are not comfortable handling 32-bit devices, and the TTM DMA pool code would
> > only turn itself on when the radeon|nouveau was 32-bit _and_ SWIOTLB was enabled.
> > 
> > .. Testing the patches you posted on my AMD box.
> 
> Yes, swiotlb was enabled (bounce buffer message) thing is when force is
> not set usual ttm memory page allocation works properly ie on pci map
> no bounce buffer is use, but when force is set it does use a bounce
> This is due to :
> if (dma_capable(dev, dev_addr, size) && !swiotlb_force)
> in swiotlb_map_page, i am not sure how much the force argument is
> usefull.
> 
> Anyway i did few benchmark and it seems that the dma allocator isn't
> slower than the other page allocator. But this is limited testing
> (openarena, nexuiz running on composited desktop with firefox).

Hehe. I also run it with perf record to see it before and after.
Albeit with 'swiotlb=force' _everything_ is going throught the bounce buffer
- including your network packets/USB mouse/etc.

This little patch makes it easier to switch between the TTM DMA and
the default one without using the big hammer approach of 'swiotlb=force':