Nouveau - Jul 2022 - susetting the remaining swioltb couplin in DRM

Hi i915 and nouveau maintainers,

any chance I could get some help to remove the remaining direct
driver calls into swiotlb, namely swiotlb_max_segment and
is_swiotlb_active.  Either should not matter to a driver as they
should be written to the DMA API.

In the i915 case it seems like the driver should use
dma_alloc_noncontiguous and/or dma_alloc_noncoherent to allocate
DMAable memory instead of using alloc_page and the streaming
dma mapping helpers.

For the latter it seems like it should just stop passing
use_dma_alloc == true to ttm_device_init and/or that function
should switch to use dma_alloc_noncoherent.

On Mon, Jul 11, 2022 at 10:26:14AM +0200, Christoph Hellwig
wrote:
> Hi i915 and nouveau maintainers,
> 
> any chance I could get some help to remove the remaining direct
> driver calls into swiotlb, namely swiotlb_max_segment and
> is_swiotlb_active.  Either should not matter to a driver as they
> should be written to the DMA API.
Hi Christoph,

while we take a look here, could you please share the reasons
behind sunsetting this calls?
> 
> In the i915 case it seems like the driver should use
> dma_alloc_noncontiguous and/or dma_alloc_noncoherent to allocate
> DMAable memory instead of using alloc_page and the streaming
> dma mapping helpers.
> 
> For the latter it seems like it should just stop passing
> use_dma_alloc == true to ttm_device_init and/or that function
> should switch to use dma_alloc_noncoherent.

Hi! Sorry about the slow reply to this, been busy with a bunch of other
pressing nouveau work lately.

Anyway, the steps look pretty simple here so I can see if I can write up a
patch shortly :)

On Mon, 2022-07-11 at 10:26 +0200, Christoph Hellwig
wrote:
> Hi i915 and nouveau maintainers,
> 
> any chance I could get some help to remove the remaining direct
> driver calls into swiotlb, namely swiotlb_max_segment and
> is_swiotlb_active.? Either should not matter to a driver as they
> should be written to the DMA API.
> 
> In the i915 case it seems like the driver should use
> dma_alloc_noncontiguous and/or dma_alloc_noncoherent to allocate
> DMAable memory instead of using alloc_page and the streaming
> dma mapping helpers.
> 
> For the latter it seems like it should just stop passing
> use_dma_alloc == true to ttm_device_init and/or that function
> should switch to use dma_alloc_noncoherent.
> 
-- 
Cheers,
 Lyude Paul (she/her)
 Software Engineer at Red Hat