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

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.

On Mon, Jul 11, 2022 at 04:31:49PM -0400, Rodrigo Vivi
wrote:
> 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?
Because they are a completely broken layering violation.  A driver has
absolutely no business knowing the dma-mapping violation.  The DMA
API reports what we think is all useful constraints (e.g.
dma_max_mapping_size()), and provides useful APIs to (e.g.
dma_alloc_noncoherent or dma_alloc_noncontiguous) to allocate pages
that can be mapped without bounce buffering and drivers should use
the proper API instead of poking into one particular implementation
and restrict it from changing.

swiotlb_max_segment in particular returns a value that isn't actually
correct (a driver can't just use all of swiotlb) AND actually doesn't
work as is in various scenarious that are becoming more common,
most notably host with memory encryption schemes that always require
bounce buffering.