Re: [REGRESSION] Invalid gather when using Tegra210 media engines

[Mikko Perttunen <cyndis@kapsi.fi>]

On 2/4/25 4:13 AM, Jason Gunthorpe wrote:
> On Mon, Feb 03, 2025 at 06:49:07PM +0000, Robin Murphy wrote:
>> I'd hope the historical reasons for not supporting IOMMU_DOMAIN_DMA in
>> tegra-smmu no longer apply, given that all the default domain stuff has now
>> been integrated into host1x for the newer arm-smmu based Tegras.
> 
> Indeed I do see appropriate looking calls to the normal DMA API, and
> the other mapping path is conditionalized by !host->domain.
> 
> So, why didn't it work for Diogo? Even in identity mode the DMA API
> will return correct DMA addresses and the !host->domain path will skip
> mapping them.
> 
> Poking around I wonder if there is some assumption that if other parts
> of the stack, maybe the DRM driver, are using the special domain than
> everyone is? It seems to blindly pass around IOVA without really
> checking who is consuming it.

I'm not sure where that would be, but it's certainly possible given that 
this combination of code paths wouldn't have been tested.

> 
> Christian was telling me DMABUF had some drivers that made the
> (incorrect!) assumption they were all sharing translation.
> 
> It does seem like a nice project for someone who has the hardware to
> rip out all of this custom domain stuff and just have the iommu layer
> setup a shared dma-iommu domain.
> 
> Jason
> 

This has been a long-standing project. The issue is that some boot 
chains set up the display expecting identity mappings, but if dma-iommu 
domains were enabled, they would enable the IOMMU without mappings 
before the display driver gets to run. Perhaps Thierry knows what the 
missing pieces are.

See 
https://lore.kernel.org/linux-iommu/20220512190052.1152377-5-thierry.reding@gmail.com/

Cheers,
Mikko