[PATCH v5 15/21] drm/tegra: Add new UAPI to header

[Mikko Perttunen]

On 1/13/21 8:14 PM, Dmitry Osipenko wrote:
> 11.01.2021 16:00, Mikko Perttunen пишет:
>> +struct drm_tegra_submit_buf {
>> +	/**
>> +	 * @mapping_id: [in]
>> +	 *
>> +	 * Identifier of the mapping to use in the submission.
>> +	 */
>> +	__u32 mapping_id;
> 
> I'm now in process of trying out the UAPI using grate drivers and this
> becomes the first obstacle.
> 
> Looks like this is not going to work well for older Tegra SoCs, in
> particular for T20, which has a small GART.
> 
> Given that the usefulness of the partial mapping feature is very
> questionable until it will be proven with a real userspace, we should
> start with a dynamic mappings that are done at a time of job submission.
> 
> DRM already should have everything necessary for creating and managing
> caches of mappings, grate kernel driver has been using drm_mm_scan for a
> long time now for that.
> 
> It should be fine to support the static mapping feature, but it should
> be done separately with the drm_mm integration, IMO.
> 
> What do think?
> 

Can you elaborate on the requirements to be able to use GART? Are there 
any other reasons this would not work on older chips?

I think we should keep CHANNEL_MAP and mapping_ids, but if e.g. for GART 
we cannot do mapping immediately at CHANNEL_MAP time, we can just treat 
it as a "registration" call for the GEM object - potentially no-op like 
direct physical addressing is. We can then do whatever is needed at 
submit time. This way we can have the best of both worlds.

Note that partial mappings are already not present in this version of 
the UAPI.

Mikko