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

[Thierry Reding]

On Thu, Jan 14, 2021 at 12:34:22PM +0200, Mikko Perttunen wrote:
> On 1/14/21 10:36 AM, Dmitry Osipenko wrote:
> > 13.01.2021 21:56, Mikko Perttunen пишет:
> > > 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?
> > 
> > We have all DRM devices in a single address space on T30+, hence having
> > duplicated mappings for each device should be a bit wasteful.
> 
> I guess this should be pretty easy to change to only keep one mapping per
> GEM object.

The important point here is the semantics: this IOCTL establishes a
mapping for a given GEM object on a given channel. If the underlying
implementation is such that the mapping doesn't fit into the GART, then
that's an implementation detail that the driver needs to take care of.
Similarly, if multiple devices share a single address space, that's
something the driver already knows and can take advantage of by simply
reusing an existing mapping if one already exists. In both cases the
semantics would be correctly implemented and that's really all that
matters.

Overall this interface seems sound from a high-level point of view and
allows these mappings to be properly created even for the cases we have
where each channel may have a separate address space. It may not be the
optimal interface for all use-cases or any one individual case, but the
very nature of these interfaces is to abstract away certain differences
in order to provide a unified interface to a common programming model.
So there will always be certain tradeoffs.

Thierry
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