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

[Dmitry Osipenko <digetx@gmail.com>]

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 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.

I have some thoughts now, but nothing concrete yet. Maybe we will need
to create a per-SoC ops for MM.

I'll finish with trying what we currently have to see what else is
missing and then we will decide what to do about it.

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

Oh, right :) I haven't got closely to this part of reviewing yet.