[Intel-gfx] [PATCH 1/3] drm/i915/dp: Fail DP MST config when there are not enough vcpi slots

[Daniel Vetter]

On Fri, Nov 18, 2016 at 9:43 AM, Daniel Vetter <daniel at ffwll.ch> wrote:
> On Thu, Nov 17, 2016 at 06:03:46PM -0800, Dhinakaran Pandiyan wrote:
>> drm_dp_find_vcpi_slots() returns an error when there is not enough
>> available bandwidth on a link to support a mode. This error should make
>> compute_config() to fail. Not returning false could end up in a modeset
>> which will not work.
>>
>> Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan at intel.com>
>> ---
>>  drivers/gpu/drm/i915/intel_dp_mst.c | 4 ++++
>>  1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c b/drivers/gpu/drm/i915/intel_dp_mst.c
>> index e21cf08..4280a83 100644
>> --- a/drivers/gpu/drm/i915/intel_dp_mst.c
>> +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
>> @@ -63,6 +63,10 @@ static bool intel_dp_mst_compute_config(struct intel_encoder *encoder,
>>
>>       pipe_config->pbn = mst_pbn;
>>       slots = drm_dp_find_vcpi_slots(&intel_dp->mst_mgr, mst_pbn);
>> +     if (slots < 0) {
>> +             DRM_ERROR("not enough available time slots for pbn=%d", mst_pbn);
>
> No DRM_ERROR for cases that are expected to fail, i.e. DRM_DEBUG_KMS is
> the right level.
>
> And I don't think this works correctly either. Assume you do an atomic
> modeset where you enable 2 mst sinks at the same time, and the following
> happens:
> - mst connector 1 can be allocated, and passes
>   intel_dp_mst_compute_config.
> - mst connector 2 can be allocated, but not together with connector 1.
>   But drm_dp_find_vcpi_slots only checks what's available, it doesn't do a
>   temporary reservation.
>
> And we can just reserve the slot in drm_dp_find_vcpi_slots, because then
> in the above case (when connector 2 doesn't have enough slots anymore)
> we'd leak the vcpi allocation for connector 1.
>
> Even worse, when we do a TEST_ONLY atomic commit (i.e. only run
> atomic_check/compute_config code, but not commit anything) this can even
> happen for a successful commit.
>
> Long story short: To fix this properly we need to rewrite the dp helpers
> to be compliant with atomic design. I think for that we roughly need:
>
> - Exract vcpi allocations into a free-standing state structure. I'd call
>   it drm_dp_mst_state or similar. Provide duplicate(get_state)/release
>   functions like we already have for plane, connector and crtc states in
>   the core, and e.g. dpll configuration in i915/intel_dpll_mgr.c. Ander
>   Conselvan can help you with this. I'm also planning to write better
>   documentation for how to do this exactly (since you also need a ww_mutex
>   to protect this state), and I'll prioritize that work.
>
> - Wire up that dp mst state at the right places globally (we need one slot
>   per drm_dp_mst_topology_mgr, i.e. per port), and duplicate that state in
>   intel_dp_mst_compute_config. We can't wire this up anywhere in the core
>   since the dp mst library is a helper library, so all the integration
>   points need to be done explicitly in drivers.
>
> - Do the same for nouveau - nouveau is now also atomic, and it also is
>   atomic with mst support.
>
> - While doing all that make sure that the existing (non-atomic-compliant)
>   functions in the dp mst helpers keep working, we need those for amggpu.
>
> - Create a new drm_dp_state_allocate_vcpi_slots, which only touches the
>   new drm_dp_mst_state structure and allocats the vcpi slots there. Also
>   add some function to find the allocation for each sink again (we need
>   that in the modeset commit functions).
>
> - Rework our compute_config and modeset code to use this new function, and
>   not the old legacy find/allocate functions.
>
> To make this happen we need buy-in from Ben Skeggs (nouveau maintainer)
> and preferrably also from the AMD display team (since they support mst
> already, and also plan to eventually support atomic).
>
> Fixing this correctly is unfortunately a _lot_ more work than your simple
> patch here :(

Adding Ben&Alex&Harry. Alex/Harry, pls pull in your mst expert into this too.

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch