Re: [Freedreno] [PATCH 01/25] drm/msm/dpu: fix hw ctl retrieval for mixer muxing

[Sean Paul <sean@poorly.run>]

On Tue, Oct 09, 2018 at 10:46:41PM -0700, Jeykumar Sankaran wrote:
> On 2018-10-09 11:07, Sean Paul wrote:
> > On Mon, Oct 08, 2018 at 09:27:18PM -0700, Jeykumar Sankaran wrote:
> > > Layer mixer/pingpong block counts and hw ctl block counts
> > > will not be same for all the topologies (e.g. layer
> > > mixer muxing to single interface)
> > > 
> > > Use the encoder's split_role info to retrieve the
> > > respective control path for programming.
> > > 
> > > Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
> > > ---
> > >  drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c | 9 ++++++---
> > >  1 file changed, 6 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
> > b/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
> > > index 96cdf06..d12f896 100644
> > > --- a/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
> > > +++ b/drivers/gpu/drm/msm/disp/dpu1/dpu_encoder.c
> > > @@ -1060,6 +1060,7 @@ static void dpu_encoder_virt_mode_set(struct
> > drm_encoder *drm_enc,
> > > 
> > >  	for (i = 0; i < dpu_enc->num_phys_encs; i++) {
> > >  		struct dpu_encoder_phys *phys = dpu_enc->phys_encs[i];
> > > +		int ctl_index;
> > > 
> > >  		if (phys) {
> > >  			if (!dpu_enc->hw_pp[i]) {
> > > @@ -1068,14 +1069,16 @@ static void dpu_encoder_virt_mode_set(struct
> > drm_encoder *drm_enc,
> > >  				return;
> > >  			}
> > > 
> > > -			if (!hw_ctl[i]) {
> > > +			ctl_index = phys->split_role == ENC_ROLE_SLAVE ? 1
> > : 0;
> > > +
> > 
> > What if MAX_CHANNELS_PER_ENC isn't 2? Similarly, what if num_phys_encs >
> > MAX_CHANNELS_PER_ENC? It seems like there should be a more formal
> > relationship
> > between all of these verious values (num_of_h_tiles assumed to be <= 2
> > as
> > well).
> > If one of them changes beyond the assumed bound, the rest of the driver
> > falls
> > over pretty hard.
> > 
> MAX_CHANNELS_PER_ENC is set to 2 to represent HW limitation on the chipset
> as
> we cannot gang up more than 2 LM chain to an interface. Supporting more than
> 2
> might demand much larger changes than validating for boundaries.
> 
> num_phys_enc is the max no of phys encoders we create as we are looping
> through
> num_of_h_tiles which cannot be more than priv->dsi array size.
> 
> So its very unlikely we would expect these loops to go out of bound!

For now, sure. However a new revision of hardware will be a pain to add support
for if we add more assumptions, and secondly it makes it _really_ hard to
understand the code if you don't have Qualcomm employee-level access to the
hardware design :).

So this is why I'm advocating for the reduction of the number of "num_of_"
values we assume are all in the same range. It's a lot easier to understand the
hardware when you can see that a phys encoder is needed per h tile, and that a
ctl/pp is needed per phys encoder.

Anyways, just my $0.02.

Sean

> 
> Thanks,
> Jeykumar S.
> > 
> > > +			if (!hw_ctl[ctl_index]) {
> > >  				DPU_ERROR_ENC(dpu_enc, "no ctl block
> > assigned"
> > > -					     "at idx: %d\n", i);
> > > +					     "at idx: %d\n", ctl_index);
> > >  				return;
> > 
> > When you return on error here, should you give back the resources that
> > you've
> > already provisioned?
> > 
> > >  			}
> > > 
> > >  			phys->hw_pp = dpu_enc->hw_pp[i];
> > > -			phys->hw_ctl = hw_ctl[i];
> > > +			phys->hw_ctl = hw_ctl[ctl_index];
> > > 
> > >  			phys->connector = conn->state->connector;
> > >  			if (phys->ops.mode_set)
> > > --
> > > The Qualcomm Innovation Center, Inc. is a member of the Code Aurora
> > Forum,
> > > a Linux Foundation Collaborative Project
> > > 
> > > _______________________________________________
> > > Freedreno mailing list
> > > Freedreno@lists.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/freedreno
> 
> -- 
> Jeykumar S

-- 
Sean Paul, Software Engineer, Google / Chromium OS
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