linux-kernel - Re: [PATCH 3/3] drm/msm/dp: Handle aux timeouts, nacks, defers

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Date:   Mon, 24 May 2021 19:19:11 +0000
From:   Stephen Boyd <swboyd@...omium.org>
To:     khsieh@...eaurora.org
Cc:     Rob Clark <robdclark@...il.com>, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        freedreno@...ts.freedesktop.org,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Abhinav Kumar <abhinavk@...eaurora.org>,
        aravindh@...eaurora.org, Sean Paul <sean@...rly.run>
Subject: Re: [PATCH 3/3] drm/msm/dp: Handle aux timeouts, nacks, defers

Quoting khsieh@...eaurora.org (2021-05-24 09:33:49)
> On 2021-05-07 14:25, Stephen Boyd wrote:
> > @@ -367,36 +347,38 @@ static ssize_t dp_aux_transfer(struct drm_dp_aux
> > *dp_aux,
> >       }
> >
> >       ret = dp_aux_cmd_fifo_tx(aux, msg);
> > -
> >       if (ret < 0) {
> >               if (aux->native) {
> >                       aux->retry_cnt++;
> >                       if (!(aux->retry_cnt % MAX_AUX_RETRIES))
> >                               dp_catalog_aux_update_cfg(aux->catalog);
> >               }
> > -             usleep_range(400, 500); /* at least 400us to next try */
> > -             goto unlock_exit;
> > -     }
>
> 1) dp_catalog_aux_update_cfg(aux->catalog) will not work without
> dp_catalog_aux_reset(aux->catalog);
> dp_catalog_aux_reset(aux->catalog) will reset hpd control block and
> potentially cause pending hpd interrupts got lost.
> Therefore I think we should not do
> dp_catalog_aux_update_cfg(aux->catalog) for now.
> reset aux controller will reset hpd control block probolem will be fixed
> at next chipset.
> after that we can add dp_catalog_aux_update_cfg(aux->catalog) followed
> by dp_catalog_aux_reset(aux->catalog) back at next chipset.

Hmm ok. So the phy calibration logic that tweaks the tuning values is
never used? Why can't the phy be tuned while it is active? I don't
understand why we would ever want to reset the aux phy when changing the
settings for a retry. Either way, this is not actually changing in this
patch so it would be another patch to remove this code.

>
> 2) according to DP specification, aux read/write failed have to wait at
> least 400us before next try can start.
> Otherwise, DP compliant test will failed

Yes. The caller of this function, drm_dp_dpcd_access(), has the delay
already

                if (ret != 0 && ret != -ETIMEDOUT) {
                        usleep_range(AUX_RETRY_INTERVAL,
                                     AUX_RETRY_INTERVAL + 100);
                }

so this delay here is redundant.