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

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Date:   Mon, 24 May 2021 12:57:17 -0700
From:   khsieh@...eaurora.org
To:     Stephen Boyd <swboyd@...omium.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

On 2021-05-24 12:19, Stephen Boyd wrote:
> 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.
ok,
> 
>> 
>> 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.
yes, you are right. This is enough.