linux-kernel - Re: [PATCH v7 3/4] drm/msm/dp: wait for hpd high before aux transaction

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Date:   Thu, 14 Apr 2022 09:39:52 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Sankeerth Billakanti <quic_sbillaka@...cinc.com>
Cc:     dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        freedreno <freedreno@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Rob Clark <robdclark@...il.com>,
        Sean Paul <seanpaul@...omium.org>,
        Stephen Boyd <swboyd@...omium.org>,
        quic_kalyant <quic_kalyant@...cinc.com>,
        "Abhinav Kumar (QUIC)" <quic_abhinavk@...cinc.com>,
        "Kuogee Hsieh (QUIC)" <quic_khsieh@...cinc.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        quic_vproddut <quic_vproddut@...cinc.com>,
        "Aravind Venkateswaran (QUIC)" <quic_aravindh@...cinc.com>,
        Steev Klimaszewski <steev@...i.org>
Subject: Re: [PATCH v7 3/4] drm/msm/dp: wait for hpd high before aux transaction

Hi,

On Thu, Apr 14, 2022 at 5:20 AM Sankeerth Billakanti
<quic_sbillaka@...cinc.com> wrote:
>
> The source device should ensure the sink is ready before proceeding to
> read the sink capability or perform any aux transactions. The sink
> will indicate its readiness by asserting the HPD line. The controller
> driver needs to wait for the hpd line to be asserted by the sink before
> it performs any aux transactions.
>
> The eDP sink is assumed to be always connected. It needs power from the
> source and its HPD line will be asserted only after the panel is powered
> on. The panel power will be enabled from the panel-edp driver and only
> after that, the hpd line will be asserted.
>
> Whereas for DP, the sink can be hotplugged and unplugged anytime. The hpd
> line gets asserted to indicate the sink is connected and ready. Hence
> there is no need to wait for the hpd line to be asserted for a DP sink.
>
> Signed-off-by: Sankeerth Billakanti <quic_sbillaka@...cinc.com>
> ---

It might be worth mentioning "after the cut" that we may eventually
end up changing the rules if people like my proposal [1]. However,
what your code is doing here for eDP is correct as things are
currently intended to work and it would make sense to land it while we
debate about whether we want to add the is_hpd_asserted() callback
like my patch does.

[1] https://lore.kernel.org/r/20220408193536.RFC.3.Icf57bb12233a47727013c6ab69eebf803e22ebc1@changeid/


> Changes in v7:
>   - add a comment to say why the wait si done for eDP
>   - correct the commit text
>
> Changes in v6:
>   - Wait for hpd high only for eDP
>   - Split into smaller patches
>
>  drivers/gpu/drm/msm/dp/dp_aux.c     | 21 ++++++++++++++++++++-
>  drivers/gpu/drm/msm/dp/dp_aux.h     |  3 ++-
>  drivers/gpu/drm/msm/dp/dp_catalog.c | 13 +++++++++++++
>  drivers/gpu/drm/msm/dp/dp_catalog.h |  1 +
>  drivers/gpu/drm/msm/dp/dp_display.c |  2 +-
>  5 files changed, 37 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/msm/dp/dp_aux.c b/drivers/gpu/drm/msm/dp/dp_aux.c
> index 6d36f63..cf0739f 100644
> --- a/drivers/gpu/drm/msm/dp/dp_aux.c
> +++ b/drivers/gpu/drm/msm/dp/dp_aux.c
> @@ -36,6 +36,7 @@ struct dp_aux_private {
>         bool initted;
>         u32 offset;
>         u32 segment;
> +       bool is_edp;

Kinda nitty, but can you put it next to the other booleans? This will
help with structure packing.


>         struct drm_dp_aux dp_aux;
>  };
> @@ -337,6 +338,22 @@ static ssize_t dp_aux_transfer(struct drm_dp_aux *dp_aux,
>                 goto exit;
>         }
>
> +       /*
> +        * For eDP it's important to give a reasonably long wait here for HPD
> +        * to be asserted. This is because the panel driver may have _just_
> +        * turned on the panel and then tried to do an AUX transfer. The panel
> +        * driver has no way of knowing when the panel is ready, so it's up
> +        * to us to wait. For DP we never get into this situation so let's
> +        * avoid ever doing the extra long wait for DP.
> +        */
> +       if (aux->is_edp) {
> +               ret = dp_catalog_aux_wait_for_hpd_connect_state(aux->catalog);
> +               if (ret) {
> +                       DRM_DEBUG_DP("Panel not ready for aux transactions\n");
> +                       goto exit;
> +               }
> +       }
> +
>         dp_aux_update_offset_and_segment(aux, msg);
>         dp_aux_transfer_helper(aux, msg, true);
>
> @@ -491,7 +508,8 @@ void dp_aux_unregister(struct drm_dp_aux *dp_aux)
>         drm_dp_aux_unregister(dp_aux);
>  }
>
> -struct drm_dp_aux *dp_aux_get(struct device *dev, struct dp_catalog *catalog)
> +struct drm_dp_aux *dp_aux_get(struct device *dev, struct dp_catalog *catalog,
> +                               bool is_edp)

nit: I think indentation rules for this file are that the type of the
argument for the 2nd line should line up right under the 1st. Thus you
should delete one tab character and insert 6 spaces before the "bool".

Similar in other places, like your header file.


Stuff above is all nits and this looks right to me. I'm happy with:

Reviewed-by: Douglas Anderson <dianders@...omium.org>