linux-kernel - Re: [PATCH 1/3] drm/msm/dp: Avoid a long timeout for AUX transfer if nothing connected

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Message-ID: <CAD=FV=UrxOmPRUa9Cw1u+JmTepMjKfKCwFLgLjj3f2O9wSdZyg@mail.gmail.com>
Date: Tue, 12 Mar 2024 18:07:01 -0700
From: Doug Anderson <dianders@...omium.org>
To: Guenter Roeck <groeck@...gle.com>
Cc: Rob Clark <robdclark@...il.com>, Abhinav Kumar <quic_abhinavk@...cinc.com>, 
	Dmitry Baryshkov <dmitry.baryshkov@...aro.org>, Bjorn Andersson <andersson@...nel.org>, 
	Daniel Vetter <daniel@...ll.ch>, David Airlie <airlied@...il.com>, Guenter Roeck <groeck@...omium.org>, 
	Kuogee Hsieh <quic_khsieh@...cinc.com>, Marijn Suijten <marijn.suijten@...ainline.org>, 
	Sean Paul <sean@...rly.run>, Tanmay Shah <tanmay@...eaurora.org>, 
	Vinod Polimera <quic_vpolimer@...cinc.com>, dri-devel@...ts.freedesktop.org, 
	freedreno@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] drm/msm/dp: Avoid a long timeout for AUX transfer if
 nothing connected

Hi,

On Tue, Mar 12, 2024 at 5:47 PM Guenter Roeck <groeck@...gle.com> wrote:
>
> On Tue, Mar 12, 2024 at 5:14 PM Douglas Anderson <dianders@...omium.org> wrote:
> >
> > As documented in the description of the transfer() function of
> > "struct drm_dp_aux", the transfer() function can be called at any time
> > regardless of the state of the DP port. Specifically if the kernel has
> > the DP AUX character device enabled and userspace accesses
> > "/dev/drm_dp_auxN" directly then the AUX transfer function will be
> > called regardless of whether a DP device is connected.
> >
> > For eDP panels we have a special rule where we wait (with a 5 second
> > timeout) for HPD to go high. This rule was important before all panels
> > drivers were converted to call wait_hpd_asserted() and actually can be
> > removed in a future commit.
> >
> > For external DP devices we never checked for HPD. That means that
> > trying to access the DP AUX character device (AKA `hexdump -C
> > /dev/drm_dp_auxN`) would very, very slowly timeout. Specifically on my
> > system:
> >   $ time hexdump -C /dev/drm_dp_aux0
> >   hexdump: /dev/drm_dp_aux0: Connection timed out
> >
> >   real    0m8.200s
> >
> > Let's add a check for HPD to avoid the slow timeout. This matches
> > what, for instance, the intel_dp_aux_xfer() function does when it
> > calls intel_tc_port_connected_locked(). That call has a document by it
> > explaining that it's important to avoid the long timeouts.
> >
> > Fixes: c943b4948b58 ("drm/msm/dp: add displayPort driver support")
> > Signed-off-by: Douglas Anderson <dianders@...omium.org>
> > ---
> >
> >  drivers/gpu/drm/msm/dp/dp_aux.c     |  8 +++++++-
> >  drivers/gpu/drm/msm/dp/dp_catalog.c | 10 ++++++++++
> >  drivers/gpu/drm/msm/dp/dp_catalog.h |  1 +
> >  3 files changed, 18 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/gpu/drm/msm/dp/dp_aux.c b/drivers/gpu/drm/msm/dp/dp_aux.c
> > index 03f4951c49f4..de0b0eabced9 100644
> > --- a/drivers/gpu/drm/msm/dp/dp_aux.c
> > +++ b/drivers/gpu/drm/msm/dp/dp_aux.c
> > @@ -307,7 +307,8 @@ static ssize_t dp_aux_transfer(struct drm_dp_aux *dp_aux,
> >          * 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.
> > +        * avoid ever doing the extra long wait for DP and just query HPD
> > +        * directly.
> >          */
> >         if (aux->is_edp) {
> >                 ret = dp_catalog_aux_wait_for_hpd_connect_state(aux->catalog);
> > @@ -315,6 +316,11 @@ static ssize_t dp_aux_transfer(struct drm_dp_aux *dp_aux,
> >                         DRM_DEBUG_DP("Panel not ready for aux transactions\n");
> >                         goto exit;
> >                 }
> > +       } else {
> > +               if (!dp_catalog_aux_is_hpd_connected(aux->catalog)) {
> > +                       ret = -ENXIO;
> > +                       goto exit;
> > +               }
> >         }
> >
> >         dp_aux_update_offset_and_segment(aux, msg);
> > diff --git a/drivers/gpu/drm/msm/dp/dp_catalog.c b/drivers/gpu/drm/msm/dp/dp_catalog.c
> > index 5142aeb705a4..93e2d413a1e7 100644
> > --- a/drivers/gpu/drm/msm/dp/dp_catalog.c
> > +++ b/drivers/gpu/drm/msm/dp/dp_catalog.c
> > @@ -266,6 +266,16 @@ int dp_catalog_aux_wait_for_hpd_connect_state(struct dp_catalog *dp_catalog)
> >                                 2000, 500000);
> >  }
> >
> > +bool dp_catalog_aux_is_hpd_connected(struct dp_catalog *dp_catalog)
> > +{
> > +       struct dp_catalog_private *catalog = container_of(dp_catalog,
> > +                               struct dp_catalog_private, dp_catalog);
> > +
> > +       /* poll for hpd connected status every 2ms and timeout after 500ms */
>
> Maybe I am missing something, but the comment doesn't seem to match
> the code below.
>
> Guenter
>
> > +       return readl(catalog->io->dp_controller.aux.base + REG_DP_DP_HPD_INT_STATUS) &
> > +              DP_DP_HPD_STATE_STATUS_CONNECTED;

LOL. I guess I overlooked that. Thanks for catching! The comment got
copied from the dp_catalog_aux_wait_for_hpd_connect_state(). I'll
remove the comment and send a v2, but I'll wait a little bit to see if
there is additional feedback.

-Doug