linux-kernel - Re: [PATCH v2 1/4] drm/msm/dp: Avoid a long timeout for AUX transfer if nothing connected

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Date: Mon, 18 Mar 2024 11:16:45 -0700
From: Abhinav Kumar <quic_abhinavk@...cinc.com>
To: Douglas Anderson <dianders@...omium.org>, Rob Clark <robdclark@...il.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
CC: Bjorn Andersson <quic_bjorande@...cinc.com>,
        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>, Stephen Boyd
	<swboyd@...omium.org>,
        Vara Reddy <quic_varar@...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 v2 1/4] drm/msm/dp: Avoid a long timeout for AUX transfer
 if nothing connected



On 3/15/2024 2:36 PM, Douglas Anderson 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
> We want access to the drm_dp_auxN character device to fail faster than
> 8 seconds when no DP cable is plugged in.
> 
> Let's add a test to make transfers fail right away if a device isn't
> plugged in. Rather than testing the HPD line directly, we have the
> dp_display module tell us when AUX transfers should be enabled so we
> can handle cases where HPD is signaled out of band like with Type C.
> 
> Fixes: c943b4948b58 ("drm/msm/dp: add displayPort driver support")
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---
> 
> Changes in v2:
> - Don't look at the HPD line directly; have dp_display call us.
> 
>   drivers/gpu/drm/msm/dp/dp_aux.c     | 20 ++++++++++++++++++++
>   drivers/gpu/drm/msm/dp/dp_aux.h     |  1 +
>   drivers/gpu/drm/msm/dp/dp_display.c |  4 ++++
>   3 files changed, 25 insertions(+)
> 

Reviewed-by: Abhinav Kumar <quic_abhinavk@...cinc.com>