Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
MIME-Version: 1.0
References: <20220426114627.1.I2dd93486c6952bd52f2020904de0133970d11b29@changeid>
 <20220426114627.2.I4ac7f55aa446699f8c200a23c10463256f6f439f@changeid>
In-Reply-To: <20220426114627.2.I4ac7f55aa446699f8c200a23c10463256f6f439f@changeid>
From:   Doug Anderson <dianders@chromium.org>
Date:   Thu, 5 May 2022 08:44:36 -0700
Message-ID: <CAD=FV=XzGOGc8pMics4=idQeCuLYWxj=bHcic4NZa9+3qbqpbg@mail.gmail.com>
Subject: Re: [PATCH 2/2] drm/probe-helper: For DP, add 640x480 if all other
 modes are bad
To:     dri-devel <dri-devel@lists.freedesktop.org>,
        =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= <ville.syrjala@linux.intel.com>,
        "Abhinav Kumar (QUIC)" <quic_abhinavk@quicinc.com>
Cc:     Dmitry Baryshkov <dmitry.baryshkov@linaro.org>,
        Stephen Boyd <swboyd@chromium.org>,
        "Aravind Venkateswaran (QUIC)" <quic_aravindh@quicinc.com>,
        Rob Clark <robdclark@gmail.com>,
        "Kuogee Hsieh (QUIC)" <quic_khsieh@quicinc.com>,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        Sankeerth Billakanti <quic_sbillaka@quicinc.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        David Airlie <airlied@linux.ie>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        Thomas Zimmermann <tzimmermann@suse.de>,
        LKML <linux-kernel@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

Ville,

On Tue, Apr 26, 2022 at 11:47 AM Douglas Anderson <dianders@chromium.org> wrote:
>
> As per Displayport spec section 5.2.1.2 ("Video Timing Format") says
> that all detachable sinks shall support 640x480 @60Hz as a fail safe
> mode.
>
> A DP compliance test expected us to utilize the above fact when all
> modes it presented to the DP source were not achievable. It presented
> only modes that would be achievable with more lanes and/or higher
> speeds than we had available and expected that when we couldn't do
> that then we'd fall back to 640x480 even though it didn't advertise
> this size.
>
> In order to pass the compliance test (and also support any users who
> might fall into a similar situation with their display), we need to
> add 640x480 into the list of modes. However, we don't want to add
> 640x480 all the time. Despite the fact that the DP spec says all sinks
> _shall support_ 640x480, they're not guaranteed to support it
> _well_. Continuing to read the spec you can see that the display is
> not required to really treat 640x480 equal to all the other modes. It
> doesn't need to scale or anything--just display the pixels somehow for
> failsafe purposes. It should also be noted that it's not hard to find
> a display hooked up via DisplayPort that _doesn't_ support 640x480 at
> all. The HP ZR30w screen I'm sitting in front of has a native DP port
> and doesn't work at 640x480. I also plugged in a tiny 800x480 HDMI
> display via a DP to HDMI adapter and that screen definitely doesn't
> support 640x480.
>
> As a compromise solution, let's only add the 640x480 mode if:
> * We're on DP.
> * All other modes have been pruned.
>
> This acknowledges that 640x480 might not be the best mode to use but,
> since sinks are _supposed_ to support it, we will at least fall back
> to it if there's nothing else.
>
> Note that we _don't_ add higher resolution modes like 1024x768 in this
> case. We only add those modes for a failed EDID read where we have no
> idea what's going on. In the case where we've pruned all modes then
> instead we only want 640x480 which is the only defined "Fail Safe"
> resolution.
>
> This patch originated in response to Kuogee Hsieh's patch [1].
>
> [1] https://lore.kernel.org/r/1650671124-14030-1-git-send-email-quic_khsieh@quicinc.com
>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
>
>  drivers/gpu/drm/drm_probe_helper.c | 26 +++++++++++++++++++++-----
>  1 file changed, 21 insertions(+), 5 deletions(-)

I think this patch is fairly safe / non-controversial, but someone
suggested you might have an opinion on it and another patch I posted
recently [1] so I wanted to double-check. Just to be clear: I'm hoping
to land _both_ this patch and [1]. If you don't have an opinion,
that's OK too.

Abhinav: I think maybe you're happy with this now? Would you be
willing to give a Reviewed-by?

[1] https://lore.kernel.org/r/20220426132121.RFC.1.I31ec454f8d4ffce51a7708a8092f8a6f9c929092@changeid

-Doug