linux-kernel - Re: [PATCH RFC v2 0/6] ARM: qcom: initial Nexus 5 display support

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Date:   Wed, 29 May 2019 05:41:52 -0400
From:   Brian Masney <masneyb@...tation.org>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Rob Clark <robdclark@...il.com>, Sean Paul <sean@...rly.run>,
        "open list:DRM PANEL DRIVERS" <dri-devel@...ts.freedesktop.org>,
        MSM <linux-arm-msm@...r.kernel.org>,
        freedreno@...ts.freedesktop.org, Dave Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jonathan Marek <jonathan@...ek.ca>,
        Rob Herring <robh@...nel.org>
Subject: Re: [PATCH RFC v2 0/6] ARM: qcom: initial Nexus 5 display support

On Wed, May 29, 2019 at 08:23:17AM +0200, Linus Walleij wrote:
> On Wed, May 29, 2019 at 3:17 AM Brian Masney <masneyb@...tation.org> wrote:
> 
> > It's in low speed mode but its usable.
> 
> How low speed is that?

I don't have a number but my test with 4.17 is to run
'cat /etc/passwd > /dev/tty1' over a serial cable. The first 2-3 calls
will fill up the screen and the file contents appear to show up on the
screen immediately to the human eye. The next time that I run it
requires scrolling the entire console and there is a small fraction of
a second where I see the entire framebuffer contents scroll up. I
don't have a graphics background, but I believe that this is the
tearing effect that I'm seeing based on what I've read. I believe that
disp-te-gpios can be used to mitigate this tearing effect. I have a few
questions about this later once we get the basic display working.

> > I assume that it's related to the
> > vblank events not working properly?
> 
> They are only waiting for 50 ms before timing out, I raised it
> to 100ms in the -next kernel. I'm still suspicious about this
> even though I think you said this was not the problem.
> 
> For a command mode panel in LP mode it will nevertheless
> be more than 100ms for sure, the update is visible to the
> naked eye.
> 
> Raise it to 10000 ms or something and see what happens.
> drivers/gpu/drm/drm_atomic_helper.c:
>  msecs_to_jiffies(50)

I previously raised those timeouts as high as 5 seconds and it still
has the same issue. Writing to /dev/tty1 can take anywhere between a few
seconds to 30 seconds or more.

Brian