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

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Date:   Wed, 29 May 2019 17:54:21 -0400
From:   Brian Masney <masneyb@...tation.org>
To:     Jeffrey Hugo <jhugo@...eaurora.org>
Cc:     Jeffrey Hugo <jeffrey.l.hugo@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Sean Paul <sean@...rly.run>, Rob Herring <robh@...nel.org>,
        Jonathan Marek <jonathan@...ek.ca>,
        Dave Airlie <airlied@...ux.ie>,
        MSM <linux-arm-msm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:DRM PANEL DRIVERS" <dri-devel@...ts.freedesktop.org>,
        Rob Clark <robdclark@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        freedreno <freedreno@...ts.freedesktop.org>
Subject: Re: [Freedreno] [PATCH RFC v2 0/6] ARM: qcom: initial Nexus 5
 display support

On Wed, May 29, 2019 at 01:58:16PM -0600, Jeffrey Hugo wrote:
> On 5/29/2019 1:30 PM, Brian Masney wrote:
> > On Wed, May 29, 2019 at 08:41:31AM -0600, Jeffrey Hugo wrote:
> > > On Wed, May 29, 2019 at 4:28 AM Brian Masney <masneyb@...tation.org> wrote:
> > > > 
> > > > On Tue, May 28, 2019 at 08:53:49PM -0600, Jeffrey Hugo wrote:
> > > > > On Tue, May 28, 2019 at 8:46 PM Brian Masney <masneyb@...tation.org> wrote:
> > > > > > 
> > > > > > On Tue, May 28, 2019 at 07:42:19PM -0600, Jeffrey Hugo wrote:
> > > > > > > > > Do you know if the nexus 5 has a video or command mode panel?  There
> > > > > > > > > is some glitchyness with vblanks and command mode panels.
> > > > > > > > 
> > > > > > > > Its in command mode. I know this because I see two 'pp done time out'
> > > > > > > > messages, even on 4.17. Based on my understanding, the ping pong code is
> > > > > > > > only applicable for command mode panels.
> > > > > > > 
> > > > > > > Actually, the ping pong element exists in both modes, but 'pp done
> > > > > > > time out' is a good indicator that it is command mode.
> > > > > > > 
> > > > > > > Are you also seeing vblank timeouts?
> > > > > > 
> > > > > > Yes, here's a snippet of the first one.
> > > > > > 
> > > > > > [    2.556014] WARNING: CPU: 0 PID: 5 at drivers/gpu/drm/drm_atomic_helper.c:1429 drm_atomic_helper_wait_for_vblanks.part.1+0x288/0x290
> > > > > > [    2.556020] [CRTC:49:crtc-0] vblank wait timed out
> > > > > > [    2.556023] Modules linked in:
> > > > > > [    2.556034] CPU: 0 PID: 5 Comm: kworker/0:0 Not tainted 5.2.0-rc1-00178-g72c3c1fd5f86-dirty #426
> > > > > > [    2.556038] Hardware name: Generic DT based system
> > > > > > [    2.556056] Workqueue: events deferred_probe_work_func
> > > > > > ...
> > > > > > 
> > > > > > > Do you have busybox?
> > > > > > > 
> > > > > > > Can you run -
> > > > > > > sudo busybox devmem 0xFD900614
> > > > > > > sudo busybox devmem 0xFD900714
> > > > > > > sudo busybox devmem 0xFD900814
> > > > > > > sudo busybox devmem 0xFD900914
> > > > > > > sudo busybox devmem 0xFD900A14
> > > > > > 
> > > > > > # busybox devmem 0xFD900614
> > > > > > 0x00020020
> > > > > 
> > > > > Ok, so CTL_0 path, command mode, ping pong 0, with the output going to DSI 1.
> > > > > 
> > > > > Next one please:
> > > > > 
> > > > > busybox devmem 0xFD912D30
> > > > 
> > > > It's 0x00000000 on mainline and 4.17. I used the following script to
> > > > dump the entire mdp5 memory region and attached the dump from 4.17 and
> > > > 5.2rc1.
> > > > 
> > > 
> > > ok, 0 means autorefresh is not on.  Which is fine.  My next guess
> > > would be the vblank code checking the hardware vblank counter, which
> > > doesn't exist.
> > > In video mode, there is a frame counter which increments, which can be
> > > used as the vblank counter.  Unfortunately, that hardware isn't active
> > > in command mode, and there isn't an equivalent.
> > > 
> > > So, the vblank code is going to read the register, and look for an
> > > update, which will never happen, thus it will timeout.  There is a
> > > backup path which uses timestamps (no hardware), which you can
> > > activate with a quick hack - make max_vblank_count = 0 at the
> > > following line
> > > https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/msm/disp/mdp5/mdp5_kms.c#L753
> > 
> > That fixed the issue!
> 
> Awesome.  I'm glad it was something simple.
> 
> > 
> > I previously observed that mdp5_get_vblank_counter, specifically
> > mdp5_encoder_get_framecount, would always return 0.
> > 
> > What's the best way to fix this in mainline? Set that to zero if any
> > of the interface modes is MDP5_INTF_DSI_MODE_COMMAND?
> > 
> 
> Short version, yes.  Long version:
> 
> I still have that hack in my tree and haven't come back to formulating
> a proper fix yet.  Feel free to run with it.
> 
> Thinking about it briefly, we could do two things.  We could fake a
> hardware counter by just increment an int every time the vblank irq is
> processed, but that seems clunky.  Otherwise, we could force a
> fallback onto the timestamp solution, which seems less invasive.
> 
> In theory, we could service multiple displays, with different
> properties (ie a combination of command and video mode).  The hack
> then, is not good, because it would break video mode (at-least we
> wouldn't be using the register when we could).  It would be great if
> the use of the hardware register could be done per display.
> 
> Luckily, it looks like someone just made that possible -
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/gpu/drm/drm_vblank.c?h=v5.2-rc2&id=ed20151a7699bb2c77eba3610199789a126940c4

I'll work on this for the msm driver.

Thanks for the info!

Brian