Message-ID: <50D0473D.9060106@vodafone.de>
Date:	Tue, 18 Dec 2012 11:36:45 +0100
From:	Christian König <deathsimple@...afone.de>
To:	Paul Bolle <pebolle@...cali.nl>
CC:	David Airlie <airlied@...ux.ie>,
	Jerome Glisse <jglisse@...hat.com>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [RFC] drm/radeon: return 0 on successful gpu reset

On 17.12.2012 22:31, Paul Bolle wrote:
> On an (outdated) laptop the radeon driver (almost always) prints, during
> the first resume of each session:
>      [drm] crtc 1 is connected to a TV
>
> This message is a bit puzzling as, as far as I know, no TV has ever
> been connected to this laptop. Anyhow, before v3.5, if that happened the
> radeon driver then printed an error during all following resumes:
>      [drm:radeon_cs_ioctl] *ERROR* Failed to parse relocation -35!
>
> (-35 is -EDEADLK.) But the resume would succeed and the driver seemed to
> run without too much trouble. From v3.5 onwards things changed. If the
> (puzzling) message about crtc 1 was printed on first resume the laptop
> would simply hang on second resume. Only a manual power off would then
> be possible. In that case nothing of interest would be found in the
> (truncated) logs.  And, most annoyingly, the hang would never happen if
> the laptop was booted with, say, "console=ttyS0,115200n8" added to the
> kernel command line.
>
> I bisected the hang to commit 6c6f478370eccfbfafbdc6fc55c0def03e58f124
> ("drm/radeon: rework recursive gpu reset handling"), which was added in
> the v3.5 release cycle. After discovering that and poking at the driver
> it turned out that this hang is triggered by radeon_cs_handle_lockup()
> returning -EAGAIN after successfully resetting the gpu. Simply returning
> 0 makes the hang disappear (and makes the drm error reappear).
>
> Nothing in the code or the commit explanation clarifies why -EAGAIN
> should be returned on successful gpu reset. So I suggest
> radeon_cs_handle_lockup() simply returns what radeon_gpu_reset()
> returns, eg 0 (on success) or a negative error code (on failure).
>
> Signed-off-by: Paul Bolle <pebolle@...cali.nl>
> ---
> 0) This exact patch is untested (but I run something comparable).
>
> 1) Sent as an RFC because I do not understand why this laptop (almost
> always) prints the "crtc 1" message on first resume. Note that another
> workaround for this hang is simply booting with "radeon.tv=0".
Alex should probably take a look into this, since he probably is the one 
with the deepest knowledge of the display engine. My best guess is that 
it is just some error while probing for an attached TV and actually 
isn't so bad after all.

> 2) Also sent as an RFC because I have no idea whatsoever why returning
> -EAGAIN will hang the machine. I guess it's returned to userland by
> radeon_cs_ioctl(). What code uses that ioctl? And what does that code do
> on -EAGAIN that hangs this laptop?

EAGAIN just tells userspace to reissue the requested system call. When a 
system call is interrupted (either by a signal or in our case a GPU 
lockup) it aborts and EGAIN is returned to userspace, telling userspace 
that it should try again. So by just returning 0 userspace things that 
our system call was executed and doesn't try it again.

So you just prevented the normal reissuing of the system call and so 
also prevented whatever this command submission should be doing in the 
first place.

> 3) A third reason to send this as an RFC is that I also have no idea why
> this hang doesn't happen when booting with "console=ttyS0,115200n8" or
> even "console=tty0"! But I guess I'm now allowed to call this hang a
> Heisenbug.
"Heisenbug" ? LOL, I need to remember that.

But anyway it is not so unusual seeing a bug like this, cause it is 
possible (but highly unlikely) that actually trying to print an error 
message can cause a lockup.

You should definitely try Alex latest drm-fixes-3.8 branch 
(git://people.freedesktop.org/~agd5f/linux) since the possibility is 
quite high that we already have fixed that bug. If that doesn't helps 
then please open a bug report and leave me a note so that I can 
investigate further.

Christian.


>
>   drivers/gpu/drm/radeon/radeon_cs.c |    5 +----
>   1 files changed, 1 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/radeon/radeon_cs.c b/drivers/gpu/drm/radeon/radeon_cs.c
> index 41672cc..a302c00 100644
> --- a/drivers/gpu/drm/radeon/radeon_cs.c
> +++ b/drivers/gpu/drm/radeon/radeon_cs.c
> @@ -486,11 +486,8 @@ out:
>   
>   static int radeon_cs_handle_lockup(struct radeon_device *rdev, int r)
>   {
> -	if (r == -EDEADLK) {
> +	if (r == -EDEADLK)
>   		r = radeon_gpu_reset(rdev);
> -		if (!r)
> -			r = -EAGAIN;
> -	}
>   	return r;
>   }
>   

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