On 01/22/2016 04:17 AM, Michel Dänzer wrote:
On 21.01.2016 18:16, Mario Kleiner wrote:
On 01/21/2016 09:25 AM, Michel Dänzer wrote:
On 21.01.2016 17:16, Mario Kleiner wrote:
This patch replaces calls to drm_vblank_pre/post_modeset in the
drivers dpms code with calls to drm_vblank_off/on, as recommended
for drivers with hw counters that reset to zero during modeset.
Sounds like you fell for the drm_vblank_on/off propaganda. :(
This was working fine with drm_vblank_pre/post_modeset, that it broke
is simply a regression.
I agree with you that pre/post modeset breakage is a regression. It's
just that i stumbled over the on/off stuff while searching for a
solution and the other sort of hacks i could think of looked similar or
more convoluted/hacky/fragile to me.
Finding and fixing the cause of a regression isn't a hack, it's
established procedure.
That's not what i meant. I meant i couldn't find something less
complicated/risky/without new regression potential, so this looked like
a better solution. Of course i would have tested my own patches against
at least a couple bits of userspace (ati ddx, modesetting ddx, weston),
i just didn't have access to the machine yesterday.
Anyway, some more hours of thinking and code browsing later, now i think
i have a simple and safe solution which should hopefully restore the
drm_vblank_pre/post_modeset behaviour with only a few lines of core
code. At the same time it should fix up another bug in that new
drm_update_vblank_count code that i just realized, in a way simple
enough for a stable fix.
Now i just need to actually code and test it first.
And they probably wouldn't solve that other small race i found as easily
- I don't think it's likely to happen (often/at all?) in practice, but i
have trouble "forgetting" about its existence now.
That's something which should be addressed independently from the
regression fix.
Please split up the PM fixes from your patch into one or two separate
patches (which may be appropriate for 4.5 / stable trees), and leave the
switch to drm_vblank_on/off to Daniel's patch for 4.6.
I will look into that once i'm done with the above, and probably got
some sleep again.
Fixing this race regression without switching to vblank_off/on might
need a small bit of extra band aid there.
Btw. wrt. the radeon_pm.c fix: It's certainly good to fix that potential
drm_vblank_get/put imbalance. I wonder if that "might glitch" DEBUG
message makes much sense though. Can that code run during a modeset at
all? And if so, i'd almost expect that there won't be any vblank irqs
available at that point anyway - once the crtc's are off they don't
trigger vblank irqs anymore - so that code might glitch due to lack of
vblank sync regardless if drm_vblank_get is successful or not?
The other thing is my placement of the radeon_pm_compute_clocks() in the
DPMS_ON path. I moved it to fix the potential extra race i described.
But thinking about it, wouldn't be the better place at the beginning of
the DPMS on path, before the atombios calls reenable the crtcs? I don't
know the driver well enough, but it looked a bit suspicious to me that
the memory clocks, linebuffer watermarks etc. get updated for thew new
video mode after the crtc has been enabled. Won't it then potentially
start running for a moment with wrong memory bandwidth etc.? That's
probably something for you to check - no idea, just something i noticed
as slightly odd to me.
Also moving it up might avoid collisions with Daniel's patch, if that
move doesn't hurt.
I'm not against switching to drm_vblank_on/off for 4.6, but it's not a
solution for older kernels.
Linux 4.4 is an especially important stable kernel for me because it's
supposed to be the standard distro kernel for Ubuntu 16.04-LTS and
siblings/derivatives (Linux Mint) for up to the next 5 years. Having
many of my neuroscience users ending on that kernel as their very first
impression of Linux with something potentially broken in vblank land
scares me. The reliability of timing/timestamping stuff is
super-important for them, at the same time hand-holding many of them
through non-standard kernel upgrades would be so much not fun.
But fast-tracking the switch to drm_vblank_on/off, which haven't been
widely tested with this driver, all the way to 4.4 seems less risky to
you? Seriously?
It made a lot of sense after 12 hours of browsing code, thinking about
all kind of race conditions and other new personal horrors ;) - Luckily
i don't have to decide.
Just to say i'm probably way too biased wrt. what solution for this
should get backported into an older kernel.
At the risk of sounding like a broken record: There's a bug in 4.4 (and
older/newer kernels) causing the vblank counter to jump forward across
DPMS off with drm_vblank_on/off. So it sounds like you don't want to use
those at least until we've found and fixed that.
Modesetting "jumps" are not a problem for my application, as production
or test sessions won't ever run during modesets or other "disruptive"
events like dpms on/off. That's also why my own test suite didn't detect
such trouble ever despite extensive testing. Obviously not having a well
working desktop or non-working logins etc. would be a bad problem for
myself and my users. It still surprises me that the bug Vlastimil
reported never happened to myself in days, given that i loggedout/in a
lot for testing different X-Server xorg.conf configs on KDE Plasma 5.
Even now as i see the jumps during modesets in my dmesg i haven't
managed to get Plasma or glxgears etc. to lockup during dozens of modesets.
-mario
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