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. 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.
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. Just to
say i'm probably way too biased wrt. what solution for this should get
backported into an older kernel.
Anyway, urgently need to sleep.
-mario
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