Re: [PATCH 1/2] drm/radeon: Use drm_vblank_off/on to fix vblank counter trouble.

[Mario Kleiner <mario.kleiner.de@gmail.com>]

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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