linux-kernel - Re: [PATCH v3 7/7] [wip] virtio-gpu: add page flip support

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Date:	Tue, 31 May 2016 08:37:47 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Gerd Hoffmann <kraxel@...hat.com>
Cc:	dri-devel <dri-devel@...ts.freedesktop.org>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:VIRTIO GPU DRIVER" 
	<virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH v3 7/7] [wip] virtio-gpu: add page flip support

On Tue, May 31, 2016 at 8:18 AM, Gerd Hoffmann <kraxel@...hat.com> wrote:
>> > https://lists.freedesktop.org/archives/dri-devel/2016-May/108772.html
>>
>> Hm, smells more like virtio isn't too happy with the default ordering of
>> the commit operation. The default is:
>>
>> - Disable any crtc/encoders that need to be disabled/change.
>> - Bash new plane setup into hw.
>> - Enable all crtcs/encoders that need to be enabled/have changed.
>>
>> There's two problems:
>> - some hw gets real grumpy if you bash in plane state without the crtc
>>   state yet matching.
>> - if you do runtime pm nothing is enabled and the writes get lost at best,
>>   or hang your interconnect at worst.
>>
>> That's why you can overwrite atomic_commit_tail, and use something more
>> sensible. See for example what it looks like for rockchip. I have a gut
>> feeling that should also take care of your troubles. Using the old crtc is
>> definitely not what you want.
>
>> Another option is that virtio isn't happy about bashing in plane state for
>> disabled crtc. Again helpers have you covered, look at the active_only
>> parameter for drm_atomic_helper_commit_planes().
>
> virtio-gpu is a bit simplified compared to real hardware, so there isn't
> really separate plane/crtc state.
>
> Right now the virtual outputs are linked to drm_crtc.  To apply any
> changes I need to lookup the crtc to figure which virtual output should
> be updated.
>
> So, setting active_only should make sure I have a valid crtc pointer on
> plane updates, right?  It probably also skips the disable + enable crtc
> steps on commit?  What happens when outputs are disabled?
>
> Maybe it makes sense to link our virtual outputs to (primary) planes not
> crtcs?

Nah, I just misunderstood your patch. If it's all about finding the
corresponding crtc, then you're all good. I thought there was some
other reason (like the virtual hw getting upset about certain things).
I'll pick up your patch into my nonblocking atomic branch to make sure
virtio isn't accidentally broken. btw can you pls drop an ack or r-b
onto my virtio conversion? I already added your tested-by.

Thanks, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch