linux-kernel - Re: [PATCH v4 3/7] drm/fb-helper: Add fb_deferred

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Message-ID: <20160429153636.GE14148@phenom.ffwll.local>
Date:	Fri, 29 Apr 2016 17:36:37 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Tomi Valkeinen <tomi.valkeinen@...com>
Cc:	Noralf Trønnes <noralf@...nnes.org>,
	dri-devel@...ts.freedesktop.org, linux-fbdev@...r.kernel.org,
	laurent.pinchart@...asonboard.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/7] drm/fb-helper: Add fb_deferred_io support

On Fri, Apr 29, 2016 at 06:00:18PM +0300, Tomi Valkeinen wrote:
> 
> On 29/04/16 17:55, Daniel Vetter wrote:
> 
> >> Who's calling {write,fillrect,copyarea,imageblit} in an atomic context?
> >> That sounds like a very bad idea to me...
> >>
> >> If this is only for accumulating changes, I think it may be better to
> >> leave that to the driver as it may have better idea of how to accumulate.
> >>
> >> But, of course, this is a helper, so if all the drivers use this kind of
> >> accumulation, it makes sense =).
> > 
> > Apparently panic context and cursor timer and stuff like that. I think
> > this started with udl, and just to make sure (it's fbdev after all, no one
> > wants to read the code itself) we've put those checks onto all entry
> > points that draw stuff. It could be overkill, but this is what udl/qxl
> > already do, so better to keep imo for now.
> > 
> > I guess if it's really not needed we could later on change that, but not
> > sure that's worth the effort. And we can't get rid of it entirely.
> 
> Yep... Sounds fine to me.
> 
> Someone should implement (minimal) fbdev userspace API support to DRM
> without using the current fbdev, and implement fbcon on top of that. I
> don't like how DRM and fbdev gets mixed...
> 
> I offer a beer to anyone who does that =).

The big plan that has been floated years back already (before David
Herrmann disappeared into the rh systemd gang) was to have a pure
userspace kmscon for console (including vt-switching in userspace like X
does), plus a super-minimal panic console on top of kms in the kernel.
That looked like a really solid design, unfornutately it didn't go
anywhere. But basic patches are still around, and it really just boils
down to that some distro would need to integrate kmscon properly, and
polish of the kernel side too for integration.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch