linux-kernel - Re: Proposal for a low-level Linux display framework

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Message-ID: <4E724F93.1050203@gmx.de>
Date:	Thu, 15 Sep 2011 19:18:43 +0000
From:	Florian Tobias Schandinat <FlorianSchandinat@....de>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
CC:	Alex Deucher <alexdeucher@...il.com>,
	Keith Packard <keithp@...thp.com>, linux-fbdev@...r.kernel.org,
	linaro-dev@...ts.linaro.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org, Archit Taneja <archit@...com>,
	"Clark, Rob" <rob@...com>
Subject: Re: Proposal for a low-level Linux display framework

On 09/15/2011 06:58 PM, Alan Cox wrote:
>> Well, I rather think that the fb API is more user centric to allow every program
>> to use it directly in contrast to the KMS/DRM API which aims to support every
>> feature the hardware has. For this the fb API should not change much, but I
>> understand some additions were needed for some special users, probably limited
>> to X and wayland.
> 
> Wayland needs vblank frame buffer switching and the like. Likewise given
> you want to composite buffers really any serious accelerated device ends
> up needing a full memory manager and that ends up needing a buffer
> manager. Wayland needs clients to be doing their own rendering into
> objects which means authorisation and management of the render engine
> which ends up looking much like DRM.

As you have DRM now and as I'm not interested in wayland I won't discuss this,
but I guess it might be a good start for Geert's question what would be needed
to use it on dumb framebuffers.

>> One of my biggest problems with KMS is that it has (naturally) a lot more
>> complexity than the fb API which leads to instability. Basically it's very
> 
> It shouldn't do - and a sample of one (your machine) is not a
> statistically valid set. Fb is pretty much ununsable in contrast on my
> main box, but that's not a statistically valid sample either.
> 
> I'm not that convinced by the complexity either. For a simple video card
> setup such as those that the fb layer can kind of cope with (ie linear
> buffer, simple mode changes, no client rendering, no vblank flipping,
> limited mode management, no serious multi-head) a DRM driver is also
> pretty tiny and simple.

Yes, if you limit DRM to the functionality of the fb API I guess you could reach
the same stability level. But where can I do this? Where is a option to forbid
all acceleration or at least limit to the acceleration that can be done without
any risk?

>> Well, I think it's too late to really fix this thing. We now have 3 APIs in the
>> kernel that have to be kept. Probably the best we can do now is figure out how
>> we can reduce code duplication and do extensions to those APIs in a way that
>> they are compatible with each other or completely independent and can be used
>> across the APIs.
> 
> I think it comes down to 'when nobody is using the old fb drivers they can
> drop into staging and oblivion'. Right now the fb layer is essentially
> compatibility glue on most modern x86 platforms.

That's a really difficult question. Determining the users is difficult and there
are people that use their hardware very long, for example we are about to get a
new driver for i740. For the framebuffer infrastructure I guess you have to at
least wait for my death.


Regards,

Florian Tobias Schandinat
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