Re: Proposal for a low-level Linux display framework

On Thu, Sep 15, 2011 at 3:18 PM, Florian Tobias Schandinat
[off-list ref] wrote:

On 09/15/2011 06:58 PM, Alan Cox wrote:

quoted

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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.

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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?

Right now most of the KMS DRM drivers do not support accelerated fb,
so as long as you don't run accelerated X or a 3D app, it should be
just as stable as an fb driver.  You may run into modesetting fail in
certain cases due to wonky hardware or driver bugs, but you will hit
that with an fb driver as well.

Alex