Re: [RFC] enhancing the kernel's graphics subsystem

[Helge Hafting <helge.hafting@aitel.hist.no>]

Dave Airlie wrote:
> On 5/21/07, Helge Hafting <helge.hafting@aitel.hist.no> wrote:
>> Dave Airlie wrote:
>> > On 5/21/07, Jon Smirl <jonsmirl@verizon.net> wrote:
>> >> On Thu, 17 May 2007 14:23:45 -0700, Jesse Barnes wrote:
>> >>
>> >> > In collaboration with the FB guys, we've been working on 
>> enhancing the
>> >> > kernel's graphics subsystem in an attempt to bring some sanity 
>> to the
>> >> > Linux graphics world and avoid the situation we have now where 
>> several
>> >> > kernel and userspace drivers compete for control of graphics 
>> devices.
>> >>
>> >> How is supporting different users logged into each head going to 
>> work?
>> >> The original model for this was to give each head its own fbdev 
>> device.
>> >> It is important that each user be able to set their own mode without
>> >> being
>> >> root.
>> >
>> > TThe problem with that is the concept of heads is flawed... there is
>> > in reality no such
>> > thing, you have crtcs and outputs, no heads. So any attempt to enforce
>> > the head concept involves putting policy into the kernel, as if I have
>> > 3 outputs but 2 crtcs how do I decide the mappings without the admin
>> > telling the kernel,
>> >
>> Solution:
>> One device per crtc. You can then have two users, running consoles
>> or xservers on their crtcs, without having to involve root.
>
> Thats pretty much what the code does, but you still are putting a
> certain amount of policy in the kernel...
What policy would that be?
The mapping is set by root from userspace, not by the kernel.
The same for ownership to crtc devices. The kernel may have to
provide some default so "init=/bin/sh" will work, that's all.

>>
>> The crtc->output mapping must still be done by root of course.
>>
>> This solution allow the useful case where the computer boots, the boot
>> scripts
>> set up a crtc->output mapping. Then users log in through the
>> various consoles (using getty or xdm or similiar) using their grahphical
>> devices in whatever way they want.  A true multiseat setup.
>>
>>
>> And if one user needs to use all the screens for multi-display work?
>> Let root change the mappings, possibly through some sudo setup.
>>
>
> Multiseat isn't what i would want as a default on any machine, so the
Sure.  Not multiseat by default, as the kernel can't know which output
goes with which keyboard. All I want it a system that allows
multiseat for those that care to set it up.
> default setup should be to clone the single user onto as many screens
> as possible, as this is what users expect.. the system startup scripts
> can then reconfigure it, to suit the admins needs..
Sure.  Well, when using multiple screens as a single user, then I
don't want cloning. I want a multi-screen desktop.  The laptop
user with a video projector is about the one case I know
where cloning is wanted.

Unfortunately the kernel can't always know which outputs are in
use, so I see how cloning may have to be the default. :-/

Helge Hafting


>
> Dave.
>>
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