From: Daniel Vetter <daniel@ffwll.ch>
To: Alex Williamson <alex.williamson@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>,
dri-devel <dri-devel@lists.freedesktop.org>
Subject: Re: make fbdev/fbcon switchable per driver?
Date: Tue, 31 Jan 2017 09:00:19 +0100 [thread overview]
Message-ID: <20170131080019.7g6duliyfjgihti6@phenom.ffwll.local> (raw)
In-Reply-To: <20170130104522.3a814aea@t450s.home>
On Mon, Jan 30, 2017 at 10:45:22AM -0700, Alex Williamson wrote:
> On Mon, 30 Jan 2017 09:15:50 +0100
> Gerd Hoffmann <kraxel@redhat.com> wrote:
>
> > Hi,
> >
> > > The vgaarb code has a concept of a vga_default_device(), it's rather
> > > PCI-centric, but maybe better than nothing. This is typically the
> > > first VGA class code device found with I/O and MMIO enabled. If fbcon
> > > defaulted to running on the vga_default_device(), a user could select
> > > which to use by re-ordering the VM hardware.
> >
> > The qemu drivers don't register as vgaarb clients though. Which easily
> > explains why igd always wins the primary selection, no matter how you
> > order your hardware.
> >
> > So, should they register? The drivers don't need access to the vga
> > registers[1][2].
>
> The VGA arbiter sets up a notifier on the PCI bus and will add any VGA
> class code devices it finds. So even if the driver doesn't
> participate, it'll still be tracked and might be marked as primary. If
> a graphics driver claims a VGA device that does not depend on VGA
> region access then the driver should configure the device not to claim
> VGA accesses (maybe only relevant for integrated graphics - i915 gets
> this wrong), and register with the arbiter to opt-out of VGA
> arbitration. Picking a "primary" can be done w/o any of that latter if
> we agree on the arbiter algorithm of picking the first device with VGA
> routing to it (or it can be overridden by arch/platform code). Thanks,
Fyi we don't get this wrong, we indeed need vga access in some cases.
Broken hw :( The other issue is that the X server asks for vga access (or
at least did in the past) for every render op, even if it's a kms driver.
Which makes X crawl if you have multi-gpu system. Those two things
combined means vgaarb is essentially dead on any system with intel gfx.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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next prev parent reply other threads:[~2017-01-31 8:00 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-27 10:00 make fbdev/fbcon switchable per driver? Gerd Hoffmann
2017-01-27 14:02 ` Daniel Vetter
2017-01-27 14:41 ` Liviu Dudau
2017-01-27 18:30 ` Alex Williamson
2017-01-30 8:15 ` Gerd Hoffmann
2017-01-30 17:45 ` Alex Williamson
2017-01-31 8:00 ` Daniel Vetter [this message]
2017-01-31 10:06 ` Gerd Hoffmann
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