Re: [PATCH v5 2/9] video/aperture: use generic code to figure out the vga default device

[Helge Deller <deller@gmx.de>]

On 4/11/23 16:36, Daniel Vetter wrote:
> On Fri, Apr 07, 2023 at 10:54:00PM +0200, Helge Deller wrote:
>> On 4/6/23 15:21, Thomas Zimmermann wrote:
>>> From: Daniel Vetter <daniel.vetter@ffwll.ch>
>>>
>>> Since vgaarb has been promoted to be a core piece of the pci subsystem
>>> we don't have to open code random guesses anymore, we actually know
>>> this in a platform agnostic way, and there's no need for an x86
>>> specific hack. See also commit 1d38fe6ee6a8 ("PCI/VGA: Move vgaarb to
>>> drivers/pci")
>>>
>>> This should not result in any functional change, and the non-x86
>>> multi-gpu pci systems are probably rare enough to not matter (I don't
>>> know of any tbh). But it's a nice cleanup, so let's do it.
>>>
>>> There's been a few questions on previous iterations on dri-devel and
>>> irc:
>>>
>>> - fb_is_primary_device() seems to be yet another implementation of
>>>     this theme, and at least on x86 it checks for both
>>>     vga_default_device OR rom shadowing. There shouldn't ever be a case
>>>     where rom shadowing gives any additional hints about the boot vga
>>>     device, but if there is then the default vga selection in vgaarb
>>>     should probably be fixed. And not special-case checks replicated all
>>>     over.
>>>
>>> - Thomas also brought up that on most !x86 systems
>>>     fb_is_primary_device() returns 0, except on sparc/parisc. But these
>>>     2 special cases are about platform specific devices and not pci, so
>>>     shouldn't have any interactions.
>>
>> Nearly all graphics cards on parisc machines are actually PCI cards,
>> but the way we handle the handover to graphics mode with STIcore doesn't
>> conflicts with your planned aperture changes.
>> So no problem as far as I can see for parisc...
>
> Ah I thought sticore was some very special bus, if those can be pci cards

STI stands for "Standard Text Interface" [1], which is a API of ROM functions
to output text chars on a console. It's comparable to the text output functions
in a PC-BIOS on x86 and dependend on the ROM it drives any supported card which has
a parisc ROM. So, STI supports cards on PCI & AGP busses, as well on older GSC busses.
[1] https://parisc.wiki.kernel.org/images-parisc/e/e3/Sti.pdf

> underneath then I guess some cleanup eventually might be a good idea? For
> anything with a pci bus it's rather strange when vgaarb and
> fb_is_primary_device() aren't a match ...

There is no VGA on parisc, so there is no conflict. Cards come either with
a parisc STI ROM to support text mode, or they will only be used as secondary
cards only.  The graphics mode is only done in userspace by specific drivers, e.g.
by the X11 server in HP-UX.
Even on x86 the BIOS will only show text output if the graphics card comes
with a VGA-compatible BIOS.

Helge