linux-kernel - Re: [RFC] vgaarb: Add a module parm to use the first VGA device for boot

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Message-ID: <87b3e6f7-6272-aeb7-3588-b1527d2c66b6@infradead.org>
Date:   Sat, 2 Jul 2022 22:06:45 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Cal Peake <cp@...olutedigital.net>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        Huacai Chen <chenhuacai@...nel.org>
Subject: Re: [RFC] vgaarb: Add a module parm to use the first VGA device for
 boot

Hi--

On 7/2/22 21:22, Cal Peake wrote:
> Hi,
> 
> I have a dual GPU system with the GPU in the first CPU PCIe slot for the 
> host and the GPU in the second CPU PCIe slot being passed through to a 
> virtual machine.
> 
> Currently, the VGA arbiter enumerates through all VGA devices in the order 
> that they appear in on the bus with the last one ultimately getting the 
> assignment for boot device.
> 
> This is a problem because once the VFIO PCI driver claims the second GPU, 
> all display output is lost -- just empty black screens on both cards.
> 
> Setting VGA_ARB_MAX_GPUS to 1 helps a little: the X server will initialize 
> on the first GPU like it should, but there's no VTs to be found (i.e. 
> Alt-F1/F2/etc), just more black screens.
> 
> Below is an attempt at a fix by creating a kernel param to have the VGA 
> arbiter use the first VGA device it finds for the boot device and then 
> subsequently ignore the rest for this task.

So that's
  vgaarb.use_first=1
right?

Seems worthy of some documentation in
Documentation/admin-guide/kernel-parameters.txt, please.

-- 
~Randy