Re: [Intel-gfx] [PATCH v3 4/4] PCI/VGA: introduce is_boot_device function callback to vga_client_register

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Jun 09, 2023 at 10:27:39AM +0800, Sui Jingfeng wrote:
> On 2023/6/9 03:19, Bjorn Helgaas wrote:
> > On Thu, Jun 08, 2023 at 07:43:22PM +0800, Sui Jingfeng wrote:
> > > From: Sui Jingfeng <suijingfeng@loongson.cn>
> > > 
> > > The vga_is_firmware_default() function is arch-dependent, which doesn't
> > > sound right. At least, it also works on the Mips and LoongArch platforms.
> > > Tested with the drm/amdgpu and drm/radeon drivers. However, it's difficult
> > > to enumerate all arch-driver combinations. I'm wrong if there is only one
> > > exception.
> > > 
> > > With the observation that device drivers typically have better knowledge
> > > about which PCI bar contains the firmware framebuffer, which could avoid
> > > the need to iterate all of the PCI BARs.
> > > 
> > > But as a PCI function at pci/vgaarb.c, vga_is_firmware_default() is
> > > probably not suitable to make such an optimization for a specific device.
> > > 
> > > There are PCI display controllers that don't have a dedicated VRAM bar,
> > > this function will lose its effectiveness in such a case. Luckily, the
> > > device driver can provide an accurate workaround.
> > > 
> > > Therefore, this patch introduces a callback that allows the device driver
> > > to tell the VGAARB if the device is the default boot device. This patch
> > > only intends to introduce the mechanism, while the implementation is left
> > > to the device driver authors. Also honor the comment: "Clients have two
> > > callback mechanisms they can use"
> > s/bar/BAR/ (several)
> > 
> > Nothing here uses the callback.  I don't want to merge this until we
> > have a user.
> 
> This is chicken and egg question.
> 
> If you could help get this merge first, I will show you the first user.
> 
> > I'm not sure why the device driver should know whether its device is
> > the default boot device.
> 
> It's not that the device driver should know,
> 
> but it's about that the device driver has the right to override.
> 
> Device driver may have better approach to identify the default boot
> device.

The way we usually handle this is to include the new callback in the
same series as the first user of it.  That has two benefits:
(1) everybody can review the whole picture and possibly suggest
different approaches, and (2) when we merge the infrastructure,
we also merge a user of it at the same time, so the whole thing can be
tested and we don't end up with unused code.

Bjorn