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

[Sui Jingfeng]

On 2023/6/10 00:48, Bjorn Helgaas wrote:
> 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@xxxxxxxxxxx>
> > > >
> > > > 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.

OK, acceptable

I will try to prepare the user code of this callback and respin the patch.

I may resend it with another patch set in the future, this series 
already drop it,

see v5[1]

[1] https://patchwork.freedesktop.org/series/119134/

> Bjorn