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 <suijingf...@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
