Re: [PATCH] arm64: Relocate screen_info.lfb_base on PCI BAR allocation

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On 04/28/2016 06:20 PM, Bjorn Helgaas wrote:
On Thu, Apr 28, 2016 at 12:22:24AM +0200, Alexander Graf wrote:
When booting with efifb, we get a frame buffer address passed into the system.
This address can be backed by any device, including PCI devices.
I guess we get the frame buffer address via EFI, but it doesn't tell
us what PCI device it's connected to?

Pretty much, yes. We can get the frame buffer address from a multitude of sources using various boot protocols, but the case where I ran into this was with efi on arm64.

This same thing could happen on any EFI arch, I guess.  Maybe even on

Yes and no :). I would've put it into whatever code "owns" screen_info, but I couldn't find any. So instead I figured I'd make the approach as generic as I could and implemented the calculation for the case where I saw it break.

The reason we don't see this on x86 (if I understand all pieces of the puzzle correctly) is that we get the BAR allocation from firmware using _CRS attributes in ACPI, so firmware tells the OS where to put the BARs. In the device tree case (which is what I'm running on arm64) we however allocate BARs dynamically.

non-EFI arches, if there's a way to discover the frame buffer address
as a bare address rather than a "offset X into BAR Y of PCI device Z"
sort of thing.

It'd be perfectly doable today - we do get a cpu physical address and use that in the notifier. All we would need to do is move the code that I added in arm64/efi.c to something more generic that "owns" the frame buffer address. Then any boot protocol that passes a screen_info in would get the frame buffer relocated on BAR remap. Drivers like vesafb might benefit from this as well - though apparently x86 fixed this using ACPI.

I'm not sure if offb could potentially also break. At the end of the day, it might, depending on how it's backed. For that we would then need another callback, since it doesn't use screen_info.

I don't feel incredibly comfortable preemptively fixing issues people haven't seen yet. There's just a good chance we end up breaking more than we fix :). But if you like and can point me to a better place for the screen_info modification, I'm happy to move it there.


Alex

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