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

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Apr 28, 2016 at 06:41:42PM +0200, Alexander Graf wrote:
> 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. 

I think the real reason is that on x86, firmware typically assigns all
the BARs and Linux typically doesn't change them.  PCI host bridges
have _CRS, which tells us where the host bridge windows are.  PCI
devices themselves don't normally have _CRS; we just make sure their
BARs are inside the ranges of an upstream _CRS.  If/when we get x86
boxes where firmware doesn't assign all the BARs, we should see the
same problem there.

> 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.

We could consider a quirk that would mark any BAR that happened to
contain the frame buffer address as IORESOURCE_PCI_FIXED.  That would
(in theory, anyway) keep the PCI core from moving it.

Is there any run-time EFI (or other firmware) dependency on the frame
buffer address?  If there is, things will break when we move it, even
if we have your notifier to tell efifb about it.

> Drivers like vesafb might benefit from this as well - though
> apparently x86 fixed this using ACPI.

Where is this x86 vesafb ACPI fix?  I don't see anything ACPI-related
in drivers/video/fbdev/vesafb.c.  I'm just curious what this fix looks
like.

> 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.

If firmware is giving us a bare address of something, that seems like
a clue that it might depend on that address staying the same.

Bjorn