drivers/fbdev/efifb: Allow BAR to be moved instead of claiming it

drivers/fbdev/efifb: Allow BAR to be moved instead of claiming it

On UEFI systems, the firmware may expose a Graphics Output Protocol (GOP)
instance to which the efifb driver attempts to attach in order to provide
a minimal, unaccelerated framebuffer. The GOP protocol itself is not very
sophisticated, and only describes the offset and size of the framebuffer
in memory, and the pixel format.

If the GOP framebuffer is provided by a PCI device, it will have been
configured and enabled by the UEFI firmware, and the GOP protocol will
simply point into a live BAR region. However, the GOP protocol itself does
not describe this relation, and so we have to take care not to reconfigure
the BAR without taking efifb's dependency on it into account.

Commit:

  55d728a40d36 ("efi/fb: Avoid reconfiguration of BAR that covers the framebuffer")

attempted to do so by claiming the BAR resource early on, which prevents the
PCI resource allocation routines from changing it.  However, it turns out
that this only works if the PCI device is not behind any bridges, since
the bridge resources need to be claimed first.

So instead, allow the BAR to be moved, but make the efifb driver deal
with that gracefully. So record the resource that covers the BAR early
on, and if it turns out to have moved by the time we probe the efifb
driver, update the framebuffer address accordingly.

While this is less likely to occur on x86, given that the firmware's
PCI resource allocation is more likely to be preserved, this is a
worthwhile sanity check to have in place, and so let's remove the
preprocessor conditional that makes it !X86 only.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Peter Jones <pjones@redhat.com>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20170818194947.19347-8-ard.biesheuvel@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>