Re: [PATCH 2/2] efifb: Avoid reconfiguration of BAR that covers the framebuffer

[Ard Biesheuvel]

On 19 May 2017 at 21:44, Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> On Fri, May 19, 2017 at 05:37:30PM +0100, Ard Biesheuvel wrote:
>> Hi Bjorn,
>>
>> On 19 May 2017 at 17:27, Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>> > [+cc linux-pci]
>> >
>> > On Tue, Apr 04, 2017 at 04:27:44PM +0100, Ard Biesheuvel wrote:
>> >> On UEFI systems, the PCI subsystem is enumerated by the firmware,
>> >> and if a graphical framebuffer is exposed by a PCI device, its base
>> >> address and size are exposed to the OS via the Graphics Output
>> >> Protocol (GOP).
>> >>
>> >> On arm64 PCI systems, the entire PCI hierarchy is reconfigured from
>> >> scratch at boot. This may result in the GOP framebuffer address to
>> >> become stale, if the BAR covering the framebuffer is modified. This
>> >> will cause the framebuffer to become unresponsive, and may in some
>> >> cases result in unpredictable behavior if the range is reassigned to
>> >> another device.
>> >>
>> >> So add a non-x86 quirk to the EFI fb driver to find the BAR associated
>> >> with the GOP base address, and claim the BAR resource so that the PCI
>> >> core will not move it.
>> >
>> > I know this has already been merged as 55d728a40d36 ("efi/fb: Avoid
>> > reconfiguration of BAR that covers the framebuffer"), and I'm not
>> > suggesting that we revert it, but I have some misgivings.
>> ...
>
>> > Another is the use of pci_claim_resource() to express the idea that "this
>> > BAR can not be moved".  We have IORESOURCE_PCI_FIXED for that purpose, and
>> > previous versions of the patch used that.  I understand there was some
>> > problem with that, but I wish we could figure out and fix that problem
>> > instead of using a different mechanism.
>>
>> OK. The problem was that IORESOURCE_PCI_FIXED does not prevent the PCI
>> subsystem from handing out the same range to another device.
>
> Yes, this would definitely be a problem.  There must be a path where
> we start doing the reassignment before we claim the resources that
> have already been assigned.  That's what seems backwards to me -- it
> seems like we should claim things that are valid first so we know to
> avoid them.
>
>> > I'm not even completely sold on the idea that we need to prevent the BAR
>> > from being moved.  I'm not a UEFI expert, but if this requirement is in the
>> > spec, we should reference it.  If not, it should be sufficient to remember
>> > the boot-time BAR value, match the GOP base to *that*, and map it to
>> > whatever the current BAR value is.
>>
>> There is no such requirement in the spec. The graphics output protocol
>> is not described in terms of PCI, BARs etc. The framebuffer is simply
>> a memory range with side effects that is left enabled when handing
>> over to the OS.
>>
>> In summary, I am as unhappy with the patch as you are, but it is still
>> an improvement over the previous situation, so let's simply
>> collaborate to get this into shape going forward.
>>
>> My preference would be to investigate IORESOURCE_PCI_FIXED again,
>> because that still seems like the best way to deal with a live
>> framebuffer on a PCI device that has memory decoding enabled. It
>> should also address the issue with the current version of the patch,
>> i.e., that claiming resources at this point is not possible if the
>> device resides behind a bridge.
>>
>> So is there any guidance you can give as to how to proceed? If we set
>> IORESOURCE_PCI_FIXED, how should be prevent the PCI layer from
>> assigning this resource elsewhere if we cannot claim it yet?
>
> My preference would actually be to remember the boot BAR values and
> map to the current values because that avoids the unnecessary
> restriction.  The IORESOURCE_PCI_FIXED uses that seem legitimate to me
> are the legacy VGA and IDE things (stuff that's not described by BARs
> and *can't* be moved) and the new "Enhanced Allocation" stuff
> (basically a way to describe more non-BAR resources).
>
> We do something sort of similar with pci_revert_fw_address(), although
> it's currently only implemented for x86.  I could imagine a more
> generic solution that could be used for this GOP issue and could also
> replace pci_revert_fw_address().
>

I already proposed something like this a while ago:
http://marc.info//?l=linux-fbdev&m=149190021316410&w=2

> Conceptually it could be as simple as adding 7 u64 boot-time BAR
> values (6 BARs + the ROM) to struct pci_dev.  We went to a lot of
> trouble to implement the pcibios_fwaddrmap stuff on x86, and I can't
> remember why it's x86-specific.  Maybe we thought the extra 56 bytes
> per dev was too much overhead (it does seem like a lot for such a
> limited use case).  Maybe there's a clever way to store just the BARs
> we actually change and keep that long enough for efifb to use it.
>
> It *would* be nice to fix the problem with IORESOURCE_PCI_FIXED, and I
> think it would help clean up PCI resource management a lot.  Ideally
> we would be able to claim host bridge resources even before scanning
> the bus, then claim BARs immediately when we discover them.  That
> would allow us to automatically use any setup done by firmware, and
> reassign anything that we couldn't claim.
>
> But I think this will end up being difficult because we currently do
> the PCI bus scan before looking at ACPI resources, and sometimes those
> ACPI resources overlap with the host bridge windows.  Every time I
> look at this, I get discouraged.
>
> Bjorn