[PATCH v3] efifb: avoid reconfiguration of BAR that covers the framebuffer

[Ard Biesheuvel]

On 28 March 2017 at 22:27, Sinan Kaya <okaya at codeaurora.org> wrote:
> Hi Lorenzo,
>
> On 3/23/2017 6:57 AM, Lorenzo Pieralisi wrote:
>>> Correct. But on x86 (or rather, on a PC), you can be sure that UEFI
>>> (or the legacy PCI bios) performed the resource assignment already.
>>> One could argue that this is equally the case when running arm64 in
>>> ACPI mode, but in general, you cannot assume the presence of firmware
>>> on ARM/arm64 that has already taken care of this, and so the state of
>>> the BARs has to be presumed invalid.
>> The story is a bit more convoluted than that owing to x86 (and other
>> arches) legacy.
>>
>> x86 tries to claim all PCI resources (in two passes - first enabled
>> devices, second disabled devices) and that predates ACPI/UEFI.
>>
>> Mind, x86 reassign resources that can't be claimed too, the only
>> difference with ARM64 is that, for the better or the worse, we
>> have decided not to claim the FW PCI set-up on ARM64 even if it
>> is sane, we do not even try, it was a deliberate choice.
>>
>> This patch should be harmless on x86 since if the FB PCI BAR is set
>> up sanely, claiming it again should be a nop (to be checked).
>>
>> For all the talk about PCI being arch agnostic as I said manifold
>> times before, that's just theory. In practice different arches
>> treat PCI FW set-up differently, it would be ideal to make them
>> uniform but legacy is huge and there is a massive risk of triggering
>> regressions, it is no mean feat (if achievable at all).
>
> Can we explore having a uniform behavior across ALL ACPI bases systems
> by trying to reuse the resources assigned by firmware first and reassign
> them only if something is wrong?
>
> There are protocols like hotplug reservation in UEFI. It looks like Linux
> is not honoring any of these protocols by being too smart.
>

Could you be more specific? Which protocol do you mean exactly, and
how does not reusing resources interfere with it?