[edk2-devel] [PATCH v2 0/4] Add extra pci roots support for Arm

Thu Dec 17 13:52:17 UTC 2020

On 2020/12/17 21:23, Laszlo Ersek wrote:
> On 12/15/20 13:52, Jiahui Cen wrote:
>> For x86, Linux does not handle the pci resource assignment. But for arm, it
>> would assign all the pci resources, unless we explicitly let Linux preserve
>> PCI resource alignment made by firmware, using "PCI Boot Configuration"
>> _DSM function.
> This difference between x86 and arm seems inexplicable to me.
> 
>> What do you think of adding "PCI Boot Configuration" _DSM function into
>> dsdt table to make kernel use firmware's resource configuration?
> The relevant kernel commits seem to be:
> 
> - 18e94a338436 ("PCI: Make a shareable UUID for PCI firmware ACPI _DSM",
> 2015-04-08)
> 
> - a78cf9657ba5 ("PCI/ACPI: Evaluate PCI Boot Configuration _DSM",
> 2019-06-21)
> 
> I've also read now section "4.6.5. _DSM for Ignoring PCI Boot
> Configurations" in the "PCI(TM) Firmware Specification,
> Revision 3.1, December 13, 2010".
> 
> Basically if this _DSM#5 function exists, it tells the OS whether it is
> required to honor the firmware-assigned resources (value 0), or if it is
> free to reassign (value 1). If the function does not exist at all, then,
> "the operating system may continue to use the legacy handling regarding
> the boot configuration".
> 
> Without knowing more, I consider it a bug that aarch64 Linux uses a
> default strategy (in the absence of the _DSM#5 function) that is the
> opposite of the x86 default. But, you seem to be right that there is a
> specified way to convince arm64 Linux otherwise. Can you indeed add this
> _DSM#5 function to the "virt" machine's ACPI generator, returning value
> 0, and see if it makes a difference?

OK, I've tested adding _DSM#5 function in QEMU, and it seems to work well for
ARM "virt" machine.

Thanks,
Jiahui

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