Re: AHCI SATA Runtime PM

Hi,

On 3/4/21 4:34 PM, Alexander Monakov wrote:

Hello, Hans, Linux PM folks,

I'm looking for clarification regarding this patch discussion:

https://patchwork.kernel.org/project/linux-pm/patch/20180420101834.15783-2-0v3rdr0n3@gmail.com/

Hans said,

quoted

Ah, so the AHCI code has runtime pm enabled by default (so there
is another pm_runtime_allow() somewhere, but then disables it for
unused ports to make hotpluging something into those ports work.

I have a laptop where two unused AHCI SATA controllers are present (but
obviously nothing can be hotplugged into them). Apparently due to the above,
they do not enter runtime autosuspend.

The problem is, these "ATA port" nodes don't seem to participate in udev
hierarchy, so it's unclear how I'm supposed to automatically re-enable
runtime PM for them. For PCI device nodes, I have

ACTION=="add", SUBSYSTEM=="pci", TEST=="power/control", ATTR{power/control}="auto"

but ata1/uevent is empty, so there's no obvious way to write the corresponding
UDev rule.

You can enable runtime-pm from udev using the following rule:

ACTION=="add", SUBSYSTEM=="ata_port", ATTR{../../power/control}="auto"

Prior to discovering the above patch discussion, I have filed a bug:
https://bugzilla.kernel.org/show_bug.cgi?id=211837

Does the above correctly reflect how AHCI PM is supposed to be?

This is a complicated question, I just checked and according to:
https://www.intel.com/content/www/us/en/io/serial-ata/serial-ata-ahci-spec-rev1-3-1.html

AHCI 1.3 compliant controllers should raise a PME signal on hotplug
changes even when in D3. So perhaps the solution is to catch this signal
and then wakeup the controller; and remove the pm_runtime_forbid(dev); call
from ata_tport_add().

I don't have much experience with dealing with the PME stuff, so you will
need to do some research there, or perhaps someone else can help?

This seems to be more of a sata/ahci question then a generic linux-pm question
though, so I have added Jebns and linux-ide to the To / Cc.

If so, what is
the proper way to enable runtime PM for unused ATA ports?

Regards,

Hans