Re: [PATCH v2 2/2] ALSA: hda: Allow HDA to be runtime suspended when dGPU is not bound

[Bjorn Helgaas <helgaas@kernel.org>]

On Thu, Aug 29, 2019 at 02:01:28AM +0800, Kai-Heng Feng wrote:
> It's a common practice to let dGPU unbound and use PCI platform power
> management to disable its power through _OFF method of power resource,
> which is listed by _PR3.
> When the dGPU comes with an HDA function, the HDA won't be suspended if
> the dGPU is unbound, so the power resource can't be turned off.

I'm not involved in applying this patch, but from the peanut gallery:

  - The above looks like it might be two paragraphs missing a blank
    line between?  Or maybe it's one paragraph that needs to be
    rewrapped?

  - I can't parse the first sentence.  I guess "dGPU" and "HDA" are
    hardware devices, but I don't know what "unbound" means.  Is that
    something to do with a driver being bound to the dGPU?  Or is it
    some connection between the dGPU and the HDA?

  - "PCI platform power management" is still confusing -- I think we
    either have "PCI power management" that uses the PCI PM Capability
    (i.e., writing PCI_PM_CTRL as in pci_raw_set_power_state()) OR we
    have "platform power management" that uses some other method,
    typically ACPI.  Since you refer to _OFF and _PR3, I guess you're
    referring to platform power management here.

  - "It's common practice to let dGPU unbound" -- does that refer to
    some programming technique common in drivers, or some user-level
    thing like unloading a driver, or ...?  My guess is it probably
    means "unbind the driver from the dGPU" but I still don't know
    what makes it common practice.

This probably all makes perfect sense to the graphics cognoscenti, but
for the rest of us there are a lot of dots here that are not
connected.

> Commit 37a3a98ef601 ("ALSA: hda - Enable runtime PM only for
> discrete GPU") only allows HDA to be runtime-suspended once GPU is
> bound, to keep APU's HDA working.
> 
> However, HDA on dGPU isn't that useful if dGPU is unbound. So let's
> relax the runtime suspend requirement for dGPU's HDA function, to save
> lots of power.
> 
> BugLink: https://bugs.launchpad.net/bugs/1840835
> Fixes: b516ea586d71 ("PCI: Enable NVIDIA HDA controllers”)
> Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
> ---
> v2:
> - Change wording.
> - Rebase to Tiwai's branch.
> 
>  sound/pci/hda/hda_intel.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/sound/pci/hda/hda_intel.c b/sound/pci/hda/hda_intel.c
> index 91e71be42fa4..c3654d22795a 100644
> --- a/sound/pci/hda/hda_intel.c
> +++ b/sound/pci/hda/hda_intel.c
> @@ -1284,7 +1284,11 @@ static void init_vga_switcheroo(struct azx *chip)
>  		dev_info(chip->card->dev,
>  			 "Handle vga_switcheroo audio client\n");
>  		hda->use_vga_switcheroo = 1;
> -		chip->bus.keep_power = 1; /* cleared in either gpu_bound op or codec probe */
> +
> +		/* cleared in either gpu_bound op or codec probe, or when its
> +		 * root port has _PR3 (i.e. dGPU).
> +		 */
> +		chip->bus.keep_power = !pci_pr3_present(p);
>  		chip->driver_caps |= AZX_DCAPS_PM_RUNTIME;
>  		pci_dev_put(p);
>  	}
> -- 
> 2.17.1
>