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

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Message-ID: <20190827223125.GB9987@google.com>
Date:   Tue, 27 Aug 2019 17:31:25 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Kai-Heng Feng <kai.heng.feng@...onical.com>
Cc:     tiwai@...e.com, linux-pci@...r.kernel.org,
        alsa-devel@...a-project.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] ALSA: hda: Allow HDA to be runtime suspended when
 dGPU is not bound

On Tue, Aug 27, 2019 at 09:47:56PM +0800, Kai-Heng Feng wrote:
> It's a common practice to let dGPU unbound and use PCI port PM to
> disable its power through _PR3. When the dGPU comes with an HDA
> function, the HDA won't be suspended if the dGPU is unbound, so the dGPU
> power can't be disabled.

Just a terminology question:

I thought "using PCI port PM" meant using the PCI Power Management
Capability in config space to directly change the device's power
state, e.g., in pci_raw_set_power_state().

And I thought using _PS3, _PR3, etc would be part of "platform power
management"?

And AFAICT, _PR3 merely returns a list of power resources; it doesn't
disable power itself.

> 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 relax
> the runtime suspend requirement for dGPU's HDA function, to save lots of
> power.
> 
> BugLink: https://bugs.launchpad.net/bugs/1840835
> Signed-off-by: Kai-Heng Feng <kai.heng.feng@...onical.com>
> ---
>  sound/pci/hda/hda_intel.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/sound/pci/hda/hda_intel.c b/sound/pci/hda/hda_intel.c
> index 99fc0917339b..d4ee070e1a29 100644
> --- a/sound/pci/hda/hda_intel.c
> +++ b/sound/pci/hda/hda_intel.c
> @@ -1285,7 +1285,8 @@ static void init_vga_switcheroo(struct azx *chip)
>  		dev_info(chip->card->dev,
>  			 "Handle vga_switcheroo audio client\n");
>  		hda->use_vga_switcheroo = 1;
> -		hda->need_eld_notify_link = 1; /* cleared in gpu_bound op */
> +		/* cleared in gpu_bound op */
> +		hda->need_eld_notify_link = !pci_pr3_present(p);
>  		chip->driver_caps |= AZX_DCAPS_PM_RUNTIME;
>  		pci_dev_put(p);
>  	}
> -- 
> 2.17.1
>