linux-kernel - Re: [PATCH] drm: amd: amdgpu: ACPI: Add comment about ACPI_FADT_LOW_POWER

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Date:   Wed, 24 Aug 2022 15:25:03 -0500
From:   "Limonciello, Mario" <mario.limonciello@....com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Alex Deucher <alexander.deucher@....com>
Cc:     Christian König <christian.koenig@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>, amd-gfx@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>
Subject: Re: [PATCH] drm: amd: amdgpu: ACPI: Add comment about
 ACPI_FADT_LOW_POWER_S0

On 8/24/2022 12:32, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> According to the ACPI specification [1], the ACPI_FADT_LOW_POWER_S0
> flag merely means that it is better to use low-power S0 idle on the
> given platform than S3 (provided that the latter is supported) and it
> doesn't preclude using either of them (which of them will be used
> depends on the choices made by user space).
> 
> However, on some systems that flag is used to indicate whether or not
> to enable special firmware mechanics allowing the system to save more
> energy when suspended to idle.  If that flag is unset, doing so is
> generally risky.
> 
> Accordingly, add a comment to explain the ACPI_FADT_LOW_POWER_S0 check
> in amdgpu_acpi_is_s0ix_active(), the purpose of which is otherwise
> somewhat unclear.
> 
> Link: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fuefi.org%2Fspecs%2FACPI%2F6.4%2F05_ACPI_Software_Programming_Model%2FACPI_Software_Programming_Model.html%23fixed-acpi-description-table-fadt&amp;data=05%7C01%7Cmario.limonciello%40amd.com%7Cf43320dda5114deeb16908da85f69d3b%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637969591512297179%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=xp8pNzsXCkLcIJOJFY77yaLkMrvz5he3S%2Bi%2FwaxTwwg%3D&amp;reserved=0 # [1]
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Reviewed-by: Mario Limonciello <mario.limonciello@....com>

> ---
>   drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c |    6 ++++++
>   1 file changed, 6 insertions(+)
> 
> Index: linux-pm/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
> ===================================================================
> --- linux-pm.orig/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
> +++ linux-pm/drivers/gpu/drm/amd/amdgpu/amdgpu_acpi.c
> @@ -1066,6 +1066,12 @@ bool amdgpu_acpi_is_s0ix_active(struct a
>   	    (pm_suspend_target_state != PM_SUSPEND_TO_IDLE))
>   		return false;
>   
> +	/*
> +	 * If ACPI_FADT_LOW_POWER_S0 is not set in the FADT, it is generally
> +	 * risky to do any special firmware-related preparations for entering
> +	 * S0ix even though the system is suspending to idle, so return false
> +	 * in that case.
> +	 */
>   	if (!(acpi_gbl_FADT.flags & ACPI_FADT_LOW_POWER_S0)) {
>   		dev_warn_once(adev->dev,
>   			      "Power consumption will be higher as BIOS has not been configured for suspend-to-idle.\n"
> 
> 
>