linux-kernel - Re: [PATCH v3] ata: ahci: Do not check ACPI_FADT_LOW_POWER

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Date:   Thu, 25 Aug 2022 13:48:11 -0500
From:   "Limonciello, Mario" <mario.limonciello@....com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Damien Le Moal <damien.lemoal@...nsource.wdc.com>
Cc:     linux-ide@...r.kernel.org, Linux ACPI <linux-acpi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] ata: ahci: Do not check ACPI_FADT_LOW_POWER_S0

On 8/25/2022 13:47, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> 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).
> 
> For this reason, there is no benefit from checking that flag in
> ahci_update_initial_lpm_policy().
> 
> First off, it cannot be a bug to do S3 with policy set to either
> ATA_LPM_MIN_POWER_WITH_PARTIAL or ATA_LPM_MIN_POWER, because S3 can be
> used on systems with ACPI_FADT_LOW_POWER_S0 set and it must work if
> really supported, so the ACPI_FADT_LOW_POWER_S0 check is not needed to
> protect the S3-capable systems from failing.
> 
> Second, suspend-to-idle can be carried out on a system with
> ACPI_FADT_LOW_POWER_S0 unset and it is expected to work, so if setting
> policy to either ATA_LPM_MIN_POWER_WITH_PARTIAL or ATA_LPM_MIN_POWER is
> needed to handle that case correctly, it should be done regardless of
> the ACPI_FADT_LOW_POWER_S0 value.
> 
> Accordingly, replace the ACPI_FADT_LOW_POWER_S0 check in
> ahci_update_initial_lpm_policy() with pm_suspend_default_s2idle()
> which is more general and also takes the user's preference into
> account and drop the CONFIG_ACPI #ifdef around it that is not necessary
> any more.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

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

> ---
> 
> v2 -> v3:
>     * Use pm_suspend_default_s2idle() instead of the check being dropped.
>     * Update the changelog.
> 
> v1 -> v2:
>     * Adjust subject (Damien).
>     * Drop #ifdef CONFIG_ACPI that is not necessary any more (Mario).
>     * Update the changelog.
> 
> ---
>   drivers/ata/ahci.c |    5 +----
>   1 file changed, 1 insertion(+), 4 deletions(-)
> 
> Index: linux-pm/drivers/ata/ahci.c
> ===================================================================
> --- linux-pm.orig/drivers/ata/ahci.c
> +++ linux-pm/drivers/ata/ahci.c
> @@ -1609,15 +1609,12 @@ static void ahci_update_initial_lpm_poli
>   		goto update_policy;
>   	}
>   
> -#ifdef CONFIG_ACPI
> -	if (policy > ATA_LPM_MED_POWER &&
> -	    (acpi_gbl_FADT.flags & ACPI_FADT_LOW_POWER_S0)) {
> +	if (policy > ATA_LPM_MED_POWER && pm_suspend_default_s2idle()) {
>   		if (hpriv->cap & HOST_CAP_PART)
>   			policy = ATA_LPM_MIN_POWER_WITH_PARTIAL;
>   		else if (hpriv->cap & HOST_CAP_SSC)
>   			policy = ATA_LPM_MIN_POWER;
>   	}
> -#endif
>   
>   update_policy:
>   	if (policy >= ATA_LPM_UNKNOWN && policy <= ATA_LPM_MIN_POWER)
> 
> 
>