linux-kernel - Re: [PATCH v2] ata: ahci: Do not check ACPI_FADT_LOW_POWER

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Date:   Thu, 25 Aug 2022 13:16:19 -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 v2] ata: ahci: Do not check ACPI_FADT_LOW_POWER_S0

On 8/25/2022 13:01, 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, drop the ACPI_FADT_LOW_POWER_S0 check from
> ahci_update_initial_lpm_policy() along with the CONFIG_ACPI #ifdef
> around it that is not necessary any more.

Looking at the source commit for this behavior:

b1a9585cc396 ("ata: ahci: Enable DEVSLP by default on x86 with SLP_S0")

It was trying to set a policy tied to when the system is defaulting to
suspend to idle.

To try to match the spirit of the original request but not tying it to 
the FADT, how about using pm_suspend_default_s2idle()?

> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
> 
> 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) {
>   		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)
> 
> 
>