linux-kernel - Re: [PATCH] ATA: ACPI: Do not check ACPI_FADT_LOW_POWER

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Message-ID: <9c479bb6-bb57-9b5a-2fa5-7b4d1b840245@opensource.wdc.com>
Date:   Wed, 24 Aug 2022 16:54:42 -0700
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     linux-ide@...r.kernel.org, Linux ACPI <linux-acpi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mario Limonciello <mario.limonciello@....com>
Subject: Re: [PATCH] ATA: ACPI: Do not check ACPI_FADT_LOW_POWER_S0

On 2022/08/24 10:29, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

The patch title should be "ata: ahci: ..."

> 
> 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().
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>  drivers/ata/ahci.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> Index: linux-pm/drivers/ata/ahci.c
> ===================================================================
> --- linux-pm.orig/drivers/ata/ahci.c
> +++ linux-pm/drivers/ata/ahci.c
> @@ -1610,8 +1610,7 @@ static void ahci_update_initial_lpm_poli
>  	}
>  
>  #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)
> 
> 
> 


-- 
Damien Le Moal
Western Digital Research