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

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Date:   Wed, 24 Aug 2022 17:13:36 -0700
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     Mario Limonciello <mario.limonciello@....com>,
        "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>
Subject: Re: [PATCH] ATA: ACPI: Do not check ACPI_FADT_LOW_POWER_S0

On 2022/08/24 16:55, Mario Limonciello wrote:
> On 8/24/22 18:54, Damien Le Moal wrote:
>> 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 making this change, perhaps the #ifdef CONFIG_ACPI can drop too.

Indeed.

> 
>>>   		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