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

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Date:   Wed, 24 Aug 2022 18:55:50 -0500
From:   Mario Limonciello <mario.limonciello@....com>
To:     Damien Le Moal <damien.lemoal@...nsource.wdc.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 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.

>>   		if (hpriv->cap & HOST_CAP_PART)
>>   			policy = ATA_LPM_MIN_POWER_WITH_PARTIAL;
>>   		else if (hpriv->cap & HOST_CAP_SSC)
>>
>>
>>
> 
>