ATA: ACPI: Do not check ACPI_FADT_LOW_POWER_S0

From: Rafael J. Wysocki <rafael.j.wysocki@intel.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().

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
 drivers/ata/ahci.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

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@intel.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@intel.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)
>>
>>
>>
> 
>