Re: [PATCH v2] ata: ahci: Do not enable LPM if no LPM states are supported by the HBA

[Mika Westerberg <mika.westerberg@linux.intel.com>]

On Tue, Jun 18, 2024 at 05:28:29PM +0200, Niklas Cassel wrote:
> LPM consists of HIPM (host initiated power management) and DIPM
> (device initiated power management).
> 
> ata_eh_set_lpm() will only enable HIPM if both the HBA and the device
> supports it.
> 
> However, DIPM will be enabled as long as the device supports it.
> The HBA will later reject the device's request to enter a power state
> that it does not support (Slumber/Partial/DevSleep) (DevSleep is never
> initiated by the device).
> 
> For a HBA that doesn't support any LPM states, simply don't set a LPM
> policy such that all the HIPM/DIPM probing/enabling will be skipped.
> 
> Not enabling HIPM or DIPM in the first place is safer than relying on
> the device following the AHCI specification and respecting the NAK.
> (There are comments in the code that some devices misbehave when
> receiving a NAK.)
> 
> Performing this check in ahci_update_initial_lpm_policy() also has the
> advantage that a HBA that doesn't support any LPM states will take the
> exact same code paths as a port that is external/hot plug capable.
> 
> Side note: the port in ata_port_dbg() has not been given a unique id yet,
> but this is not overly important as the debug print is disabled unless
> explicitly enabled using dynamic debug. A follow-up series will make sure
> that the unique id assignment will be done earlier. For now, the important
> thing is that the function returns before setting the LPM policy.
> 
> Fixes: 7627a0edef54 ("ata: ahci: Drop low power policy board type")
> Cc: stable@vger.kernel.org
> Signed-off-by: Niklas Cassel <cassel@kernel.org>

Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>