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: 7627a0ed ("ata: ahci: Drop low power policy board type")
Cc: stable@vger.kernel.org
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Link: https://lore.kernel.org/r/20240618152828.2686771-2-cassel@kernel.orgSigned-off-by: Niklas Cassel <cassel@kernel.org>