Re: [PATCH v2] ata: ahci: Allow ignoring the external/hotplug capability of ports

[Niklas Cassel <cassel@kernel.org>]

On Fri, Aug 22, 2025 at 02:34:54PM +0200, Niklas Cassel wrote:

(snip)

> But do we really want a introduce a module parameter for this?
> I know that commit 4edf1505b76d ("ata: ahci: Disallow LPM policy control for
> external ports")
> if external, both:
> 1) sets initial lpm policy to MAX_POWER (so that hot plug works by default)
> 2) sets ATA_FLAG_NO_LPM, so that you cannot change the LPM policy using sysfs
> 
> I think that 1) is good.
> 
> However, why should we forbid the user to override to policy via sysfs just
> because the port is external?
> If a system admin has installed a udev rule or similar to set a lpm policy,
> why should we not respect that?
> 
> Yes, I know that many distros supply a rule that just enables LPM
> unconditionally for all disks...
> 
> But... instead of forbidding the user to change to policy using sysfs, perhaps
> a better way would be for the system admin/distros to improve their udev rules?
> 
> We have a sysfs property that says if the port supports LPM.
> Perhaps we should have a sysfs attribute that says if the port is external.
> 
> The udev rules can then be smarter and just set the LPM policy if the port is
> not external. But the user would still have the option to set a LPM policy
> (using the same interface), if they don't care about hotplug.
> 
> It seems more user friendly for a user that has a laptop with a docking
> station with hotplug capable ports, to install a udev rule to set an LPM
> policy, than to set a kernel module param.
> 
> What do you think?

Another idea: perhaps we could add something like:
"hotplug_supported" and "hotplug_enable" to sysfs.

For ports marked as external/hotplug capable, we set
"hotplug_supported" to true, and for ports that support
hotplug, we set "hotplug_enable" to true by default.

We can then continue to disallow (return -EOPNOTSUPP) when the user tries
to write to /sys/class/scsi_host/hostX/link_power_management_policy
for a port that has "hotplug_enable" == true.

If a user sets "hotplug_enable" = false, we allow writing to
/sys/class/scsi_host/hostX/link_power_management_policy

What do you think? Better or worse idea?


Kind regards,
Niklas