Re: [PATCH v6] ACPI: device_sysfs: Add sysfs support for _PLD

[Heikki Krogerus]

On Mon, Feb 14, 2022 at 02:58:44PM -0800, Won Chung wrote:
> On Mon, Feb 14, 2022 at 12:30 PM Won Chung <wonchung@xxxxxxxxxx> wrote:
> >
> > On Mon, Feb 14, 2022 at 11:12 AM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> > >
> > > On Fri, Feb 11, 2022 at 3:30 AM Won Chung <wonchung@xxxxxxxxxx> wrote:
> > > >
> > > > When ACPI table includes _PLD fields for a device, create a new
> > > > directory (pld) in sysfs to share _PLD fields.
> > >
> > > This version of the patch loos better to me, but I'm not sure if it
> > > goes into the right direction overall.
> > >
> > > > Currently without PLD information, when there are multiple of same
> > > > devices, it is hard to distinguish which device corresponds to which
> > > > physical device in which location. For example, when there are two Type
> > > > C connectors, it is hard to find out which connector corresponds to the
> > > > Type C port on the left panel versus the Type C port on the right panel.
> > >
> > > So I think that this is your primary use case and I'm wondering if
> > > this is the best way to address it.
> > >
> > > Namely, by exposing _PLD information under the ACPI device object,
> > > you'll make user space wanting to use that information depend on this
> > > interface, but the problem is not ACPI-specific (inevitably, it will
> > > appear on systems using DT, sooner or later) and making the user space
> > > interface related to it depend on ACPI doesn't look like a perfect
> > > choice.
> > >
> > > IOW, why don't you create a proper ABI for this in the Type C
> > > subsystem and expose the information needed by user space in a generic
> > > way that can be based on the _PLD information on systems with ACPI?
> >
> > Hi Rafael,
> >
> > Thank you for the review.
> >
> > I was thinking that _PLD info is specific to ACPI since it is part of
> > the ACPI table. Could you explain a little bit more on why you think
> > exposing _PLD fields is not an ACPI-specific problem?
> 
> Hi Rafael again,
> 
> Sorry for the silly question here. I misunderstood your comment a bit,
> but I talked to Benson and Prashant for clarification. I understand
> now what you mean by it is not an ACPI-specific problem and exposing
> PLD would depend on ACPI.
> 
> >
> > I gave an example of how _PLD fields can be used for specifying Type C
> > connectors, but it is not Type C specific. For Chrome OS, we plan to
> > initially add PLD to not only Type C connectors but also USB port
> > devices (including Type C and Type A). Also, PLD can be used in the
> > future for describing other types of ports too like HDMI. (Benson and
> > Prashant, please correct or add if I am wrong or missing some
> > information) Maybe my commit message was not detailed enough..
> >
> > I am also curious what Heikki thinks about this. Heikki, can you take
> > a look and share your thoughts?
> 
> I am still curious what you and Heikki think about this since it may
> not be a Type C specific issue. We can start from adding generic
> location info to Type C subsystem first, as you suggested, then
> consider how to do the same for USB devices and Type A ports
> afterwards. I would appreciate sharing any thoughts or feedback. Thank
> you very much!

Like you said, _PLD is not Type-C specific. We can't limit it to any
specific device class. For example, I'm pretty ssure that sooner or
later we want to get this information in user space also with camera
sensors, and probable with a few other things as well.

I think the question here is, can we create a some kind of an
abstraction layer for the user space that exposes the device location
details in generic Linux specific way - so with ACPI it would utilise
the _PLD, and with DT something else (today AFAIK DT does not have
any way to describe locations of the devices). Maybe I'm wrong?

But if that is the question, then IMO the answer is: maybe one day,
but not today, and even if we one day can come up with something like
that, we still should expose the _PLD as ACPI specific information to
the user space as is.

Even if one day we have common sysfs attributes for all the devices
that contain the location of the device in some form, those attributes
will almost certainly have only a sub-set of the _PLD details, a
sub-set that works also with DT.
IMO the user space should always have access to all the necessary _PLD
details in their raw form if needed, even if those common device
location attributes exist - duplicated information or not. And debugfs
unfortunately is also not OK for that, because the user space needs to
be able to also rely on access to the additional details if needed.

We can limit the _PLD fields that we expose to the ones that we know
we need today (and probable should limit them to those), and we can of
course have a Kconfig option for the _PLD sysfs information if we want
to, but let's not start this by trying to figure out what kind of
abstraction we want for this. Right now we simply can not do that.

thanks,

-- 
heikki