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

From: Won Chung <wonchung@google.com>
To: "Rafael J. Wysocki" <rafael@kernel.org>
Cc: Len Brown <lenb@kernel.org>,
	Heikki Krogerus <heikki.krogerus@linux.intel.com>,
	Benson Leung <bleung@chromium.org>,
	Prashant Malani <pmalani@chromium.org>,
	ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v6] ACPI: device_sysfs: Add sysfs support for _PLD
Date: Mon, 14 Feb 2022 12:30:17 -0800	[thread overview]
Message-ID: <CAOvb9yjpruiHxkZyZ8BOT0Hi_iV7xMOnBCr59BZX3eah_Zcy_w@mail.gmail.com> (raw)
In-Reply-To: <CAJZ5v0gD4zs3uBAYv6M4_1gNpkZ-g9XKOywJnf5007e6GwoGVA@mail.gmail.com>

On Mon, Feb 14, 2022 at 11:12 AM Rafael J. Wysocki <rafael@kernel.org> wrote:
>
> On Fri, Feb 11, 2022 at 3:30 AM Won Chung <wonchung@google.com> 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?

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?

Thank you,
Won