Re: [PATCH v6 1/4] HID: playstation: add DualSense lightbar support

From: Marek Behun <marek.behun@nic.cz>
To: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: Roderick Colenbrander <roderick@gaikai.com>,
	Jiri Kosina <jikos@kernel.org>, Pavel Machek <pavel@ucw.cz>,
	Dmitry Torokhov <dmitry.torokhov@gmail.com>,
	"open list:HID CORE LAYER" <linux-input@vger.kernel.org>,
	linux-leds@vger.kernel.org,
	Roderick Colenbrander <roderick.colenbrander@sony.com>
Subject: Re: [PATCH v6 1/4] HID: playstation: add DualSense lightbar support
Date: Tue, 16 Feb 2021 18:56:02 +0100	[thread overview]
Message-ID: <20210216185602.48a980f6@nic.cz> (raw)
In-Reply-To: <CAO-hwJ+=_fjHgenXvHv45sHgzwiG2z9vGeq7fmMqj2=BeYCF1Q@mail.gmail.com>

On Tue, 16 Feb 2021 18:29:46 +0100
Benjamin Tissoires <benjamin.tissoires@redhat.com> wrote:

> > So all HIDs can be uniqely determined via this atomic_inc_return(&id),
> > but it is only stored in string form as part of device name.  
> 
> Yes and no. This atomic_inc is only used to allow a sysfs tree,
> because you can have several HID devices below the same USB, I2C or
> UHID physical device. From the userspace, no-one cares about that ID,
> because all HID devices are exported as input, IIO or hidraw nodes.
> 
> So using this "id" would not allow for a direct mapping HID device ->
> sysfs entry because users will still have to walk through the tree to
> find out which is which.

So you are saying that the fact that userspace cannot take the number
from "hidN" string and simply do a lookup /sys/bus/hid/devices/hidN is
the problem here.

This is not a problem in my opinion, because userspace can simply
access the parent HID device via /sys/class/leds/hidN:color:func/parent.

In fact we did something similar for LEDs connected to ethernet PHYs.
To summarize:
  - ethernet PHYs are identified by long, sometimes crazy strings like
      d0032004.mdio-mii:01
    or even
      /soc/internal-regs@d0000000/mdio@32004/switch0@10/mdio:08
  - for the purposes of having a sane devicename part in LED names, I
    sent this patch
    https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg2301470.html
    which adds a simple incrementing integer ID to each PHY device.
    (The code is not in upstream yet because there is other work needed
     and because I decided that some functionality has to be available
     via a different mechanism, but this part is complete and reviewed.)

> An actual one-to-one mapping would using 'hidrawX' because there is a
> one-to-one mapping between /dev/hidrawX for HID devices. However, this
> means that we consider the bus to be hidraw which is plain wrong too.
> 
> The unique ID of HID devices (in /sys/bus/hid/devices) is in the form
> `BUS:VID:PID.XXXX`. I understand the need to not have colons, so could
> we standardize LEDs on the HID subsystem to be named
> `hid-bus_vid_pid_xxxx:color:fun(-n)?`? That would allow a mapping
> between the LED and the sysfs, and would also allow users to quickly
> filter out the playstation ones.

As I wrote in other e-mail some minutes ago, this just means that we
need to wait for other people's opinions. Please do not send this
pull-request with the LED patches until this is resolved.

Marek