linux-kernel - Re: [RFC PATCH 0/4] USB Power Delivery character device interface

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Message-ID: <YXkx0uG3Z539XJWo@kuha.fi.intel.com>
Date:   Wed, 27 Oct 2021 14:02:42 +0300
From:   Heikki Krogerus <heikki.krogerus@...ux.intel.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Prashant Malani <pmalani@...omium.org>,
        Benson Leung <bleung@...gle.com>,
        Adam Thomson <Adam.Thomson.Opensource@...semi.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Badhri Jagan Sridharan <badhri@...gle.com>,
        Jack Pham <jackp@...eaurora.org>,
        "Gopal, Saranya" <saranya.gopal@...el.com>,
        "Regupathy, Rajaram" <rajaram.regupathy@...el.com>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/4] USB Power Delivery character device interface

Hi Greg,

On Tue, Oct 26, 2021 at 05:06:28PM +0200, Greg KH wrote:
> So, why not sysfs?  :)

This is about allowing the user space to take over the USB Power
Delivery communication and policy decisions in some cases. The user
space needs to be able to send and receive raw USB Power Delivery
messages one way or the other. I don't care about what's the interface
that we use.

Here we are talking about the PDOs, so basically the power contract.
Even if we figured out a way how to expose all the information from
the Capability, Status, Alert and what ever messages you need to the
user space via sysfs, and then allow the user to separately send the
Request Message, we would have only covered the power contract. That
does not cover everything, but it would also be unnecessarily
complicated to handle with separate sysfs files IMO.

Even with the power contract it would make more sense to me to just
allow the user space to simply read and write the raw messages, but
when we go the other things like Vendor Specific Messages, I don't
think there is any other way.

So we really do need to be able to tap into the USB Power Delivery
protocol layer directly from user space. I don't care about how we do
that - character device is just a suggestion, although, it does still
feel correct to me. Is there some other way we could do this?

thanks,

-- 
heikki