linux-kernel - Re: [PATCH v2 1/1] usb: gadget: add raw-gadget interface

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Message-ID: <CAAeHK+y6mStjFJ-S0eN5qhBmik87ZMtnteRQVmjVbNRWRjLQUg@mail.gmail.com>
Date:   Fri, 15 Nov 2019 17:04:42 +0100
From:   Andrey Konovalov <andreyknvl@...gle.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     USB list <linux-usb@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Corbet <corbet@....net>,
        Felipe Balbi <balbi@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Marco Elver <elver@...gle.com>
Subject: Re: [PATCH v2 1/1] usb: gadget: add raw-gadget interface

On Fri, Nov 15, 2019 at 4:38 PM Alan Stern <stern@...land.harvard.edu> wrote:
>
> On Fri, 15 Nov 2019, Andrey Konovalov wrote:
>
> > USB Raw Gadget is a kernel module that provides a userspace interface for
> > the USB Gadget subsystem. Essentially it allows to emulate USB devices
> > from userspace. Enabled with CONFIG_USB_RAW_GADGET. Raw Gadget is
> > currently a strictly debugging feature and shouldn't be used in
> > production.
> >
> > Raw Gadget is similar to GadgetFS, but provides a more low-level and
> > direct access to the USB Gadget layer for the userspace. The key
> > differences are:
> >
> > 1. Every USB request is passed to the userspace to get a response, while
> >    GadgetFS responds to some USB requests internally based on the provided
> >    descriptors. However note, that the UDC driver might respond to some
> >    requests on its own and never forward them to the Gadget layer.
> >
> > 2. GadgetFS performs some sanity checks on the provided USB descriptors,
> >    while Raw Gadget allows you to provide arbitrary data as responses to
> >    USB requests.
> >
> > 3. Raw Gadget provides a way to select a UDC device/driver to bind to,
> >    while GadgetFS currently binds to the first available UDC.
> >
> > 4. Raw Gadget uses predictable endpoint names (handles) across different
> >    UDCs (as long as UDCs have enough endpoints of each required transfer
> >    type).
> >
> > 5. Raw Gadget has ioctl-based interface instead of a filesystem-based one.
>
> ...
>
> > --- /dev/null
> > +++ b/Documentation/usb/raw-gadget.rst
>
> > +Userspace interface
> > +~~~~~~~~~~~~~~~~~~~
> > +
> > +To create a Raw Gadget instance open /sys/kernel/debug/usb/raw-gadget
> > +(debugfs should be enabled and mounted). Multiple raw-gadget instances
>
> Looks like the documentation hasn't kept up with the more recent
> changes to the driver.

Right, will fix in v3, thanks!

>
> > --- /dev/null
> > +++ b/drivers/usb/gadget/legacy/raw.c
> > @@ -0,0 +1,1057 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * USB Raw Gadget driver.
> > + * See Documentation/usb/raw-gadget.rst for more details.
> > + *
> > + * Andrey Konovalov <andreyknvl@...il.com>
> > + */
> > +
> > +#define pr_fmt(fmt) "raw: %s: " fmt, __func__
>
> This macro isn't used anywhere now.

It's used internally by pr_debug(). I didn't remove all of the
debugging messages, I've left the ones that are printed in cases or
errors, since ftrace might not always help to distinguish between some
of those.