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

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Date:   Fri, 15 Nov 2019 10:38:40 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Andrey Konovalov <andreyknvl@...gle.com>
cc:     linux-usb@...r.kernel.org, <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, 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.

> --- /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.

Alan Stern