97df5e5758
Currently automatic gadget endpoint selection based on required features
doesn't work. Raw Gadget tries iterating over the list of available
endpoints and finding one that has the right direction and transfer type.
Unfortunately selecting arbitrary gadget endpoints (even if they satisfy
feature requirements) doesn't work, as (depending on the UDC driver) they
might have fixed addresses, and one also needs to provide matching
endpoint addresses in the descriptors sent to the host.
The composite framework deals with this by assigning endpoint addresses
in usb_ep_autoconfig() before enumeration starts. This approach won't work
with Raw Gadget as the endpoints are supposed to be enabled after a
set_configuration/set_interface request from the host, so it's too late to
patch the endpoint descriptors that had already been sent to the host.
For Raw Gadget we take another approach. Similarly to GadgetFS, we allow
the user to make the decision as to which gadget endpoints to use.
This patch adds another Raw Gadget ioctl USB_RAW_IOCTL_EPS_INFO that
exposes information about all non-control endpoints that a currently
connected UDC has. This information includes endpoints addresses, as well
as their capabilities and limits to allow the user to choose the most
fitting gadget endpoint.
The USB_RAW_IOCTL_EP_ENABLE ioctl is updated to use the proper endpoint
validation routine usb_gadget_ep_match_desc().
These changes affect the portability of the gadgets that use Raw Gadget
when running on different UDCs. Nevertheless, as long as the user relies
on the information provided by USB_RAW_IOCTL_EPS_INFO to dynamically
choose endpoint addresses, UDC-agnostic gadgets can still be written with
Raw Gadget.
Fixes: f2c2e71764 ("usb: gadget: add raw-gadget interface")
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>
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==============
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USB Raw Gadget
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==============
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USB Raw Gadget is a kernel module that provides a userspace interface for
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the USB Gadget subsystem. Essentially it allows to emulate USB devices
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from userspace. Enabled with CONFIG_USB_RAW_GADGET. Raw Gadget is
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currently a strictly debugging feature and shouldn't be used in
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production, use GadgetFS instead.
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Comparison to GadgetFS
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~~~~~~~~~~~~~~~~~~~~~~
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Raw Gadget is similar to GadgetFS, but provides a more low-level and
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direct access to the USB Gadget layer for the userspace. The key
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differences are:
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1. Every USB request is passed to the userspace to get a response, while
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GadgetFS responds to some USB requests internally based on the provided
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descriptors. However note, that the UDC driver might respond to some
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requests on its own and never forward them to the Gadget layer.
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2. GadgetFS performs some sanity checks on the provided USB descriptors,
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while Raw Gadget allows you to provide arbitrary data as responses to
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USB requests.
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3. Raw Gadget provides a way to select a UDC device/driver to bind to,
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while GadgetFS currently binds to the first available UDC.
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4. Raw Gadget explicitly exposes information about endpoints addresses and
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capabilities allowing a user to write UDC-agnostic gadgets.
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5. Raw Gadget has ioctl-based interface instead of a filesystem-based one.
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Userspace interface
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~~~~~~~~~~~~~~~~~~~
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To create a Raw Gadget instance open /dev/raw-gadget. Multiple raw-gadget
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instances (bound to different UDCs) can be used at the same time. The
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interaction with the opened file happens through the ioctl() calls, see
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comments in include/uapi/linux/usb/raw_gadget.h for details.
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The typical usage of Raw Gadget looks like:
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1. Open Raw Gadget instance via /dev/raw-gadget.
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2. Initialize the instance via USB_RAW_IOCTL_INIT.
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3. Launch the instance with USB_RAW_IOCTL_RUN.
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4. In a loop issue USB_RAW_IOCTL_EVENT_FETCH calls to receive events from
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Raw Gadget and react to those depending on what kind of USB device
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needs to be emulated.
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Potential future improvements
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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- Implement ioctl's for setting/clearing halt status on endpoints.
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- Reporting more events (suspend, resume, etc.) through
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USB_RAW_IOCTL_EVENT_FETCH.
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- Support O_NONBLOCK I/O.
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