usb: gadget: f_fs: add capability for dfu functional descriptor

Add the ability for the USB FunctionFS (FFS) gadget driver to be able
to create Device Firmware Upgrade (DFU) functional descriptors. [1]

This patch allows implementation of DFU in userspace using the
FFS gadget. The DFU protocol uses the control pipe (ep0) for all
messaging so only the addition of the DFU functional descriptor
is needed in the kernel driver.

The DFU functional descriptor is written to the ep0 file along with
any other descriptors during FFS setup. DFU requires an interface
descriptor followed by the DFU functional descriptor.

This patch includes documentation of the added descriptor for DFU
and conversion of some existing documentation to kernel-doc format
so that it can be included in the generated docs.

An implementation of DFU 1.1 that implements just the runtime descriptor
using the FunctionFS gadget (with rebooting into u-boot for DFU mode)
has been tested on an i.MX8 Nano.

An implementation of DFU 1.1 that implements both runtime and DFU mode
using the FunctionFS gadget has been tested on Xilinx Zynq UltraScale+.
Note that for the best performance of firmware update file transfers, the
userspace program should respond as quick as possible to the setup packets.

[1] https://www.usb.org/sites/default/files/DFU_1.1.pdf

Signed-off-by: David Sands <david.sands@biamp.com>
Co-developed-by: Chris Wulff <crwulff@gmail.com>
Signed-off-by: Chris Wulff <crwulff@gmail.com>
Link: https://lore.kernel.org/r/20240811000004.1395888-2-crwulff@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

3 files changed, 42 insertions, 0 deletions

diff --git a/Documentation/usb/functionfs-desc.rst b/Documentation/usb/functionfs-desc.rst
new file mode 100644
index 000000000000..39649774da54
--- /dev/null
+++ b/Documentation/usb/functionfs-desc.rst
@@ -0,0 +1,39 @@
+======================
+FunctionFS Descriptors
+======================
+
+Some of the descriptors that can be written to the FFS gadget are
+described below. Device and configuration descriptors are handled
+by the composite gadget and are not written by the user to the
+FFS gadget.
+
+Descriptors are written to the "ep0" file in the FFS gadget
+following the descriptor header.
+
+.. kernel-doc:: include/uapi/linux/usb/functionfs.h
+   :doc: descriptors
+
+Interface Descriptors
+---------------------
+
+Standard USB interface descriptors may be written. The class/subclass of the
+most recent interface descriptor determines what type of class-specific
+descriptors are accepted.
+
+Class-Specific Descriptors
+--------------------------
+
+Class-specific descriptors are accepted only for the class/subclass of the
+most recent interface descriptor. The following are some of the
+class-specific descriptors that are supported.
+
+DFU Functional Descriptor
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+When the interface class is USB_CLASS_APP_SPEC and the interface subclass
+is USB_SUBCLASS_DFU, a DFU functional descriptor can be provided.
+The DFU functional descriptor is a described in the USB specification for
+Device Firmware Upgrade (DFU), version 1.1 as of this writing.
+
+.. kernel-doc:: include/uapi/linux/usb/functionfs.h
+   :doc: usb_dfu_functional_descriptor
diff --git a/Documentation/usb/functionfs.rst b/Documentation/usb/functionfs.rst
index d05a775bc45b..f7487b0d8057 100644
--- a/Documentation/usb/functionfs.rst
+++ b/Documentation/usb/functionfs.rst
@@ -25,6 +25,8 @@ interface numbers starting from zero).  The FunctionFS changes
 them as needed also handling situation when numbers differ in
 different configurations.
 
+For more information about FunctionFS descriptors see :doc:`functionfs-desc`
+
 When descriptors and strings are written "ep#" files appear
 (one for each declared endpoint) which handle communication on
 a single endpoint.  Again, FunctionFS takes care of the real
diff --git a/Documentation/usb/index.rst b/Documentation/usb/index.rst
index 27955dad95e1..826492c813ac 100644
--- a/Documentation/usb/index.rst
+++ b/Documentation/usb/index.rst
@@ -11,6 +11,7 @@ USB support
     dwc3
     ehci
     functionfs
+    functionfs-desc
     gadget_configfs
     gadget_hid
     gadget_multi