69 lines
3.0 KiB
ReStructuredText
69 lines
3.0 KiB
ReStructuredText
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====================
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How FunctionFS works
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====================
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From kernel point of view it is just a composite function with some
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unique behaviour. It may be added to an USB configuration only after
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the user space driver has registered by writing descriptors and
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strings (the user space program has to provide the same information
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that kernel level composite functions provide when they are added to
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the configuration).
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This in particular means that the composite initialisation functions
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may not be in init section (ie. may not use the __init tag).
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From user space point of view it is a file system which when
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mounted provides an "ep0" file. User space driver need to
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write descriptors and strings to that file. It does not need
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to worry about endpoints, interfaces or strings numbers but
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simply provide descriptors such as if the function was the
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only one (endpoints and strings numbers starting from one and
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interface numbers starting from zero). The FunctionFS changes
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them as needed also handling situation when numbers differ in
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different configurations.
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When descriptors and strings are written "ep#" files appear
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(one for each declared endpoint) which handle communication on
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a single endpoint. Again, FunctionFS takes care of the real
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numbers and changing of the configuration (which means that
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"ep1" file may be really mapped to (say) endpoint 3 (and when
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configuration changes to (say) endpoint 2)). "ep0" is used
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for receiving events and handling setup requests.
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When all files are closed the function disables itself.
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What I also want to mention is that the FunctionFS is designed in such
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a way that it is possible to mount it several times so in the end
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a gadget could use several FunctionFS functions. The idea is that
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each FunctionFS instance is identified by the device name used
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when mounting.
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One can imagine a gadget that has an Ethernet, MTP and HID interfaces
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where the last two are implemented via FunctionFS. On user space
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level it would look like this::
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$ insmod g_ffs.ko idVendor=<ID> iSerialNumber=<string> functions=mtp,hid
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$ mkdir /dev/ffs-mtp && mount -t functionfs mtp /dev/ffs-mtp
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$ ( cd /dev/ffs-mtp && mtp-daemon ) &
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$ mkdir /dev/ffs-hid && mount -t functionfs hid /dev/ffs-hid
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$ ( cd /dev/ffs-hid && hid-daemon ) &
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On kernel level the gadget checks ffs_data->dev_name to identify
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whether it's FunctionFS designed for MTP ("mtp") or HID ("hid").
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If no "functions" module parameters is supplied, the driver accepts
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just one function with any name.
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When "functions" module parameter is supplied, only functions
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with listed names are accepted. In particular, if the "functions"
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parameter's value is just a one-element list, then the behaviour
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is similar to when there is no "functions" at all; however,
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only a function with the specified name is accepted.
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The gadget is registered only after all the declared function
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filesystems have been mounted and USB descriptors of all functions
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have been written to their ep0's.
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Conversely, the gadget is unregistered after the first USB function
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closes its endpoints.
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