2012-06-05

Dynamic USB device ID

Dynamic USB device ID


Reference designs

Nowadays many DVB UDB devices are actually chip vendor reference designs which means those are very similar. Used chips are same and wired together similarly, meaning only new device id is needed for the driver in order to get it working.

It takes too long

There is always significant delay from the device release to the point support is arrived for the distribution. That delay is coming from the different schedules. First it takes time until driver author gets info about new device is needed to add driver. He has to find out hardware or at least someone who can test patch. After that there is Kernel schedule. Kernel has own merge window and release candidate cycle that takes many months. In my understanding simple USB IDs are allowed to add driver during release candidate phase, but usually it still goes through merge window which adds significant delay. And finally there distribution schedule. It is usually needed to upgrade used distribution as they do not upgrade new Kernel version during release cycle. All-in-all, it could take year or so.


Promote the device ID to driver


There is feature called dynamic USB device ID to tackle that delay. User can tell for the Kernel that he wants try if given device driver could drive his new device. For example we could say load dvb_usb_af9015 driver for the device having USB ID 15a4:9016:
echo 15a4 9016 > /sys/bus/usb/drivers/dvb_usb_af9015/new_id


DVB USB and dynamic IDs

Currently DVB USB doesn't support dynamic IDs. Today I did some work in order to add support for it. Unfortunately I did not find out as nice solution as I was hoping. As for now I use .driver_info field from the struct usb_device_id to pass all needed to data to the DVB USB. In practice .driver_info field carries pointer to the struct dvb_usb_device_properties. In normal case all needed data is inside MODULE_DEVICE_TABLE() but in case of dynamic ID there is no entry inside MODULE_DEVICE_TABLE() and thus no needed pointer inside .driver_info.

Solution is to implement own .probe() and set .driver_info for dynamic ID. And finally pass that all to the DVB USB. Not very ugly, nor nice.

What I would like to see is dynamic ID entry which could be added to the driver MODULE_DEVICE_TABLE() similarly as others. Maybe there is even some reserved USB vendor IDs for special purposes that could be used.

struct usb_device_id {
    .idVendor = VENDOR_ID_DYNAMIC
    .idProduct = PRODUCT_ID_DYNAMIC
    .driver_info = <own data>
}

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