On Sat, Jan 07, 2006 at 12:05:48AM +0100, Marco d'Itri wrote:
(#104) How the kernel firmware loader works
fEnIo[0] learnt an important lesson about the kernel firmware loader:
it (usually) does not work as expected for non-modular drivers.
Yeah... thanks a lot for your explanation. I'm now a little smarter.
The reason is that the request_firmware()[1] interface is synchronous.
Since it's usually called in the initialisation section of drivers, the
userspace firmware loader is not available yet if the calling driver is
built-in in the kernel. The request_firmware_nowait()[2] asynchronous
interface was designed to replace it, but most drivers have not been
ported yet.
When a driver calls request_firmware(), a uevent[3] is sent by the
kernel to udev over a netlink(7) socket, requesting that a specific
file is uploaded. udevd runs /lib/udev/firmware.agent, a simple shell
script which will look for the $FIRMWARE file in a few directories and
then copy it to the designated place in the driver $DEVPATH in sysfs.
If the driver is initialised before userspace is started then the
loader will not be available, and the request will fail. A possible
solution is to run udev in the early userspace environment (initramfs),
but just compiling the driver as a module is usually simpler.
So I suppose that it shouldn't be possible to compile in such drivers, if
they work only as a module. At least since they aren't ported to new
interface.
Anyway, once again thanks for explanation, and now I'm glad that I posted
this question in blog, otherwise I would probably lost more time to figure
out what's going on.
regards
fEnIo
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