USB Device Add reading wrong attributes on Linux

[Kay Sievers]

On Wed, Dec 3, 2008 at 18:21, Drew Moseley <dmoseley at mvista.com> wrote:
> I've noticed a case with a USB device on a linux host where some of the
> attributes are not being properly initialized.  Specifically the
> can_wake_up and is_self_powered fields are incorrect because when HAL
> reads the bmAttributes sysfs file it gets a read error.  A restart of
> hal usually brings the correct values in.
>
> In the kernel code for the USB sysfs files there is a comment that
> "Unforunately these attributes cannot be created before the uevent is
> broadcast".  I do see in the kernel where the sysfs file is being read
> by HAL but getting an error.  Has anyone else dealt with this?  Should
> there be a retry of some kind in HAL to handle this?

Yeah, that's a known issue. Things like this can not work without
tight integration with the kernel, which must send an update event for
every change. This is just not implemented, not in the kernel and not
in HAL. Retry implemented in HAL would only help in some cases.

To make it kind of work, all these values could be updated when the
child devices are created/removed. This would fix a bunch of different
bugs in this area, like when a device is reconfigured, and HAL will
not notice it, and exports the wrong values from the initial config.
In most cases when child devices come and go, the parent device has,
at that time, all values properly populated. So, per example, "usb
interface" events could trigger an update of the "usb device" values.

Btw, It was a mistake to mirror any such kernel values in a daemon, we
didn't know that the time HAL was created. It's really hard to get
things like this right, and the pseudo "platform abstraction" at that
fine-grained level just doesn't work in reality. DeviceKit, which will
replace HAL, will not even try that. It will just use sysfs directly
on Linux, without any such caching behavior like HAL has.

Thanks,
Kay