Kernel bugs are bad news. Among the worst bugs are regressions -
situations where a once-working system breaks after a kernel upgrade. The
kernel developers have been taking an increasingly hard line against
regressions; patches which break working systems will usually be reverted,
even if those patches fix other problems. The idea, as pushed by Linus, is
that once a system works, it should
continue to work into the future.
As it happens, a number of USB users have found that, on upgrading to
2.6.16, their systems do not work anymore. But, in this case, this
"regression" is not seen as such by the developers and is not likely to
change. This issue is a good demonstration of the sort of tradeoffs which
operating systems developers must make.
USB ports can supply power to the devices plugged into them; this power is
sufficient to drive many devices, as well as totally unrelated items (such
as USB-powered LED lamps). There are limits to the amount of power which
can be supplied, however. USB devices will communicate their maximum
current draw to the host, which can then decide whether it has the capacity
available or not. If sufficient power is not available, the device will
not be allowed to configure itself and operate.
There are many rules in the USB specification on how power configuration
should work. One of those applies to unpowered USB hubs - the ones which
lack a power supply of their own. The total current drawn by an unpowered
hub cannot be allowed to exceed what the host can supply; in particular,
the USB specification limits devices on unpowered USB hubs to 100 mA of
current. Even if only one hub port is in use, that single port is limited
to that value, despite the fact that a larger draw should work in that situation.
Prior to 2.6.16, the Linux kernel did not actually check power requirements
before configuring devices. With 2.6.16, however, any device whose stated
maximum power requirement exceeds 100 mA will not be allowed to
configure itself on an unpowered hub. Thus, devices which worked in that
mode in earlier kernels now fail to operate; not all users are entirely
The argument has been made that, since these configurations almost always
work in the real world, the kernel should not be shutting them down now.
The fact is, however, that running hardware outside of its specifications
is always a dangerous thing to do. Often one will get away with it, but
sometimes things can fail badly. A fairly large class of USB devices are
mass storage devices; the consequences of power-related problems with these
include corrupted data and damaged hardware. These are not consequences
which the USB developers wish to inflict on their users, so, instead, they
refuse to operate devices out of their specifications.
To the developers, the fact that some previously-working hardware now fails
to operate is not a regression. It is a bug fix, with the kernel finally
performing some due diligence which should have been happening all along.
They do not intend to change this behavior.
As it happens, it is possible to convince the kernel to override its
good sense and configure the device anyway. It is not easy, however.
Essentially, the steps are this:
Needless to say, this sequence of steps is not entirely easy - and it must
be repeated each time the device is plugged in. For those who are
comfortable writing udev rules, this configuration change can be
automated without too much trouble. Perhaps the desktop environments will
eventually be made smart enough to detect this situation and offer (with
suitable scary warnings) to override the kernel for specific devices. But
it might just be better to buy a powered hub or plug the device directly
into the host.
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