Small usability improvements for devbind

Over the course of using devbind, i find myself frequently bumping up against two common errors (with the assumption being that i'm not the only person who hits these errors).

First happens when i forget to specify the driver. The error message in this case looks something like the following:

anatoly@xxxx:~$ sudo DPDK/usertools/dpdk-devbind.py -b 08:00.0 08:00.1
Error: bind failed for 0000:08:00.1 - Cannot open /sys/bus/pci/drivers/08:00.0/bind
Error: unbind failed for 0000:08:00.1 - Cannot open /sys/bus/pci/drivers//unbind

This is confusing to anyone who isn't intimately familiar with how driver binding through sysfs works. The first patch in this series changes the error message to instead look like the following:

anatoly@xxxx:~$ sudo DPDK/usertools/dpdk-devbind.py -b 08:00.0 08:00.1
ERROR: Driver '08:00.0' does not look like a valid driver. Did you forget to specify the driver to bind devices to?

We do that by assuming that no one in their right mind will name their PCI driver with something that looks like a PCI address, so we check if the driver string is actually a valid device string. If it is, we error out.

The second error i often come across is forgetting to load the driver. This error looks something like this:

anatoly@xxxx:~$ sudo DPDK/usertools/dpdk-devbind.py -b vfio-pci 08:00.1
Error: bind failed for 0000:08:00.1 - Cannot open /sys/bus/pci/drivers/vfio-pci/bind

This too isn't very informative. The second patch in this patchset changes this error to look like this instead:

anatoly@xxxx:~$ sudo DPDK/usertools/dpdk-devbind.py -b vfio-pci 08:00.1
ERROR: Driver 'vfio-pci' is not loaded.

Nice and informative!

Additionally, since we're outputting our new error messages to stderr, i took the liberty of adjusting all the rest of the error messages to also output to stderr.

v4:
- Corrected mixing of tabs and spaces in the first patch as well

v3:
- Corrected mixing of tabs and spaces. Spaces all the way down!

v2:
- Addressed Stephen's feedback
- Added new patch adjusting error output to stderr

Anatoly Burakov (3):
usertools/devbind: add error on forgetting to specify driver usertools/devbind: check if module is loaded before binding usertools/devbind: print all errors to stderr

 usertools/dpdk-devbind.py | 144 ++++++++++++++++++++++----------------
 1 file changed, 83 insertions(+), 61 deletions(-)

2.17.1