Re: [RFC] libusb / in-kernel usb driver criteria (was: USB driver for talking to the Microchip PIC18 boot loader)

[Paulo Marques <pmarques@grupopie.com>]

Xiaofan Chen wrote:
> On Dec 30, 2007 11:53 AM, mgross <640e9920@gmail.com> wrote:
>> [...]
>> What is the linux-usb policies on new drivers that could be
>> implemented in user space?  When does a kernel driver make sense over
>> a libusb one?
> 
> That would be interesting to know.

I myself have been faced with this question before, and I think we 
should try to clarify this by adding a document with some guidelines to 
Documentation/usb.

So, to get the ball rolling, here are some factors that IMHO help decide 
in which side to implement a driver:

  - if the driver ties a hardware device to an existing in-kernel 
interface (network, block, serial, bluetooth, video4linux, etc.), it 
should probably be implemented in-kernel.

  - on the other hand, if the driver doesn't use an existing kernel 
interface and creates a new user-visible interface that is going to be 
used by a single userspace application, it should probably be done in 
userspace.

  - if it is going to be used by several applications it could still be 
implemented as a library, but it starts moving into the gray area.

  - performance might be a reason to move to kernel space, but I don't 
think it matters for transfer rates below 10Mbytes/sec or so.

Anyway, this is just MHO, so feel free to discuss this further. I'm 
simply volunteering to sum up this thread into a patch to add a 
Documentation/usb/userspace_drivers.txt (or something like that), so 
that we can help future developers decide where to write their drivers.

-- 
Paulo Marques - www.grupopie.com

"Very funny Scotty. Now beam up my clothes."