Re: Improving kernel -> userspace (usbfs) usb device hand off

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On Fri, 10 Jun 2011, Felipe Balbi wrote:

> I don't see any problems in this situation. If, for that particular
> product, webcam and still image functionality are mutually exclusive,
> then that's how the product (and their drivers) have to work.
> 
> If the linux community decided to put webcam functionality in kernel and
> still image functionality on a completely separate driver, that's
> entirely our problem.

And the problem is how to coordinate the two of them.

> > 2. Until recently in the history of Linux, there was an irreconcilable 
> > conflict. If a kernel driver for the video streaming mode was present and 
> > installed, it was not possible to use the camera in stillcam mode at all. 
> > Thus the only solution to the problem was to blacklist the kernel module 
> > so that it would not get loaded automatically and only to install said 
> > module by hand if the camera were to be used in video streaming mode, then 
> > to rmmod it immediately afterwards. Very cumbersome, obviously. 
> 
> true... but why couldn't we combine both in kernel or in userspace
> altogether ? Why do we have this split ? (words from a newbie in V4L2,
> go easy :-p)

I think the problem may be that the PTP protocol used in the still-cam
mode isn't suitable for a kernel driver.  Or if it is suitable, it
would have to be something like a shared-filesystem driver -- nothing
like a video driver.  You certainly wouldn't want to put it in V4L2.

> Or, to move the libgphoto2 driver to kernel, combine it in the same
> driver that handles streaming. No ?

No.  Something else is needed.

Alan Stern

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