Re: Autosuspend problem - some explanation needed.

[Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>]

On Mon, 18 Jun 2007, Dave Mielke wrote:

> [quoted lines by Alan Stern on 2007/06/18 at 12:03 -0400]
> 
> >I shouldn't think it would be too much harder to read several sysfs
> >files than a single usbfs file.  
> 
> Except that it starts to get messy if each system call is protected with error
> checking and each piece of data read in as text is tested for validity. Perhaps
> some or all such carefulness isn't necessary, but it does tend to protect
> against unforeseen problems. It's much simpler to read it all in at once.
> 
> >Do you really need anything other than
> >the vendor ID, the product ID, and perhaps the bcdDevice value?
> 
> That's mostly true for device selection. Once we know we have the right device,
> though, we do use the string index fields to fetch the strings for logging. I
> know the strings teemselves are in sysfs, but to keep that code portable we
> read the string desccriptors from the selected device.
> 
> It was just an idea which would make switching back to a truly passive device
> descriptor read (like it has been before autosuspend) easy, not only for us but
> also for other applications.

Greg, what do you think?  Is it reasonable to add a binary sysfs
attribute file containing the device descriptor?  (And perhaps also one 
containing the current configuration descriptor, and the interface 
descriptors in their individual subdirectories.  Maybe even the 
endpoint descriptors too.)

Even though the binary descriptors are available in usbfs, when a
device's usbfs file is opened the device is automatically resumed.  
This means that a program looking for a particular device will wake up
a bunch of unrelated devices.  With sysfs there is no such problem.

Alan Stern


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