[Nut-upsdev] usbhid-ups fails with Belkin F6C120-UNV in nut-2.4.3

Arjen de Korte nut+devel at de-korte.org
Sat Mar 20 21:13:47 UTC 2010

Citeren Charles Lepple <clepple op gmail.com>:

> Several years ago on the linux-usb list, it was often pointed out  
> that the kernel was supposed to honor the wMaxPacketSize field, and  
> fragment the requests accordingly. While this was suggested for  
> performance reasons (fewer round trips between user and kernel  
> space), it should still hold true for less frequent transfers.

This was my understanding too, but for instance the blazer_usb (and  
megatec_usb) also fragment transfers in 8-byte chunks themselves. A  
while back we tried to query for larger chunks, but this wasn't  
reliable at all.

It looks like we can't trust that the kernel will do this for us, so I  
guess it would be better to do this in the libusb.c module. If memory  
serves, it would be possible to get wMaxPacketSize somehow from the  
device handle.

Best regards, Arjen
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