* Alan Stern <[EMAIL PROTECTED]> [2005-11-27 09:14]:
> There's no mechanism to report back that an URB is too large,
> and no driver tries to break URBs up into smaller pieces.
Most drivers won't submit URBs larger than 8KB, so usb-storage
was my primary focus. With a better understanding of how scatter
gather lists map to URBs, however, I don't think drivers using sg
should worry about URB sizes.
> There are several other related properties of block request
> queues; you would interested in the one that controls the
> maximum size of a scatter_gather hardware segment.
> Unfortunately usb-storage doesn't provide any way to set that
> property.
Yes, I am interested in that. Thanks for letting me know.
> > usb-storage doesn't check for dma mapping failures either.
>
> Nonsense. What about this code from usb_sg_init():
I apologize for incorrectly determining usb-storage was at fault.
The problem lies with a broken API, where dma_map_sg() can't
report errors. usb_buffer_map_sg() notes this problem:
// FIXME generic api broken like pci, can't report errors
return dma_map_sg (controller, sg, nents,
usb_pipein (pipe) ? DMA_FROM_DEVICE : DMA_TO_DEVICE);
This is very problematic for my memory constrained HC, where
-ENOMEM is common. Pretending to be a PIO controller and
handling DMA in-house is looking like a better solution.
> > This wouldn't require modification to the USB core if I
> > temporarily replace the USB completion handler, but this seems
> > like a hack. Ideally, struct usb_operations would have a
> > 'complete_urb' or 'giveback_urb' function I would use to unmap
> > dma memory.
>
> Why do you need to replace anything? Can't you do the unmapping within
> the HCD's URB-completion routine?
The problem is I'm not writing a HCD, but a bus glue for OHCI,
and the OHCI core doesn't let the bus glue know about URB
completions. You're right, though, I should handle this with the
HCD's completion routine. I just need a hook to use.
Thanks,
Chris Humbert
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