Re: [linux-usb-devel] Re: [Bluetooth] HCI USB driver update. Support for SCO over HCI USB.

[Max Krasnyansky <maxk@qualcomm.com>]

On Fri, 2003-05-09 at 12:06, David Brownell wrote:
> Hi Max,
> 
> > Do you think we can add this 
> >         struct urb {
> >                 ...
> >                 struct list_head drv_list;
> >                 char drv_cb[X];
> >                 char hcd_cb[X]; 
> >                 ...
> >         };
> 
> Only with kerneldoc ... :)
> 
:) 

> I'd certainly like the list_head.  Patch attached,
> in case Greg agrees enough.  On x86, this makes
> sizeof(struct urb) == 120, so it's using space
> that was previously wasted.
> 
> 
> As for the skb->cb analogue(s), details need working.
> 
>   - What should "X" be?  skb->cb is 48 bytes.
I'd say 16 will be enough for drivers. 

>   - Should the cb[] arrays be "long"?  They tend to
>     be used for pointers...
Probably doesn't matter as long as ->cb is aligned.

>   - The HCDs want different amounts of per-urb data.
>     Sizes on x86:
>       * UHCI wants a lot -- 60 bytes!
>       * OHCI typically uses 16 bytes, but more for
>         multi-TD urbs (control 24 bytes, ISO often 36).
>       * EHCI doesn't allocate such extra data.
> 
>   - The HCDs would need conversion to use hcd_cb[].
>     I once had a patch that did that, but it's not
>     current.  It made urb->cb replace urb->hcpriv.
>
> I suppose X=60 for hcd_cb[] will be enough, at least
> on 32 bit CPUs.  But you start to see why in the
> new "USB Gadget" API, the analogue of "urb" gets
> allocated by the USB (device) controller rather
> than by generic code:  so that in typical cases,
> no additional per-request allocations are needed.

Ok. Sounds like it should be
	uint32_t hcd_cb[16]; // 64 bytes for internal use by HCD
	uint32_t drv_cb[2];  // 8  bytes for internal use by USB driver

?

Max