Re: [PATCH] USB: EHCI: inflate max_tt_usecs and implement sitd backpointers

[Brent Page <brentfpage@gmail.com>]

> That loop adds three new "blank" sitds to iso_sched.  At the end of 
> sitd_urb_transaction(), the picture looks like this:
> 
> 	+-> iso_sched <--> new2 <--> new1 <--> new0 <-+
> 	|				                              |
> 	+---------------------------------------------+

But for an iso endpoint, the schedule should never be empty (except for
when the very first urb comes in).  So, there are sitds that are already
scheduled before the new sitds come in, and I think in total the picture
will look like, e.g.,
	+-> stream <--> new2  <--> new1  <--> new0  <--
	|		        --> prev2 <--> prev1 <--> prev0 <-+
	|	                        					  |
	+-------------------------------------------------+
Then, after

2177  sitd = list_entry(iso_sched->td_list.next,
2178    struct ehci_itd, sitd_list);
2179  list_move_tail(&sitd->sitd_list, &stream->td_list);

is executed 3x, the schedule will look like
	+-> stream <--> prev2 <--> prev1 <--> prev0 <--
	|		        -->  new2 <--> new1  <--> new0  <-+
	|	                        					  |
	+-------------------------------------------------+

> As far as I can see, there's nothing wrong with 
> allowing the first uframe to be partially occupied as long as the new 
> sitd is added to the end of the uframe's queue, so it doesn't delay the 
> sitd's already there.
Oh yeah, I see that – because then the large sitd can partially occupy
the uframe without needing to be broken up.

> But in general, things will be simpler if each new sitd is always added
> to the end of the queue.  The code should be changed to do this.
To be conservative, I didn't change sitd_link() in the patch.  I just
submitted the newest version to this thread.  The whitespace formatting
should be correct now.

From,
Brent Page