Re: some question about ehci-sched.c

[yoma sophian]

hi Alan:
> We don't _have_ to do it.  It is a small optimization; an attempt to
> avoid schedule collisions between interrupt transactions and
> isochronous transactions.  The effect of the "-" is that the code tries
> frames in backward order when it is scheduling isochronous URBs -- as
> opposed to trying frames in forward order when scheduling periodic
> URBs.
appreciate ur kind explanation.

Re: some question about ehci-sched.c

[Alan Stern]

On Fri, 21 Sep 2018, yoma sophian wrote:

> hi all:
> in iso_stream_schedule of ehci-sched.c, why we have to add the "-" in
> (-(++ehci->random_frame))  to calculate the start uframe?
> thanks for ur help in advance.
> 
> if (stream->ps.phase == NO_FRAME) {
> int done = 0;
> struct ehci_tt  *tt = find_tt(stream->ps.udev);
> 
> if (IS_ERR(tt)) {
> status = PTR_ERR(tt);
> goto fail;
> }
> compute_tt_budget(ehci->tt_budget, tt);
> 
> start = ((-(++ehci->random_frame)) << 3) & (period - 
> 1);
>   ^^ Here it is.

We don't _have_ to do it.  It is a small optimization; an attempt to
avoid schedule collisions between interrupt transactions and
isochronous transactions.  The effect of the "-" is that the code tries
frames in backward order when it is scheduling isochronous URBs -- as
opposed to trying frames in forward order when scheduling periodic
URBs.

It isn't important.

Alan Stern

some question about ehci-sched.c

[yoma sophian]

hi all:
in iso_stream_schedule of ehci-sched.c, why we have to add the "-" in
(-(++ehci->random_frame))  to calculate the start uframe?
thanks for ur help in advance.

if (stream->ps.phase == NO_FRAME) {
int done = 0;
struct ehci_tt  *tt = find_tt(stream->ps.udev);

if (IS_ERR(tt)) {
status = PTR_ERR(tt);
goto fail;
}
compute_tt_budget(ehci->tt_budget, tt);

start = ((-(++ehci->random_frame)) << 3) & (period - 1);
  ^^ Here it is.