On Fri, 29 Oct 2010, Gavin Guo wrote:
> Hi all,
> The following code in qh_link_async:
>
> if (!head->qh_next.qh) {
> u32 cmd = ehci_readl(ehci, &ehci->regs->command);
>
> if (!(cmd & CMD_ASE)) {
> /* in case a clear of CMD_ASE didn't take yet */
> (void)handshake(ehci, &ehci->regs->status,
> STS_ASS, 0, 150);
> cmd |= CMD_ASE | CMD_RUN;
> ehci_writel(ehci, cmd, &ehci->regs->command);
> ehci_to_hcd(ehci)->state = HC_STATE_RUNNING;
> /* posted write need not be known to HC yet ... */
> }
> }
> Why only the null queue head is checked? I think it is
> possible that non-empty queue head link list also have asynchronous
> list disabled
No, it is not possible.
> (hc encounter queue head of which H=1 and scan_async havent's
> unlink the queue head).
Only one QH, ehci->async, ever has H set to 1. It is never removed
from the async schedule.
> If the non-empty queue head list condition is met, and
> qh_link_async add new queue head directly without enable CMD_ASE &
> CMD_RUN.
> The host controller will not go ahead to process the new queue
> head?? or if it's possible that hc could enable CMD_ASE & CMD_RUN in
> other place ??
> Could someone please tell me how this work ??
The async schedule is turned off in only one place: when
ehci_watchdog() calls start_unlink_async(ehci, ehci->async). That
happens only when the timer_action(ehci, TIMER_ASYNC_OFF) is called,
and that happens only in end_unlink_async() when the async list is
empty.
Alan Stern
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