On Wed, 9 Feb 2011, loody wrote:
> Hi all:
> I have some questions about usb driver:
> 1. what is qh->stamp used for?
It is used to prevent the CPU from doing unnecessary and unwanted work.
> I trace the source and I found it seems only used to increase the
> performance such that we will avoid re-scan the previous qh.
That's right.
> But if we don't check qh->stamp, in qh_completions, it will return
> immediately if the qh has been scanned before, since the qtd_list of
> the qh is empty, right.
What makes you think the qtd_list will be empty? Didn't you read the
code in scan_async?
/* clean any finished work for this qh */
if (!list_empty (&qh->qtd_list)
&& qh->stamp != ehci->stamp) {
Obviously the call to qh_completions won't occur if the list is empty.
> if so, why we still need to check qh->stamp?
> Meanwhile, there will be 1/8192 chance that 2 stamps be the same,
> suppose my frame list size is 1024, right?
That's sort of true. The probability isn't really 1/8192 because of
other factors. Still, this could be a bug. It could be fixed by
masking out the higher bits from the frame index register and
incrementing the higher bits of ehci->stamp. For example:
ehci->stamp = (ehci->stamp & ~0x1fff) + 0x2000 +
(ehci_readl(ehci, &ehci->regs->frame_index) & 0x1fff);
> 2. what is the max value of "Frame index register"
> I set my frame list size as 1024, in my opinion the max value of
> "Frame index register" should be 8192, 0x2000.
You probably mean 8191 (0x1fff), not 8192.
> But I found sometimes it will over 0x2000.
> Does that I calculate the wrong value?
The spec merely says that the upper bits are reserved; it doesn't say
that they have to be 0.
Alan Stern
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