On Thu, Dec 25, 2008 at 08:01:29PM +0100, Pavel Machek wrote:
> On Mon 2008-12-22 15:11:57, Andrew Patterson wrote:
> > ASPM: Use msleep instead of cpu_relax during link retraining
> >
> > The cpu_relax() function can be a noop on certain architectures
> > like IA-64 when CPU threads are disabled, so use msleep instead
> > during link retraining busy/wait loop.
>
> Author clearly wanted to do a busy loop... why do you think 10msec
> delay here is acceptable?
10ms? I see a 1ms sleep.
> > @@ -217,16 +220,18 @@ static void pcie_aspm_configure_common_clock(struct pci_dev *pdev)
> > pci_write_config_word(pdev, pos + PCI_EXP_LNKCTL, reg16);
> >
> > /* Wait for link training end */
> > - /* break out after waiting for 1 second */
> > + /* break out after waiting for timeout */
> > start_jiffies = jiffies;
> > - while ((jiffies - start_jiffies) < HZ) {
> > + for (;;) {
> > pci_read_config_word(pdev, pos + PCI_EXP_LNKSTA, ®16);
> > if (!(reg16 & PCI_EXP_LNKSTA_LT))
> > break;
> > - cpu_relax();
> > + if ((jiffies - start_jiffies) >= LINK_RETRAIN_TIMEOUT)
> > + break;
> > + msleep(1);
>
> Is this safe w.r.t. jiffie wraparounds?
Definitely needs to be time_before/time_after.
> > }
> > /* training failed -> recover */
> > - if ((jiffies - start_jiffies) >= HZ) {
> > + if ((jiffies - start_jiffies) >= LINK_RETRAIN_TIMEOUT) {
> > dev_printk (KERN_ERR, &pdev->dev, "ASPM: Could not configure"
> > " common clock\n");
> > i = 0;
>
> AFAICT this can trigger false positives. !reg16 test succeeds and then
> jiffies tick.
>
> ...it could happen before but you make it way more probable...
No, because the test moved.
I came up with this loop (off the top of my head):
<willy> for (;;) {
<willy> pci_read_config_word(pdev, pos + PCI_EXP_LNKSTA, ®16);
<willy> if (!(reg16 & PCI_EXP_LNKSTA_LT))
<willy> break;
<willy> if ((jiffies - start_jiffies) >= HZ)
<willy> break;
<willy> msleep(1);
<willy> }
Andrew has mostly followed that ... improving it to LINK_RETRAIN_TIMEOUT
instead of HZ.
Yes, the subsequent test should be of reg16 instead of jiffies.
And we should be using time_before/after instead of the explicit
comparison.
--
Matthew Wilcox Intel Open Source Technology Centre
"Bill, look, we understand that you're interested in selling us this
operating system, but compare it to ours. We can't possibly take such
a retrograde step."
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