Re: [PATCH V12 4/5] PCI: Handle CRS ("device not ready") returned by device after FLR

Bjorn Helgaas
2017-08-23T22:24:41Z

On Wed, Aug 23, 2017 at 05:51:19PM -0400, Sinan Kaya wrote:
> On 8/23/2017 5:38 PM, Bjorn Helgaas wrote:
> > If we increase the timeout, is there still value in adding the
> > pci_bus_wait_crs() stuff? I'm not sure there is.
>
> I agree increasing the timeout is more than enough for FLR case.

If that's the case, I think there's no value in adding additional
complexity to the path. If we increase the timeout, we might pretty
it up a little along the lines of the patch below.

> However, I was considering the wait and pending functions as a utility
> that I can reuse towards fixing CRS in other conditions like secondary
> bus reset and potentially PM.
>
> I'm planning to have a CRS session in the Linux Plumbers Conference
> to talk about CRS use cases.

Great idea. I'm kind of confused on its value in general myself. And
there's now a new mechanism (Function Readiness Status) that I don't
think we have any support for at all.

> I was going to follow up this series with secondary bus reset next once
> this goes in.

I think I'm OK with everything except the pci_flr_wait() change. The
rest of it makes sense (although I don't think there are any users
outside probe.c, so maybe should be static for now).

> I'm unable to test PM. I can't promise how I fix/test it.

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index af0cc3456dc1..b04c99e60b77 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -3811,27 +3811,50 @@ int pci_wait_for_pending_transaction(struct pci_dev *dev)
}
EXPORT_SYMBOL(pci_wait_for_pending_transaction);

-/*
- * We should only need to wait 100ms after FLR, but some devices take longer.
- * Wait for up to 1000ms for config space to return something other than -1.
- * Intel IGD requires this when an LCD panel is attached. We read the 2nd
- * dword because VFs don't implement the 1st dword.
- */
static void pci_flr_wait(struct pci_dev *dev)
{
- int i = 0;
+ int delay = 1, timeout = 60000;
u32 id;

- do {
- msleep(100);
+ /*
+ * Per PCIe r3.1, sec 6.6.2, a device must complete an FLR within
+ * 100ms, but may silently discard requests while the FLR is in
+ * progress. Wait 100ms before trying to access the device.
+ */
+ msleep(100);
+
+ /*
+ * After 100ms, the device should not silently discard config
+ * requests, but it may still indicate that it needs more time by
+ * responding to them with CRS completions. The Root Port will
+ * generally synthesize ~0 data to complete the read (except when
+ * CRS SV is enabled and the read was for the Vendor ID; in that
+ * case it synthesizes 0x0001 data).
+ *
+ * Wait for the device to return a non-CRS completion. Read the
+ * Command register instead of Vendor ID so we don't have to
+ * contend with the CRS SV value.
+ */
+ pci_read_config_dword(dev, PCI_COMMAND, &id);
+ while (id == ~0) {
+ if (delay > timeout) {
+ dev_warn(&dev->dev, "not ready %dms after FLR; giving up\n",
+ delay - 1);
+ return;
+ }
+
+ if (delay > 1000)
+ dev_info(&dev->dev, "not ready %dms after FLR; waiting\n",
+ delay - 1);
+
+ msleep(delay);
+ delay *= 2;
+
pci_read_config_dword(dev, PCI_COMMAND, &id);
- } while (i++ < 10 && id == ~0);
+ }

- if (id == ~0)
- dev_warn(&dev->dev, "Failed to return from FLR\n");
- else if (i > 1)
- dev_info(&dev->dev, "Required additional %dms to return from FLR\n",
- (i - 1) * 100);
+ if (delay > 1000)
+ dev_info(&dev->dev, "ready %dms after FLR\n", delay - 1);
}

/**