Re: [PATCH V10 1/3] PCI: introduce pci_bus_wait_crs() function

Bjorn Helgaas
2017-08-18T03:00:09Z

On Fri, Aug 11, 2017 at 12:56:34PM -0400, Sinan Kaya wrote:
> Kernel is hiding Configuration Request Retry Status (CRS) inside
> pci_bus_read_dev_vendor_id() function. We are looking to add support for
> Function Level Reset (FLR) where vendor id read returns ~0.
>
> Move CRS handling into its own function so that it can be called from other
> places as well.

I think this is a much better idea than what I proposed. I still have
a few questions proposals. I'll post a v11 to show what I'm thinking.

> Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
> ---
> drivers/pci/pci.h | 2 ++
> drivers/pci/probe.c | 44 ++++++++++++++++++++++++++++++--------------
> 2 files changed, 32 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> index 22e0617..1bbe851 100644
> --- a/drivers/pci/pci.h
> +++ b/drivers/pci/pci.h
> @@ -235,6 +235,8 @@ enum pci_bar_type {
> pci_bar_mem64, /* A 64-bit memory BAR */
> };
>
> +bool pci_bus_wait_crs(struct pci_bus *bus, int devfn, u32 *l,
> + int crs_timeout);
> bool pci_bus_read_dev_vendor_id(struct pci_bus *bus, int devfn, u32 *pl,
> int crs_timeout);
> int pci_setup_device(struct pci_dev *dev);
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index c31310d..b1cb7bd 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -1824,29 +1824,17 @@ struct pci_dev *pci_alloc_dev(struct pci_bus *bus)
> }
> EXPORT_SYMBOL(pci_alloc_dev);
>
> -bool pci_bus_read_dev_vendor_id(struct pci_bus *bus, int devfn, u32 *l,
> - int crs_timeout)
> +bool pci_bus_wait_crs(struct pci_bus *bus, int devfn, u32 *l, int crs_timeout)
> {
> int delay = 1;
>
> - if (pci_bus_read_config_dword(bus, devfn, PCI_VENDOR_ID, l))
> - return false;
> -
> - /* some broken boards return 0 or ~0 if a slot is empty: */
> - if (*l == 0xffffffff || *l == 0x00000000 ||
> - *l == 0x0000ffff || *l == 0xffff0000)
> - return false;
> -
> /*
> * Configuration Request Retry Status. Some root ports return the
> * actual device ID instead of the synthetic ID (0xFFFF) required
> * by the PCIe spec. Ignore the device ID and only check for
> * (vendor id == 1).
> */
> - while ((*l & 0xffff) == 0x0001) {
> - if (!crs_timeout)
> - return false;
> -
> + do {
> msleep(delay);
> delay *= 2;
> if (pci_bus_read_config_dword(bus, devfn, PCI_VENDOR_ID, l))
> @@ -1858,6 +1846,34 @@ bool pci_bus_read_dev_vendor_id(struct pci_bus *bus, int devfn, u32 *l,
> PCI_FUNC(devfn));
> return false;

While staring at this, I think I found a pre-existing bug in
pci_bus_read_dev_vendor_id(). It looks like this:

while ((*l & 0xffff) == 0x0001) {
pci_bus_read_config_dword(bus, devfn, PCI_VENDOR_ID, l);
if (delay > crs_timeout)
return false;
}

The problem is that the config read may have *succeeded* that last
time before we time out. I think the correct sequence is:

- check for timeout
- read PCI_VENDOR_ID
- check for CRS

> }
> + } while ((*l & 0xffff) == 0x0001);
> +
> + return true;
> +}
> +EXPORT_SYMBOL(pci_bus_wait_crs);

I don't think we need EXPORT_SYMBOL here, do we?

> +bool pci_bus_read_dev_vendor_id(struct pci_bus *bus, int devfn, u32 *l,
> + int crs_timeout)
> +{
> + if (pci_bus_read_config_dword(bus, devfn, PCI_VENDOR_ID, l))
> + return false;
> +
> + /* some broken boards return 0 or ~0 if a slot is empty: */
> + if (*l == 0xffffffff || *l == 0x00000000 ||
> + *l == 0x0000ffff || *l == 0xffff0000)
> + return false;
> +
> + /*
> + * Configuration Request Retry Status. Some root ports return the
> + * actual device ID instead of the synthetic ID (0xFFFF) required
> + * by the PCIe spec. Ignore the device ID and only check for
> + * (vendor id == 1).
> + */
> + if ((*l & 0xffff) == 0x0001) {
> + if (!crs_timeout)
> + return false;

One thing I don't like is that every caller of pci_bus_wait_crs() has
to know about the 0x0001 value.

> + return pci_bus_wait_crs(bus, devfn, l, crs_timeout);
> }
>
> return true;
> --
> 1.9.1
>