Re: [PATCH v4 10/12] pci: add pci_find_common_upstream_dev()

[Bjorn Helgaas <bhelgaas@google.com>]

On Wed, Jul 22, 2015 at 04:39:55PM -0500, Will Davis wrote:
> Adds an interface to find the first device which is upstream of both
> devices.
> 
> Signed-off-by: Will Davis <wdavis@nvidia.com>
> ---
>  drivers/pci/search.c | 25 +++++++++++++++++++++++++
>  include/linux/pci.h  |  2 ++
>  2 files changed, 27 insertions(+)
> 
> diff --git a/drivers/pci/search.c b/drivers/pci/search.c
> index a20ce7d..cbd8163 100644
> --- a/drivers/pci/search.c
> +++ b/drivers/pci/search.c
> @@ -384,3 +384,28 @@ int pci_dev_present(const struct pci_device_id *ids)
>  	return 0;
>  }
>  EXPORT_SYMBOL(pci_dev_present);
> +
> +/**
> + * pci_find_common_upstream_dev - Returns the first common upstream device
> + * @from: the PCI device to search for a common upstream device from
> + * @to: the PCI device to search for a common upstream device to
> + *
> + * Walks up the bus hierarchy from the @from device, looking for the first bus
> + * which the @to device is downstream of. Returns %NULL if the devices do not
> + * share any upstream devices.

This interface looks like it should be symmetric: you should get the same
answer regardless of which device is "from" and which is "to".  I would
rename the arguments to something more neutral to reflect that.

> + */
> +struct pci_dev *pci_find_common_upstream_dev(struct pci_dev *from,
> +					     struct pci_dev *to)
> +{
> +	struct pci_dev *pdev = from;
> +
> +	while (pdev != NULL) {
> +		if ((to->bus->number >= pdev->bus->busn_res.start) &&
> +		    (to->bus->number <= pdev->bus->busn_res.end))
> +			return pdev;
> +		pdev = pdev->bus->self;

I think you want "pdev = pci_upstream_bridge(pdev)" here; otherwise you'll
exit the loop early if "from" is a VF on a virtual bus.

> +	}
> +
> +	return NULL;
> +}
> +EXPORT_SYMBOL(pci_find_common_upstream_dev);
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index d58063e..8262b9e 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -860,6 +860,8 @@ static inline struct pci_dev *pci_get_bus_and_slot(unsigned int bus,
>  }
>  struct pci_dev *pci_get_class(unsigned int class, struct pci_dev *from);
>  int pci_dev_present(const struct pci_device_id *ids);
> +struct pci_dev *pci_find_common_upstream_dev(struct pci_dev *from,
> +					     struct pci_dev *to);
>  
>  int pci_bus_read_config_byte(struct pci_bus *bus, unsigned int devfn,
>  			     int where, u8 *val);
> -- 
> 2.4.6
>