Re: [PATCH v2 1/4] PCI: Clean up pci_scan_slot()

Niklas Schnelle
2022-04-12T15:24:42Z

On Tue, 2022-04-12 at 16:30 +0200, Niklas Schnelle wrote:
> While determining the next PCI function is factored out of
> pci_scan_slot() into next_fn() the former still handles the first
> function as a special case duplicating the code from the scan loop and
> splitting the condition that the first function exits from it being
> multifunction which is tested in next_fn().
>
> Furthermore the non ARI branch of next_fn() mixes the case that
> multifunction devices may have non-contiguous function ranges and dev
> may thus be NULL with the multifunction requirement. It also signals
> that no further functions need to be scanned by returning 0 which is
> a valid function number.
>
> Improve upon this by moving all conditions for having to scan for more
> functions into next_fn() and make them obvious and commented.
>
> By changing next_fn() to return -ENODEV instead of 0 when there is no
> next function we can then handle the initial function inside the loop
> and deduplicate the shared handling.
>
> No functional change is intended.
>
> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
> ---
> drivers/pci/probe.c | 41 +++++++++++++++++++----------------------
> 1 file changed, 19 insertions(+), 22 deletions(-)
>
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 17a969942d37..389aa1f9cb2c 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -2579,33 +2579,35 @@ struct pci_dev *pci_scan_single_device(struct pci_bus *bus, int devfn)
> }
> EXPORT_SYMBOL(pci_scan_single_device);
>
> -static unsigned int next_fn(struct pci_bus *bus, struct pci_dev *dev,
> - unsigned int fn)
> +static int next_fn(struct pci_bus *bus, struct pci_dev *dev, int fn)
> {
> int pos;
> u16 cap = 0;
> unsigned int next_fn;
>
> - if (pci_ari_enabled(bus)) {
> - if (!dev)
> - return 0;

This part here theoretically changes the behavior slightly. If the ARI
information is wrong/lands us in a "hole" we may look for more
functions via the non-ARI path. Not sure if that is relevant though as
in the worst case we might find functions that we otherwise wouldn't
have seen. Seems rather obsure to me but I might be wrong, we currently
don't see the ARI capability in Linux on IBM Z so I have less
experience with this. I did of course boot test on my x86_64
workstation.

> + if (dev && pci_ari_enabled(bus)) {
> pos = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_ARI);
> if (!pos)
> - return 0;
> + return -ENODEV;
>
> pci_read_config_word(dev, pos + PCI_ARI_CAP, &cap);
> next_fn = PCI_ARI_CAP_NFN(cap);
> if (next_fn <= fn)
> - return 0; /* protect against malformed list */
> + return -ENODEV; /* protect against malformed list */
>
> return next_fn;
> }
>
> - /* dev may be NULL for non-contiguous multifunction devices */
> - if (!dev || dev->multifunction)
> - return (fn + 1) % 8;
> -
> - return 0;
> + /* only multifunction devices may have more functions */
> + if (dev && !dev->multifunction)
> + return -ENODEV;
> + /*
> + * A function 0 is required but multifunction devices may
> + * be non-contiguous so dev can be NULL otherwise.
> + */
> + if (!fn && !dev)
> + return -ENODEV;
> + return (fn <= 6) ? fn + 1 : -ENODEV;
> }
>
>
---8<---