linux-kernel - Re: [PATCH v6 1/5] PCI: Clean up pci_scan

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Message-ID: <17c30662-7285-0e1a-91fb-071fa2cfc733@linux.ibm.com>
Date:   Thu, 30 Jun 2022 14:40:34 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     Jan Kiszka <jan.kiszka@...mens.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        linux-s390@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/5] PCI: Clean up pci_scan_slot()



On 6/28/22 16:30, 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. This duplicates the code from the scan loop.
> 
> Furthermore the non ARI branch of next_fn() is generally hard to
> understand and especially the check for multifunction devices is hidden
> in the handling of NULL devices for non-contiguous multifunction. It
> also signals that no further functions need to be scanned by returning
> 0 via wraparound and this is a valid function number.
> 
> Improve upon this by transforming the conditions in next_fn() to be
> easier to understand.
> 
> 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. This also makes it more explicit
> that only function 0 must exist.
> 
> No functional change is intended.
> 
> Cc: Jan Kiszka <jan.kiszka@...mens.com>
> Signed-off-by: Niklas Schnelle <schnelle@...ux.ibm.com>
> ---
>   drivers/pci/probe.c | 38 +++++++++++++++++++-------------------
>   1 file changed, 19 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 17a969942d37..b05d0ed83a24 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -2579,8 +2579,7 @@ 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;
> @@ -2588,24 +2587,26 @@ static unsigned int next_fn(struct pci_bus *bus, struct pci_dev *dev,
>   
>   	if (pci_ari_enabled(bus)) {
>   		if (!dev)
> -			return 0;
> +			return -ENODEV;
>   		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;
> +	if (fn >= 7)
> +		return -ENODEV;
> +	/* only multifunction devices may have more functions */
> +	if (dev && !dev->multifunction)
> +		return -ENODEV;
>   
> -	return 0;
> +	return fn + 1;

No more % 8 ?
Even it disapear later shouldn't we keep it ?



>   }
>   
>   static int only_one_child(struct pci_bus *bus)
> @@ -2643,26 +2644,25 @@ static int only_one_child(struct pci_bus *bus)
>    */
>   int pci_scan_slot(struct pci_bus *bus, int devfn)
>   {
> -	unsigned int fn, nr = 0;
>   	struct pci_dev *dev;
> +	int fn = 0, nr = 0;
>   
>   	if (only_one_child(bus) && (devfn > 0))
>   		return 0; /* Already scanned the entire slot */
>   
> -	dev = pci_scan_single_device(bus, devfn);
> -	if (!dev)
> -		return 0;
> -	if (!pci_dev_is_added(dev))
> -		nr++;
> -
> -	for (fn = next_fn(bus, dev, 0); fn > 0; fn = next_fn(bus, dev, fn)) {
> +	do {
>   		dev = pci_scan_single_device(bus, devfn + fn);
>   		if (dev) {
>   			if (!pci_dev_is_added(dev))
>   				nr++;
> -			dev->multifunction = 1;
> +			if (fn > 0)
> +				dev->multifunction = 1;
> +		} else if (fn == 0) {
> +			/* function 0 is required */
> +			break;
>   		}
> -	}
> +		fn = next_fn(bus, dev, fn);
> +	} while (fn >= 0);
>   
>   	/* Only one slot has PCIe device */
>   	if (bus->self && nr)
> 

Otherwise LGTM


-- 
Pierre Morel
IBM Lab Boeblingen