linux-kernel - Re: [PATCH 25/44] drivers/pci: use min() instead of min

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Message-ID: <20251124210410.GA2708124@bhelgaas>
Date: Mon, 24 Nov 2025 15:04:10 -0600
From: Bjorn Helgaas <helgaas@...nel.org>
To: david.laight.linux@...il.com
Cc: linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
	Bjorn Helgaas <bhelgaas@...gle.com>
Subject: Re: [PATCH 25/44] drivers/pci: use min() instead of min_t()

On Wed, Nov 19, 2025 at 10:41:21PM +0000, david.laight.linux@...il.com wrote:
> From: David Laight <david.laight.linux@...il.com>
> 
> min_t(unsigned int, a, b) casts an 'unsigned long' to 'unsigned int'.
> Use min(a, b) instead as it promotes any 'unsigned int' to 'unsigned long'
> and so cannot discard significant bits.
> 
> In this case although pci_hotplug_bus_size is 'long' it is constrained
> to be <= 255.
> 
> Detected by an extra check added to min_t().

I don't mind applying this, but it sure would be nice to have a hint
at the max() and max_t() definitions about when and why to prefer one
over the other.

Applied to pci/misc for v6.19 with the following commit log:

  PCI: Use max() instead of max_t() to ease static analysis

  In this code:

    used_buses = max_t(unsigned int, available_buses,
                       pci_hotplug_bus_size - 1);

  max_t() casts the 'unsigned long' pci_hotplug_bus_size (either 32 or 64
  bits) to 'unsigned int' (32 bits) result type, so there's a potential of
  discarding significant bits.

  Instead, use max(a, b), which casts 'unsigned int' to 'unsigned long' and
  cannot discard significant bits.

  In this case, pci_hotplug_bus_size is constrained to <= 0xff by pci_setup()
  so this doesn't fix a bug, but it makes static analysis easier.

> Signed-off-by: David Laight <david.laight.linux@...il.com>
> ---
>  drivers/pci/probe.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 0ce98e18b5a8..0f0d1b44d8c2 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -3163,8 +3163,7 @@ static unsigned int pci_scan_child_bus_extend(struct pci_bus *bus,
>  	 * bus number if there is room.
>  	 */
>  	if (bus->self && bus->self->is_hotplug_bridge) {
> -		used_buses = max_t(unsigned int, available_buses,
> -				   pci_hotplug_bus_size - 1);
> +		used_buses = max(available_buses, pci_hotplug_bus_size - 1);
>  		if (max - start < used_buses) {
>  			max = start + used_buses;
>  
> -- 
> 2.39.5
>