Re: [PATCH 2/2] PCI: Skip resource distribution when no hotplug bridges

[Logan Gunthorpe <logang@deltatee.com>]

On 2019-06-22 3:03 p.m., Bjorn Helgaas wrote:
> From: Bjorn Helgaas <bhelgaas@google.com>
> 
> If "hotplug_bridges == 0", "!dev->is_hotplug_bridge" is always true, so the
> loop that divides the remaining resources among hotplug-capable bridges
> does nothing.
> 
> Check for "hotplug_bridges == 0" earlier, so we don't even have to compute
> the amount of remaining resources.  No functional change intended.

Makes sense.

Reviewed-by: Logan Gunthorpe <logang@deltatee.com>

> ---
> 
> I'm pretty sure this patch preserves the previous behavior of
> pci_bus_distribute_available_resources(), but I'm not sure that
> behavior is what we want.
> 
> For example, in the following topology, when we process bus 10, we
> find two non-hotplug bridges and no hotplug bridges, so IIUC we return
> without distributing any resources to them.  But I would think we
> should try to give 10:1c.0 more space if possible because it has a
> hotplug bridge below it.
> 
>   00:1c.0: hotplug bridge to [bus 10-2f]
>     10:1c.0: non-hotplug bridge to [bus 11-2e]
>       11:00.0: hotplug bridge to [bus 12-2e]
>     10:1c.1: non-hotplug bridge to [bus 2f]
> ---
>  drivers/pci/setup-bus.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> index af28af898e42..04adeebe8866 100644
> --- a/drivers/pci/setup-bus.c
> +++ b/drivers/pci/setup-bus.c
> @@ -1887,6 +1887,9 @@ static void pci_bus_distribute_available_resources(struct pci_bus *bus,
>  		return;
>  	}
>  
> +	if (hotplug_bridges == 0)
> +		return;
> +
>  	/*
>  	 * Calculate the total amount of extra resource space we can
>  	 * pass to bridges below this one.  This is basically the
> @@ -1936,8 +1939,6 @@ static void pci_bus_distribute_available_resources(struct pci_bus *bus,
>  		 * Distribute available extra resources equally between
>  		 * hotplug-capable downstream ports taking alignment into
>  		 * account.
> -		 *
> -		 * Here hotplug_bridges is always != 0.
>  		 */
>  		align = pci_resource_alignment(bridge, io_res);
>  		io = div64_ul(available_io, hotplug_bridges);
>