linux-kernel - Re: [PATCH v9 4/4] PCI: Allow extend_bridge_window() to shrink resource if necessary

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Date:   Wed, 30 Oct 2019 14:07:05 +0200
From:   "mika.westerberg@...ux.intel.com" <mika.westerberg@...ux.intel.com>
To:     Nicholas Johnson <nicholas.johnson-opensource@...look.com.au>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "bhelgaas@...gle.com" <bhelgaas@...gle.com>,
        "corbet@....net" <corbet@....net>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        "logang@...tatee.com" <logang@...tatee.com>
Subject: Re: [PATCH v9 4/4] PCI: Allow extend_bridge_window() to shrink
 resource if necessary

On Tue, Oct 29, 2019 at 03:29:21PM +0000, Nicholas Johnson wrote:
> Remove checks for resource size in extend_bridge_window(). This is
> necessary to allow the pci_bus_distribute_available_resources() to
> function when the kernel parameter pci=hpmemsize=nn[KMG] is used to
> allocate resources. Because the kernel parameter sets the size of all
> hotplug bridges to be the same, there are problems when nested hotplug
> bridges are encountered. Fitting a downstream hotplug bridge with size X
> and normal bridges with non-zero size Y into parent hotplug bridge with
> size X is impossible, and hence the downstream hotplug bridge needs to
> shrink to fit into its parent.
> 
> Add check for if bridge is extended or shrunken and adjust pci_dbg to
> reflect this.
> 
> Reset the resource if its new size is zero (if we have run out of a
> bridge window resource) to prevent the PCI resource assignment code from
> attempting to assign a zero-sized resource.
> 
> Rename extend_bridge_window() to adjust_bridge_window() to reflect the
> fact that the window can now shrink.
> 
> Signed-off-by: Nicholas Johnson <nicholas.johnson-opensource@...look.com.au>
> ---
>  drivers/pci/setup-bus.c | 23 +++++++++++++++--------
>  1 file changed, 15 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> index fe8b2c715..f8cd54584 100644
> --- a/drivers/pci/setup-bus.c
> +++ b/drivers/pci/setup-bus.c
> @@ -1814,7 +1814,7 @@ void __init pci_assign_unassigned_resources(void)
>  	}
>  }
>  
> -static void extend_bridge_window(struct pci_dev *bridge, struct resource *res,
> +static void adjust_bridge_window(struct pci_dev *bridge, struct resource *res,
>  				 struct list_head *add_list,
>  				 resource_size_t new_size)
>  {
> @@ -1823,13 +1823,20 @@ static void extend_bridge_window(struct pci_dev *bridge, struct resource *res,
>  	if (res->parent)
>  		return;
>  
> -	if (resource_size(res) >= new_size)
> -		return;
> +	if (new_size > resource_size(res)) {
> +		add_size = new_size - resource_size(res);
> +		pci_dbg(bridge, "bridge window %pR extended by %pa\n", res,
> +			&add_size);
> +	} else if (new_size < resource_size(res)) {
> +		add_size = resource_size(res) - new_size;
> +		pci_dbg(bridge, "bridge window %pR shrunken by %pa\n", res,
> +			&add_size);
> +	}

Do we need to care about new_size == resource_size(res)?

Also there are several calls of resource_size(res) above so probably
worth storing it into a helper variable.

> -	add_size = new_size - resource_size(res);
> -	pci_dbg(bridge, "bridge window %pR extended by %pa\n", res, &add_size);
>  	res->end = res->start + new_size - 1;
>  	remove_from_list(add_list, res);
> +	if (!new_size)
> +		reset_resource(res);
>  }