Re: [PATCH v1 4/4] PCI: Allow extend_bridge_window() to shrink resource if necessary

[Bjorn Helgaas <helgaas@kernel.org>]

On Mon, Jan 06, 2020 at 03:48:06PM +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.

s/extend_bridge_window()/adjust_bridge_window()/ above
s/to allow the/to allow/

If this patch allows pci_bus_distribute_available_resources() to
function when pci=hpmemsize=nn is used, what happens *before* this
patch?  The text implies that pci_bus_distribute_available_resources()
doesn't function, but what happens?  Do we try to assign a downstream
bridge requiring X+n inside an upstream window of size X and the
assignment fails, leaving the downstream bridge unusable?

> Add check for if bridge is extended or shrunken and reflect that in the
> call to pci_dbg().
> 
> 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.
> 
> Signed-off-by: Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
> ---
>  drivers/pci/setup-bus.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> index 0c51f4937..e7e57bf72 100644
> --- a/drivers/pci/setup-bus.c
> +++ b/drivers/pci/setup-bus.c
> @@ -1836,18 +1836,25 @@ static void adjust_bridge_window(struct pci_dev *bridge, struct resource *res,
>  				 struct list_head *add_list,
>  				 resource_size_t new_size)
>  {
> -	resource_size_t add_size;
> +	resource_size_t add_size, size = resource_size(res);
>  
>  	if (res->parent)
>  		return;
>  
> -	if (resource_size(res) >= new_size)
> -		return;
> +	if (new_size > size) {
> +		add_size = new_size - size;
> +		pci_dbg(bridge, "bridge window %pR extended by %pa\n", res,
> +			&add_size);
> +	} else if (new_size < size) {
> +		add_size = size - new_size;
> +		pci_dbg(bridge, "bridge window %pR shrunken by %pa\n", res,
> +			&add_size);
> +	}

Where's the patch that changes the caller so "new_size" may be smaller
than "size"?  I guess it must be "[3/3] PCI: Consider alignment of
hot-added bridges ..." because that's the only one that makes a
non-trivial change, right?

> -	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);

I consider reset_resource() to be deprecated because it throws away
res->flags, which tells us what kind of resource it is
(mem/io/32-bit/64-bit/prefetchable).  We learn this during
enumeration, and we shouldn't forget the information until we remove
the device.

If the resource assignment code doesn't do the right thing with a
zero-sized resource, I think we should fix that code.  Clearing the
resource struct does nothing with the hardware BAR or window
registers, so the BAR/window remains enabled unless we do something
more.  If we don't need a window and we want to disable it, we can do
that, but it requires writing special values to the hardware
registers.

Bjorn