linux-kernel - Re: [PATCH v2 1/3] PCI: Relaxed tail alignment should never increase min

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Message-ID: <20250821151035.GA674429@bhelgaas>
Date: Thu, 21 Aug 2025 10:10:35 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc: linux-pci@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Tudor Ambarus <tudor.ambarus@...aro.org>, Rio <rio@....me>,
	D Scott Phillips <scott@...amperecomputing.com>,
	linux-kernel@...r.kernel.org,
	Christian König <christian.koenig@....com>,
	stable@...r.kernel.org
Subject: Re: [PATCH v2 1/3] PCI: Relaxed tail alignment should never increase
 min_align

On Mon, Jun 30, 2025 at 05:26:39PM +0300, Ilpo Järvinen wrote:
> When using relaxed tail alignment for the bridge window,
> pbus_size_mem() also tries to minimize min_align, which can under
> certain scenarios end up increasing min_align from that found by
> calculate_mem_align().
> 
> Ensure min_align is not increased by the relaxed tail alignment.
> 
> Eventually, it would be better to add calculate_relaxed_head_align()
> similar to calculate_mem_align() which finds out what alignment can be
> used for the head without introducing any gaps into the bridge window
> to give flexibility on head address too. But that looks relatively
> complex algorithm so it requires much more testing than fixing the
> immediate problem causing a regression.
> 
> Fixes: 67f9085596ee ("PCI: Allow relaxed bridge window tail sizing for optional resources")
> Reported-by: Rio <rio@....me>

Was there a regression report URL we could include here?

> Tested-by: Rio <rio@....me>
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> Cc: <stable@...r.kernel.org>
> ---
>  drivers/pci/setup-bus.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> index 07c3d021a47e..f90d49cd07da 100644
> --- a/drivers/pci/setup-bus.c
> +++ b/drivers/pci/setup-bus.c
> @@ -1169,6 +1169,7 @@ static int pbus_size_mem(struct pci_bus *bus, unsigned long mask,
>  	resource_size_t children_add_size = 0;
>  	resource_size_t children_add_align = 0;
>  	resource_size_t add_align = 0;
> +	resource_size_t relaxed_align;
>  
>  	if (!b_res)
>  		return -ENOSPC;
> @@ -1246,8 +1247,9 @@ static int pbus_size_mem(struct pci_bus *bus, unsigned long mask,
>  	if (bus->self && size0 &&
>  	    !pbus_upstream_space_available(bus, mask | IORESOURCE_PREFETCH, type,
>  					   size0, min_align)) {
> -		min_align = 1ULL << (max_order + __ffs(SZ_1M));
> -		min_align = max(min_align, win_align);
> +		relaxed_align = 1ULL << (max_order + __ffs(SZ_1M));
> +		relaxed_align = max(relaxed_align, win_align);
> +		min_align = min(min_align, relaxed_align);
>  		size0 = calculate_memsize(size, min_size, 0, 0, resource_size(b_res), win_align);
>  		pci_info(bus->self, "bridge window %pR to %pR requires relaxed alignment rules\n",
>  			 b_res, &bus->busn_res);
> @@ -1261,8 +1263,9 @@ static int pbus_size_mem(struct pci_bus *bus, unsigned long mask,
>  		if (bus->self && size1 &&
>  		    !pbus_upstream_space_available(bus, mask | IORESOURCE_PREFETCH, type,
>  						   size1, add_align)) {
> -			min_align = 1ULL << (max_order + __ffs(SZ_1M));
> -			min_align = max(min_align, win_align);
> +			relaxed_align = 1ULL << (max_order + __ffs(SZ_1M));
> +			relaxed_align = max(min_align, win_align);
> +			min_align = min(min_align, relaxed_align);
>  			size1 = calculate_memsize(size, min_size, add_size, children_add_size,
>  						  resource_size(b_res), win_align);
>  			pci_info(bus->self,
> -- 
> 2.39.5
>