Re: [PATCH 1/2] drivers: pci: do not disregard parent resources starting at 0x0

From: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
To: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: catalin.marinas@arm.com, graeme.gregory@linaro.org,
	linux-pci@vger.kernel.org, will.deacon@arm.com,
	leif.lindholm@linaro.org, okaya@codeaurora.org, tn@semihalf.com,
	bhelgaas@google.com, yinghai@kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/2] drivers: pci: do not disregard parent resources starting at 0x0
Date: Wed, 12 Apr 2017 14:24:23 +0100	[thread overview]
Message-ID: <20170412132423.GA6584@red-moon> (raw)
In-Reply-To: <20170411163313.18577-2-ard.biesheuvel@linaro.org>

[+Yinghai, Bjorn]

On Tue, Apr 11, 2017 at 05:33:12PM +0100, Ard Biesheuvel wrote:
> Commit f44116ae8818 ("PCI: Remove pci_find_parent_resource() use for
> allocation") updated the logic that iterates over all bus resources
> and compares them to a given resource, in order to decide whether one
> is the parent of the latter.
> 
> This change inadvertently causes pci_find_parent_resource() to disregard
> resources starting at address 0x0, resulting in an error such as the one
> below on ARM systems whose I/O window starts at 0x0.
> 
> pci_bus 0000:00: root bus resource [mem 0x10000000-0x3efeffff window]
> pci_bus 0000:00: root bus resource [io  0x0000-0xffff window]
> pci_bus 0000:00: root bus resource [mem 0x8000000000-0xffffffffff window]
> pci_bus 0000:00: root bus resource [bus 00-0f]
> pci 0000:00:01.0: PCI bridge to [bus 01]
> pci 0000:00:02.0: PCI bridge to [bus 02]
> pci 0000:00:03.0: PCI bridge to [bus 03]
> pci 0000:00:03.0: can't claim BAR 13 [io  0x0000-0x0fff]: no compatible bridge window
> pci 0000:03:01.0: can't claim BAR 0 [io  0x0000-0x001f]: no compatible bridge window
> 
> While this never happens on x86, it is perfectly legal in general for a
> PCI MMIO or IO window to start at address 0x0, and it was supported in
> the code before commit f44116ae8818.
> 
> So let's drop the test for res->start != 0; resource_contains() already
> checks whether [start, end) completely covers the resource, and so it
> should be redundant.
> 
> Fixes: f44116ae8818 ("PCI: Remove pci_find_parent_resource() use for allocation")

I know this code fixes IO claiming on ARM/ARM64 (well, it fixes nothing
because we never claim resources on ARM/ARM64 apart from kvmtool and
generic host bridge), my _big_ worry is that it can cause endless
regressions on other arches, in any case I would be really really
careful about adding a Fixes: tag to it.

Yinghai, Bjorn ?

Thanks,
Lorenzo

> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> ---
>  drivers/pci/pci.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 7904d02ffdb9..53a41b1f7ef7 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -454,7 +454,7 @@ struct resource *pci_find_parent_resource(const struct pci_dev *dev,
>  	pci_bus_for_each_resource(bus, r, i) {
>  		if (!r)
>  			continue;
> -		if (res->start && resource_contains(r, res)) {
> +		if (resource_contains(r, res)) {
>  
>  			/*
>  			 * If the window is prefetchable but the BAR is
> -- 
> 2.9.3
> 

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