linux-kernel - Re: [PATCH V4 05/16] soc: tegra: pmc: Avoid extra remapping of PMC registers

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Date:	Thu, 14 Jan 2016 14:45:26 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Jon Hunter <jonathanh@...dia.com>
Cc:	Philipp Zabel <p.zabel@...gutronix.de>,
	Stephen Warren <swarren@...dotorg.org>,
	Alexandre Courbot <gnurou@...il.com>,
	Rafael Wysocki <rjw@...ysocki.net>,
	Kevin Hilman <khilman@...nel.org>,
	Ulf Hansson <ulf.hansson@...aro.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Vince Hsu <vinceh@...dia.com>, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-tegra@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [PATCH V4 05/16] soc: tegra: pmc: Avoid extra remapping of PMC
 registers

On Fri, Dec 04, 2015 at 02:57:06PM +0000, Jon Hunter wrote:
> During early initialisation, the PMC registers are mapped and the PMC SoC
> data is populated in the PMC data structure. This allows other drivers
> access the PMC register space, via the public tegra PMC APIs, prior to
> probing the PMC device.
> 
> When the PMC device is probed, the PMC registers are mapped again and if
> successful the initial mapping is freed. If the probing of the PMC device
> fails after the registers are remapped, then the registers will be
> unmapped and hence the pointer to the PMC registers will be invalid. This
> could lead to a potential crash, because once the PMC SoC data pointer is
> populated, the driver assumes that the PMC register mapping is also valid
> and a user calling any of the public tegra PMC APIs could trigger an
> exception because these APIs don't check that the mapping is still valid.
> 
> Rather than adding a test to see if the PMC register mapping is valid,
> fix this by removing the second mapping of the PMC registers and reserve
> the memory region for the PMC registers during early initialisation where
> the initial mapping is created. During the probing of the PMC simply check
> that the PMC registers have been mapped.
> 
> Signed-off-by: Jon Hunter <jonathanh@...dia.com>
> ---
>  drivers/soc/tegra/pmc.c | 19 +++++++++----------
>  1 file changed, 9 insertions(+), 10 deletions(-)

devm_ioremap_resource() was used on purpose to make sure we get a proper
name assigned to the memory region in /proc/iomem. As it is, there will
be no name associated with it, which I think is unfortunate. As such I'd
prefer keeping that call and instead fix the issue with the invalid
mapping by making sure that pmc->base is assigned only after nothing can
fail in probe anymore.

> diff --git a/drivers/soc/tegra/pmc.c b/drivers/soc/tegra/pmc.c
> index e60fc2d33c94..fdd1a8d0940f 100644
> --- a/drivers/soc/tegra/pmc.c
> +++ b/drivers/soc/tegra/pmc.c
> @@ -807,22 +807,17 @@ out:
>  
>  static int tegra_pmc_probe(struct platform_device *pdev)
>  {
> -	void __iomem *base = pmc->base;

The alternative that I proposed above would entail not setting this...

> -	struct resource *res;
>  	int err;
>  
> +	if (!pmc->base) {
> +		dev_err(&pdev->dev, "base address is not configured\n");
> +		return -ENXIO;
> +	}
> +
>  	err = tegra_pmc_parse_dt(pmc, pdev->dev.of_node);
>  	if (err < 0)
>  		return err;
>  
> -	/* take over the memory region from the early initialization */
> -	res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
> -	pmc->base = devm_ioremap_resource(&pdev->dev, res);

... and storing the result of the mapping in "base" instead...

> -	if (IS_ERR(pmc->base))
> -		return PTR_ERR(pmc->base);
> -
> -	iounmap(base);

... and move the unmap to the very end of the probe function, which
would look something like:

	/* take over the memory region from the early initialization */
	iounmap(pmc->base);
	pmc->base = base;

That way the mapping in "base" will automatically be undone upon error
and the pmc->base will only be overridden when it's certain that the
probe will succeed.

What do you think?

Thierry

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