netdev - Re: [PATCH 3/4] of: Convert to using %pOFn instead of device

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Date:   Fri, 7 Sep 2018 14:29:28 +0200
From:   Thierry Reding <thierry.reding@...il.com>
To:     Rob Herring <robh@...nel.org>
Cc:     Frank Rowand <frowand.list@...il.com>, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH 3/4] of: Convert to using %pOFn instead of
 device_node.name

On Tue, Aug 28, 2018 at 10:52:53AM -0500, Rob Herring wrote:
> In preparation to remove the node name pointer from struct device_node,
> convert printf users to use the %pOFn format specifier.
> 
> Cc: Frank Rowand <frowand.list@...il.com>
> Cc: Andrew Lunn <andrew@...n.ch>
> Cc: Florian Fainelli <f.fainelli@...il.com>
> Cc: devicetree@...r.kernel.org
> Cc: netdev@...r.kernel.org
> Signed-off-by: Rob Herring <robh@...nel.org>
> ---
>  drivers/of/device.c   |  4 ++--
>  drivers/of/of_mdio.c  | 12 ++++++------
>  drivers/of/of_numa.c  |  4 ++--
>  drivers/of/overlay.c  |  4 ++--
>  drivers/of/platform.c |  8 ++++----
>  drivers/of/unittest.c | 12 ++++++------
>  6 files changed, 22 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/of/device.c b/drivers/of/device.c
> index 5957cd4fa262..daa075d87317 100644
> --- a/drivers/of/device.c
> +++ b/drivers/of/device.c
> @@ -219,7 +219,7 @@ static ssize_t of_device_get_modalias(struct device *dev, char *str, ssize_t len
>  		return -ENODEV;
>  
>  	/* Name & Type */
> -	csize = snprintf(str, len, "of:N%sT%s", dev->of_node->name,
> +	csize = snprintf(str, len, "of:N%pOFnT%s", dev->of_node,
>  			 dev->of_node->type);
>  	tsize = csize;
>  	len -= csize;

This seems to cause the modalias to be improperly constructed. As a
consequence, automatic module loading at boot time is now broken. I
think the reason why this fails is because vsnprintf() will skip all
alpha-numeric characters after a call to pointer(). Presumably this
is meant to be a generic way of skipping whatever specifiers we throw
at it.

Unfortunately for the case of OF modaliases, this means that the 'T'
character gets eaten, so we end up with something like this:

	# udevadm info /sys/bus/platform/devices/54200000.dc
	[...]
	E: MODALIAS=of:Ndc<NULL>Cnvidia,tegra124-dc
	[...]

instead of this:

	# udevadm info /sys/bus/platform/devices/54200000.dc
	[...]
	E: MODALIAS=of:NdcT<NULL>Cnvidia,tegra124-dc
	[...]

Everything is back to normal if I revert this patch. However, since
that's obviously not what we want, I think perhaps what we need is a
way for pointer() (and its implementations) to report back how many
characters in the format string it consumed so that we can support
these kinds of back-to-back strings.

If nobody else has the time I can look into coding up a fix, but in the
meantime it might be best to back this one out until we can handle the
OF modalias format string.

Thierry

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