linux-kernel - Re: [PATCH 1/2] PWM: let of_xlate handlers check args count

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Date:	Thu, 23 Jan 2014 11:56:32 +0100
From:	Lothar Waßmann <LW@...O-electronics.de>
To:	Sascha Hauer <s.hauer@...gutronix.de>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Shawn Guo <shawn.guo@...aro.org>,
	Thierry Reding <thierry.reding@...il.com>,
	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>,
	Rob Landley <rob@...dley.net>, linux-pwm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] PWM: let of_xlate handlers check args count

Hi,

Sascha Hauer wrote:
> of_pwm_n_cells for the of_xlate handler is stored in struct pwm_chip,
> but it is only ever used by the of_xlate handler itsel. Remove
> of_pwm_n_cells from struct pwm_chip and let the handler do the argument
> count checking to simplify the code.
> 
This still does not make the PWM_POLARITY flag in the pwms node
optional as was the goal because of_parse_phandle_with_args() requires
at least #pwm-cells arguments in the node.

So, with a DT configuration like:
pwm0: pwm@0 {
	#pwm-cells = <3>;
};
backlight {
	pwms = <&pwm0 0 100000>;
};
the driver will bail out at of_parse_phandle_with_args() in
of_pwm_get() with the error message:
"/backlight: arguments longer than property" and never reach your
clever xlate function.

Thus you will still need to replace of_parse_phandle_with_args()
with different code that copies most but not all of the functionality.


Lothar Waßmann
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