linux-kernel - Re: [PATCH v3 10/24] drm/i2c: tda998x: don't read write-only registers

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Message-ID: <20140122213535.GI15937@n2100.arm.linux.org.uk>
Date:	Wed, 22 Jan 2014 21:35:35 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Jean-Francois Moine <moinejf@...e.fr>
Cc:	dri-devel@...ts.freedesktop.org, Dave Airlie <airlied@...il.com>,
	Rob Clark <robdclark@...il.com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 10/24] drm/i2c: tda998x: don't read write-only
	registers

On Sun, Jan 19, 2014 at 07:58:42PM +0100, Jean-Francois Moine wrote:
> This patch takes care of the write-only registers of the tda998x.
> 
> The value 'MAT_CONTRL_MAT_SC(1)' in the register MAT_CONTRL has been
> set as it is at reset time.
> 
> Signed-off-by: Jean-Francois Moine <moinejf@...e.fr>
> ---
> v3
>     - remarks from Russell King
> 	- don't move the sync polarity setting after the setting of the
> 	  register TBG_CNTRL_0 which must be the last setting of the
> 	  init sequence

This is better, except I find that there's an additional change in this
version which wasn't in the original patch 9:

>  	/* must be last register set: */
> -	reg_clear(priv, REG_TBG_CNTRL_0, TBG_CNTRL_0_SYNC_ONCE);
> +	reg_write(priv, REG_TBG_CNTRL_0, 0);

Register changes which have a potential effect shouldn't be part of a
patch which is really only trying to avoid reading from write only
registers.

This could be a potential functional change - and it's probably one
which Rob Clark should at least be made aware of.  As I commented last
time, when you're changing register values in an otherwise innocuous
patch, you should comment about them in the patch description.

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