linux-kernel - Re: [PATCH v5 09/23] drm/i2c: tda998x: don't read write-only registers

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Date:	Sun, 2 Feb 2014 16:23:09 +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>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Rob Clark <robdclark@...il.com>
Subject: Re: [PATCH v5 09/23] drm/i2c: tda998x: don't read write-only
	registers

On Sat, Jan 25, 2014 at 06:14:42PM +0100, Jean-Francois Moine wrote:
> This patch takes care of the write-only registers of the tda998x.
> 
> The registers SOFTRESET, TBG_CNTRL_0 and TBG_CNTRL_1 have all bits
> cleared after reset, so, they may be fully re-written.
> 
> The register MAT_CONTRL is set to
> 	MAT_CONTRL_MAT_BP | MAT_CONTRL_MAT_SC(1)
> after reset, so, it may be fully set again to this value.

I said in v3 of this patch, which seems to remain unaddressed:

>       /* 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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