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 17:57:36 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Jean-Francois Moine <moinejf@...e.fr>
Cc:	dri-devel@...ts.freedesktop.org, Rob Clark <robdclark@...il.com>,
	Dave Airlie <airlied@...il.com>, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v5 09/23] drm/i2c: tda998x: don't read write-only
	registers

On Sun, Feb 02, 2014 at 06:45:12PM +0100, Jean-Francois Moine wrote:
> On Sun, 2 Feb 2014 16:23:09 +0000
> Russell King - ARM Linux <linux@....linux.org.uk> wrote:
> 
> > 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.  
> 
> According to the tda9983b documentation, the register TBG_CNTRL_0 is
> set to 0 at reset time. I think that it is the same for all the tda998x
> family. In the other hand, this register is supposed to be write only,
> so reading it may return any value, and the reg_clear() function may
> set any other bits. Then, clearing one bit is less secure than clearing
> the full register.

Okay, in that case I'm happy with this patch.

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