Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752096AbaBBR5m (ORCPT ); Sun, 2 Feb 2014 12:57:42 -0500 Received: from gw-1.arm.linux.org.uk ([78.32.30.217]:55295 "EHLO pandora.arm.linux.org.uk" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751882AbaBBR5l (ORCPT ); Sun, 2 Feb 2014 12:57:41 -0500 Date: Sun, 2 Feb 2014 17:57:36 +0000 From: Russell King - ARM Linux To: Jean-Francois Moine Cc: dri-devel@lists.freedesktop.org, Rob Clark , Dave Airlie , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v5 09/23] drm/i2c: tda998x: don't read write-only registers Message-ID: <20140202175736.GH26684@n2100.arm.linux.org.uk> References: <5269e596e16dfe40253dce38ceb0dc4a617384c1.1390986083.git.moinejf@free.fr> <20140202162309.GF26684@n2100.arm.linux.org.uk> <20140202184512.7fa2e3cf@armhf> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140202184512.7fa2e3cf@armhf> User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 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. -- FTTC broadband for 0.8mile line: 5.8Mbps down 500kbps up. Estimation in database were 13.1 to 19Mbit for a good line, about 7.5+ for a bad. Estimate before purchase was "up to 13.2Mbit". -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/