linux-kernel - Re: [PATCH v5 2/3] I2C: mediatek: Add driver for MediaTek I2C controller

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Date:	Tue, 31 Mar 2015 15:08:35 +0800
From:	Eddie Huang <eddie.huang@...iatek.com>
To:	Sascha Hauer <s.hauer@...gutronix.de>
CC:	Mark Rutland <mark.rutland@....com>,
	Wolfram Sang <wsa@...-dreams.de>,
	<linux-kernel@...r.kernel.org>, <linux-i2c@...r.kernel.org>,
	David Box <david.e.box@...ux.intel.com>,
	Lee Jones <lee.jones@...aro.org>,
	Jean Delvare <jdelvare@...e.de>,
	Xudong Chen <xudong.chen@...iatek.com>,
	Boris BREZILLON <boris.brezillon@...e-electrons.com>,
	Arnd Bergmann <arnd@...db.de>,
	Liguo Zhang <liguo.zhang@...iatek.com>,
	Wei Yan <sledge.yanwei@...wei.com>,
	Bjorn Andersson <bjorn.andersson@...ymobile.com>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>,
	Neelesh Gupta <neelegup@...ux.vnet.ibm.com>,
	<devicetree@...r.kernel.org>, Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Beniamino Galvani <b.galvani@...il.com>,
	Rob Herring <robh+dt@...nel.org>,
	<linux-mediatek@...ts.infradead.org>,
	Matthias Brugger <matthias.bgg@...il.com>,
	<linux-arm-kernel@...ts.infradead.org>,
	<srv_heupstream@...iatek.com>,
	Anders Berg <anders.berg@...gotech.com>,
	Jim Cromie <jim.cromie@...il.com>,
	Simon Glass <sjg@...omium.org>,
	Max Schwarz <max.schwarz@...ine.de>,
	Doug Anderson <dianders@...omium.org>,
	Sascha Hauer <kernel@...gutronix.de>,
	Kumar Gala <galak@...eaurora.org>
Subject: Re: [PATCH v5 2/3] I2C: mediatek: Add driver for MediaTek I2C
 controller

Hi Sascha,

On Mon, 2015-03-30 at 19:23 +0200, Sascha Hauer wrote:
> On Mon, Mar 30, 2015 at 04:14:12PM +0800, Eddie Huang wrote:
> > Hi Sascha,
> > 
> > > 
> > > [...]
> > > 
> > > > +	if (i2c->speed_hz > 400000)
> > > > +		control_reg |= I2C_CONTROL_RS;
> > > > +	if (i2c->op == I2C_MASTER_WRRD)
> > > > +		control_reg |= I2C_CONTROL_DIR_CHANGE | I2C_CONTROL_RS;
> > > > +	mtk_i2c_writew(control_reg, i2c, OFFSET_CONTROL);
> > > > +
> > > > +	/* set start condition */
> > > > +	if (i2c->speed_hz <= 100000)
> > > > +		mtk_i2c_writew(I2C_ST_START_CON, i2c, OFFSET_EXT_CONF);
> > > > +	else
> > > > +		mtk_i2c_writew(I2C_FS_START_CON, i2c, OFFSET_EXT_CONF);
> > > > +
> > > > +	if (~control_reg & I2C_CONTROL_RS)
> > > > +		mtk_i2c_writew(I2C_DELAY_LEN, i2c, OFFSET_DELAY_LEN);
> > > 
> > > speed <= 400000 here to make this more obvious?
> > There are two cases, not only speed<=400000, but I2C_MASTER_WRRD. I tend
> > to keep it.
> 
> Still it looks strange. You only ever write this default value to the
> register. Putting this register write under an if() seems bogus since
> the same value will be in the register the next time this code is
> executed. It looks like you should move this register write to some
> initialization function.
OK, move to mtk_i2c_init_hw function

> 
> > > > +
> > > > +	/* Enable interrupt */
> > > > +	mtk_i2c_writew(I2C_HS_NACKERR | I2C_ACKERR | I2C_TRANSAC_COMP,
> > > > +		i2c, OFFSET_INTR_MASK);
> > > 
> > > Why do you enable/disable interrupts for each transfer? Enabling them
> > > once and just acknowledge them in the interrupt handler should be
> > > enough.
> > This can avoid unwanted I2C interrupt. For example, I2C transfer error,
> > and cause timeout, I2C driver report error to caller. Then I2C error
> > interrupt happen.
> 
> So isn't the same unwanted interrupt then just delayed until you enable
> the interrupts again? Is this something that really happens or just
> something you think that might happen?
> 
Clear interrupt status before enable interrupt, so won't get unwanted
interrupt again. I just think this might happen, and it's not harmful to
enable/disable interrupt in transfer function and can get extra benefit
to avoid unnecessary interrupt.  Tegra I2C driver i2c-tegra.c also do
the same thing.

Eddie



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