linux-kernel - Re: [PATCH 9/13] drivers/media/tuners/fc0011.c: use macros for i2c

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Date:	Tue, 9 Oct 2012 14:06:17 +0200
From:	Jean Delvare <khali@...ux-fr.org>
To:	Julia Lawall <julia.lawall@...6.fr>
Cc:	walter harms <wharms@....de>, Michael Buesch <m@...s.ch>,
	kernel-janitors@...r.kernel.org, rmallon@...il.com,
	shubhrajyoti@...com, Mauro Carvalho Chehab <mchehab@...radead.org>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 9/13] drivers/media/tuners/fc0011.c: use macros for
 i2c_msg initialization

On Sun, 7 Oct 2012 18:50:31 +0200 (CEST), Julia Lawall wrote:
> On Sun, 7 Oct 2012, walter harms wrote:
> > Am 07.10.2012 17:38, schrieb Julia Lawall:
> >> @@ -97,10 +96,8 @@ static int fc0011_readreg(struct fc0011_priv *priv, u8 reg, u8 *val)
> >>  {
> >>  	u8 dummy;
> >>  	struct i2c_msg msg[2] = {
> >> -		{ .addr = priv->addr,
> >> -		  .flags = 0, .buf = &reg, .len = 1 },
> >> -		{ .addr = priv->addr,
> >> -		  .flags = I2C_M_RD, .buf = val ? : &dummy, .len = 1 },
> >> +		I2C_MSG_WRITE(priv->addr, &reg, sizeof(reg)),
> >> +		I2C_MSG_READ(priv->addr, val ? : &dummy, sizeof(dummy)),
> >>  	};
> >>
> >
> > This dummy looks strange, can it be that this is used uninitialised ?
> 
> I'm not sure to understand the question.  The read, when it happens in 
> i2c_transfer will initialize dummy.  On the other hand, I don't know what 
> i2c_transfer does when the buffer is NULL and the size is 1.  It does not 
> look very elegant at least.

i2c_transfer() itself won't care, it just passes the request down to the
underlying i2c bus driver. Most driver implementations will assume
proper buffer addresses as soon as size > 0, so passing NULL instead
would crash them. In short, don't do that.

-- 
Jean Delvare
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