linux-kernel - Re: [PATCH 2/13] drivers/media/tuners/mxl5007t.c: use macros for i2c

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Date:	Tue, 9 Oct 2012 14:30:35 +0200
From:	Jean Delvare <khali@...ux-fr.org>
To:	Julia Lawall <Julia.Lawall@...6.fr>
Cc:	Michael Krufky <mkrufky@...uxtv.org>,
	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 2/13] drivers/media/tuners/mxl5007t.c: use macros for
 i2c_msg initialization

Hi Julia,

On Sun,  7 Oct 2012 17:38:33 +0200, Julia Lawall wrote:
> From: Julia Lawall <Julia.Lawall@...6.fr>
> 
> Introduce use of I2c_MSG_READ/WRITE/OP, for readability.

Next time you send this patch set, please Cc me on every post so that I
don't have to hunt for them on lkml.org.

> In each case, a length expressed as an explicit constant is also
> re-expressed as the size of the buffer, when this is possible.

This is conceptually wrong, please don't do that. It is perfectly valid
to use a buffer which is larger than the message being written or read.
When exchanging multiple messages, it is actually quite common to
declare only 2 buffers and reuse them:

	char reg;
	char val[2];

	struct i2c_msg msg[2] = {
		{ .addr = addr, .flags = 0, .buf = &reg, .len = 1 },
		{ .addr = addr, .flags = I2C_M_RD, .buf = val, .len = 1 },
	};

	reg = 0x04;
	i2c_transfer(i2c_adap, msg, 2);
	/* Do stuff with val */

	reg = 0x06;
	msg[1].len = 2;
	i2c_transfer(i2c_adap, msg, 2);
	/* Do stuff with val */

Your conversion would read 2 bytes from register 0x04 instead of 1 in
the example above.

I am not opposed to the idea of i2c_msg initialization helper macros,
but please don't mix that with actual code changes which could have bad
side effects.

Thanks,
-- 
Jean Delvare
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