Re: [PATCH 2/13] drivers/media/tuners/mxl5007t.c: use macros for i2c_msg initialization

Jean Delvare
2012-10-09T12:30:48Z

Hi Julia,

On Sun, 7 Oct 2012 17:38:33 +0200, Julia Lawall wrote:
> From: Julia Lawall <Julia.Lawall@lip6.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