Re: [PATCH 2/2] em28xx: simplify ID-reading from Micron sensors

[Mauro Carvalho Chehab <mchehab@xxxxxxxxxxxxxxxx>]

Em Sun, 19 Feb 2017 19:29:18 +0100
Frank Schäfer <fschaefer.oss@xxxxxxxxxxxxxx> escreveu:

> Use i2c_smbus_read_word_data() instead of i2c_master_send() and
> i2c_master_recv() for reading the ID of Micorn sensors.
> Bytes need to be swapped afterwards, because i2c_smbus_read_word_data()
> assumes that the received bytes are little-endian byte order (as specified
> by smbus), while Micron sensors with 16 bit register width use big endian
> byte order.
> 
> Signed-off-by: Frank Schäfer <fschaefer.oss@xxxxxxxxxxxxxx>
> ---
>  drivers/media/usb/em28xx/em28xx-camera.c | 28 ++++------------------------
>  1 file changed, 4 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/media/usb/em28xx/em28xx-camera.c b/drivers/media/usb/em28xx/em28xx-camera.c
> index 7b4129ab1cf9..4839479624e7 100644
> --- a/drivers/media/usb/em28xx/em28xx-camera.c
> +++ b/drivers/media/usb/em28xx/em28xx-camera.c
> @@ -106,8 +106,6 @@ static int em28xx_probe_sensor_micron(struct em28xx *dev)
>  {
>  	int ret, i;
>  	char *name;
> -	u8 reg;
> -	__be16 id_be;
>  	u16 id;
>  
>  	struct i2c_client *client = &dev->i2c_client[dev->def_i2c_bus];
> @@ -115,10 +113,8 @@ static int em28xx_probe_sensor_micron(struct em28xx *dev)
>  	dev->em28xx_sensor = EM28XX_NOSENSOR;
>  	for (i = 0; micron_sensor_addrs[i] != I2C_CLIENT_END; i++) {
>  		client->addr = micron_sensor_addrs[i];
> -		/* NOTE: i2c_smbus_read_word_data() doesn't work with BE data */
>  		/* Read chip ID from register 0x00 */
> -		reg = 0x00;
> -		ret = i2c_master_send(client, &reg, 1);
> +		ret = i2c_smbus_read_word_data(client, 0x00); /* assumes LE */
>  		if (ret < 0) {
>  			if (ret != -ENXIO)
>  				dev_err(&dev->intf->dev,
> @@ -126,24 +122,9 @@ static int em28xx_probe_sensor_micron(struct em28xx *dev)
>  				       client->addr << 1, ret);
>  			continue;
>  		}
> -		ret = i2c_master_recv(client, (u8 *)&id_be, 2);
> -		if (ret < 0) {
> -			dev_err(&dev->intf->dev,
> -				"couldn't read from i2c device 0x%02x: error %i\n",
> -				client->addr << 1, ret);
> -			continue;
> -		}
> -		id = be16_to_cpu(id_be);
> +		id = swab16(ret); /* LE -> BE */

That's wrong! You can't assume that CPU is BE, as some archs use LE.

You should, instead, call le16_to_cpu(), to be sure that it will be
doing the right thing.

Something like:

	id = le16_to_cpu((__le16)ret);

Regards,

Thanks,
Mauro