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

["Frank Schäfer" <fschaefer.oss@googlemail.com>]

Am 23.03.2017 um 13:56 schrieb Mauro Carvalho Chehab:
> Em Thu, 23 Mar 2017 13:01:32 +0100
> Frank Schäfer <fschaefer.oss@googlemail.com> escreveu:
>
>> Am 22.03.2017 um 15:46 schrieb Mauro Carvalho Chehab:
>>> Em Sun, 19 Feb 2017 19:29:18 +0100
>>> Frank Schäfer <fschaefer.oss@googlemail.com> 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@googlemail.com>
>>>> ---
>>>>    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);
>> SMBus read/write word transfers are always LE (see SMBus spec section
>> 6.5.5),
>> which is also what i2c_smbus_xfer_emulated() assumes:
>> http://lxr.free-electrons.com/source/drivers/i2c/i2c-core.c#L3485
> I got that part, but, if the CPU is also LE, doing swab16() is
> wrong. It should swap it *only* if the CPU is BE.
No, it should always be swapped, because the bytes are always transfered 
in the wrong order.
The cpu endianess doesn't matter, (0x12 << 8) | 0x34 is always 0x1234.

Regards,
Frank



> le16_to_cpu() should do the right thing, e. g. swap for BE
> CPUs or not swap otherwise.
>
> Thanks,
> Mauro