Am 26.03.2017 um 16:24 schrieb Frank Schäfer:
>
>
> Am 24.03.2017 um 20:16 schrieb Mauro Carvalho Chehab:
>> Em Thu, 23 Mar 2017 19:03:20 +0100
>> Frank Schäfer <fschaefer.oss@xxxxxxxxxxxxxx> escreveu:
>>
>>> Am 23.03.2017 um 13:56 schrieb Mauro Carvalho Chehab:
>>>> Em Thu, 23 Mar 2017 13:01:32 +0100
>>>> Frank Schäfer <fschaefer.oss@xxxxxxxxxxxxxx> 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@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, ®, 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.
>> You still didn't get it.
>>
>> Let's assume that the ID is 0x148c (MT9M112).
>>
>> This value, represented in low endian, is stored in memory as:
>>
>> unsigned char __id[2] = { 0x8c, 0x14 };
>>
>> If we do:
>> u16 ret = *(u16 *)__id;
>>
>> What's stored at "ret" will depend if the sistem is LE or BE:
>>
>> on LE, ret == 0x148c
>> on BE, ret == 0x8c14
>>
>> If you do:
>> u16 id = swapb16(val)
>>
>> you'll get:
>>
>> on LE, id == 0x8c14
>> on BE, id == 0x148c
>>
>> So, the value will be *wrong* at LE.
>>
>> However, if you do:
>> id = le16_to_cpu((__le16)ret);
>>
>> On LE, this will evaluate to id = ret, and on BE, to id = swab16(ret).
>> So, on both, you'll have:
>> id = 0x148c.
>
> Can you please show me the code line(s) that make the value of the
> word returned by i2c_smbus_read_word_data() cpu endianess dependent ? :)
>
Ping !?
Cheers,
Frank
