Em Thu, 23 Mar 2017 19:03:20 +0100 Frank Schäfer <[email protected]> escreveu:
> Am 23.03.2017 um 13:56 schrieb Mauro Carvalho Chehab: > > Em Thu, 23 Mar 2017 13:01:32 +0100 > > Frank Schäfer <[email protected]> escreveu: > > > >> Am 22.03.2017 um 15:46 schrieb Mauro Carvalho Chehab: > >>> Em Sun, 19 Feb 2017 19:29:18 +0100 > >>> Frank Schäfer <[email protected]> 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 <[email protected]> > >>>> --- > >>>> 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. Thanks, Mauro
