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.
Thanks,
Mauro
