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

[Mauro Carvalho Chehab <mchehab@s-opensource.com>]

Em Mon, 10 Apr 2017 20:06:03 +0200
Frank Schäfer <fschaefer.oss@googlemail.com> escreveu:

> 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@googlemail.com> 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@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.  
> >> 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 !?

Just found time today to read the code implementation for
i2c_smbus_read_word_data().

i2c_smbus_read_word_data() indeed does:

		case I2C_SMBUS_WORD_DATA:
		case I2C_SMBUS_PROC_CALL:
			data->word = msgbuf1[0] | (msgbuf1[1] << 8);

So, it indeed stores data correctly, no matter the CPU endiannes.

So, the patch itself looks correct, but I found the description
misleading.

I would describe it as:

Use i2c_smbus_read_word_data() instead of i2c_master_send() and
i2c_master_recv() for reading the ID of Micorn sensors.

i2c_smbus_read_word_data() assumes that byes are in little-endian,
so, it uses:
	data->word = msgbuf1[0] | (msgbuf1[1] << 8);

However, Micron datasheet describes the ID as if they were read
in big-endian. So, we need to change the byte order in order to
match the ID number as described on their datasheets.

With changing the description to something like that:

Acked-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Thanks,
Mauro