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

[Frank Schäfer]

Am 23.03.2017 um 13:56 schrieb Mauro Carvalho Chehab:
Em Thu, 23 Mar 2017 13:01:32 +0100
Frank Schäfer <fschaefer....@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....@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....@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