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

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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, &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.

le16_to_cpu() should do the right thing, e. g. swap for BE
CPUs or not swap otherwise.

Thanks,
Mauro



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