Am 24.03.2017 um 20:16 schrieb Mauro Carvalho Chehab:
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.
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 ? :)
Cheers,
Frank
Thanks,
Mauro
