On 2012-06-26 10:20, Laurent Pinchart wrote:
Hi Jean-Philippe,
On Tuesday 26 June 2012 16:49:35 jean-philippe francois wrote:
2012/6/26 Laurent Pinchart<laurent.pinchart@xxxxxxxxxxxxxxxx>:
Hi everybody,
While trying to use an OV7725 camera sensor with an OMAP3 system I ran
into a couple of issues related to the sensor communication protocol.
Instead of using the obiquitous I2C protocol, Omnivision invited the SCCB
communication bus, very similar to I2C but different enough not to be
compatible.
I have been using omnivision sensor without being aware of this issue,
and without any i2c reliability issue either.
Here is typical code I use :
/* This function is used to read value from register for i2c client */
static int ov2710_read(struct i2c_client *client, unsigned short reg,
unsigned char *val)
{
int err = 0;
unsigned char outbuf[] = {(reg>>8)&0xff, reg&0xff};
According to the SCCB specification (or at least the version I've found)
register addresses are 8-bit. The OV2710 might just be I2C-compatible.
unsigned char inbuf[1];
struct i2c_msg msg[] = {
{ .addr = client->addr, .flags = 0, .buf = outbuf, .len = 2 },
{ .addr = client->addr, .flags = I2C_M_RD, .buf = inbuf, .len = 1 },
};
err = i2c_transfer(client->adapter, msg, 2);
/* error handling and pass by ref handling */
....
}
The ov772x driver uses i2c_smbus_write_byte_data() and
i2c_smbus_read_byte_data(). The later calls
i2c_smbus_xfer(client->adapter, client->addr, client->flags,
I2C_SMBUS_READ, I2C_SMBUS_BYTE_DATA,&data);
which calls i2c_smbus_xfer_emulated() for hosts that don't support SMBus
transfers natively, and that's pretty much equivalent to your above code
(except for the 8/16 bit register address).
It might be a good idea to implement i2c_smbus_*-like functions for 16-bit
register addresses in the I2C core, they could be reused across drivers.
Is the point of this patch to avoid writing such functions again and again
in every ov driver ?
No, but that's a good idea as well :-)
What is solved in practice by this patch that is not solved by a regular i2c
transfer ?
The above code will not ignore acks/nacks and will not generate a stop
condition between the two messages. The SCCB specification states that
acks/nacks must be ignored and that a stop condition must be generated. The
OV7725 handles acks/nacks correctly, but chokes if no stop condition is
generated. The point of this patch set is to fix both problems (although the
acks/nacks issue might be theoretical only).
How does it "choke"? I've had success talking to the OV8820 using the
normal I2C driver as well.
--
------------------------------------------------------------
Gary Thomas | Consulting for the
MLB Associates | Embedded world
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