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).
--
Regards,
Laurent Pinchart
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