Hi Jean, On Sunday 28 October 2012 13:03:01 Jean Delvare wrote: > On Sat, 27 Oct 2012 18:41:19 +0300, Frank Schäfer wrote: > > Am 27.10.2012 18:50, schrieb Jean Delvare: > > > On Sat, 27 Oct 2012 17:17:34 +0300, Frank Schäfer wrote: > > >> the i2c interface of my device is capable of writing 2 bytes (reg + > > >> data) and reading a single data byte only. > > > > > > Are you talking about an I2C master (controller) here, or a slave > > > device? > > > > It's an em2765 USB-video-bridge with an OV2640 sensor slave. > > The i2c transfer functions I'm currently working on are not yet in the > > em28xx driver. > > > > I don't know yet if it is a general bus limitation or a client > > limitiation. > > The procedures are based on reverse-engineering work and the OV2640 is > > the only device we have seen so far. > > Maybe it has something to do with the fact that the OV2640 uses SCCB. > > Yes SCCB is apparently very limited in terms of supported transaction > types. Plus it diverges from the equivalent SMBus transactions in the > details. Note that we do have support for SCCB since kernel v3.6 > (commit d47726c52122d64253ae56e0fafdb7d0b954e97c by Laurent Pinchart.) > > > (...) > > Yes, emulating block reads/writes internally (the em28xx driver in this > > case) is not the problem. > > My question was if it makes sense to export the emulation through the > > i2c subsystem. > > If you do, you'll have to make it flexible enough that it can be used > by other drivers, such as at24 and eeprom. Is that really mandatory ? The EM2765 will only be connected to video sensors in practice. > > >> What's the right error code to return from the drivers master_xfer > > >> function if the message length is not supported ? -EMSGSIZE ? > > > > > > -EOPNOTSUPP, per Documentation/i2c/fault-codes. > > > > > > Note that ideally, the slave driver should check the bus functionality > > > and not try transactions which aren't supported. So returning > > > -EOPNOTSUPP normally never happens. > > > > What are the correct functionality flags to use in this case ? > > I2C_FUNC_I2C | I2C_FUNC_SMBUS_READ_BYTE | I2C_FUNC_SMBUS_WRITE_WORD_DATA ? > > If your controller is limited then I2C_FUNC_I2C is most certainly > wrong. From what you described, I'd say: > > I2C_FUNC_SMBUS_READ_BYTE | I2C_FUNC_SMBUS_WRITE_BYTE_DATA > > This doesn't match what Laurent said about SCCB 4 months ago though: > > "The read transaction transmits 2 2-byte messages (addr/w, reg, > followed by addr/r, data)." The EM2765 might split reg + data write operations into two transactions internally. > You can take a look at Documentation/i2c/smbus-protocol to match > transactions to function names (and from there to I2C_FUNC flags.) -- Regards, Laurent Pinchart -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
