Hi, Am 27.10.2012 18:50, schrieb Jean Delvare: > Hi Frank, > > 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. > >> A block read/write emulation function would have to do an i2c write (to >> increase the register) followed by either an i2c read or write for each >> data byte. >> The question is now: does it make sense to emulate block operations for >> the i2c/smbus layer in this case ? >> I'm not sure if an i2c read through the smbus/i2c subsystem should >> really trigger multiple low level i2c reads/writes... > No, that would be bad. There's no single way to emulate a transaction > using smaller transactions. Different slaves will react differently > (some have a notion of "current address", with or without > auto-increment, some don't.) Plus, in a multi-master environment, > splitting a transaction into small chunks is not safe. So doing that > transparently at the bus driver level would be plain wrong. Ok, thanks, that's what I expected. > What could be done (this has been discussed earlier and is already > implemented in individual drivers) is provide helper functions reading > blocks in a best-effort mode. The caller's expectations would be > different from the regular block read functions. These helper functions > would be in neutral ground to avoid any improper assumption. 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. > >> 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 ? Regards, Frank -- 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
