Re: Q: i2c block write emulation / handling of i2c message size constraints of a bus ?

[Laurent Pinchart <laurent.pinchart-ryLnwIuWjnjg/C1BVhZhaw@public.gmane.org>]

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