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

[Jean Delvare <khali-PUYAD+kWke1g9hUCZPvPmw@public.gmane.org>]

Hi Frank,

On Sun, 28 Oct 2012 15:32:16 +0200, Frank Schäfer wrote:
> Am 28.10.2012 14:03, schrieb Jean Delvare:
> > On Sat, 27 Oct 2012 18:41:19 +0300, Frank Schäfer wrote:
> >> 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)."
> >
> > You can take a look at Documentation/i2c/smbus-protocol to match
> > transactions to function names (and from there to I2C_FUNC flags.)
> 
> Ok, I've been digging deeper into this but still don't understand the
> meaning of the functionality flags I2C_FUNC_*** with regards to the
> capabilites of an master_xfer implemetation in struct i2c_algortihm...
> Are they supposed to describe the smbus operations/methods that can be
> successfully emulated by the smbus layer using i2c_xfer or do they
> describe the actual capabilities of function master_xfer / the i2c adapter ?

Most I2C_FUNC_* flags actual refer to the smbus_xfer method. A driver
implementing master_xfer is typically fully I2C capable and can thus
run almost any I2C or SMBus transaction. Such a driver will set
functionality flags to I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL. The rest of
the flags are for SMBus-only controllers. Each flag basically
corresponds to an i2c_smbus_*() function, which in turn corresponds to
the combination of an SMBus transaction type or size and a direction
(read or write).

The mapping between i2c_smbus_*() functions and functionality flags is
rather obvious, but for clarity I'll update
Documentation/i2c/smbus-protocol to mention it. Patch follows.

Also see Documentation/i2c/functionality for a detailed explanation of
how functionality flags work both on the I2C/SMBus adapter driver and
the I2C device driver sides.

-- 
Jean Delvare