Re: [PATCH 2/3] Documentation: i2c: describe the new slave mode

[Wolfram Sang]

> > +Finally, Linux can also be an I2C slave in case I2C controllers have slave
> > +support. Besides this HW requirement, one also needs a software backend
> I wouldn't have written "Finally, ...". While it's great that we have
> slave support now, being enthusiastic here looks strange if someone
> reads it while slave support has become "normal".

OK.

> > +providing the actual functionality. An example for this is the slave-eeprom
> > +driver, which acts as a dual memory driver. While another I2C master on the bus
> > +can access it like a regular eeprom, the Linux I2C slave can access the content
> > +via sysfs and retrieve/provide information as needed. The software backend
> > +driver and the I2C bus driver communicate via events. Here is a small graph
> > +visualizing the data flow and the means by which data is transported. The
> > +dotted line marks only one example. The backend could also use e.g. a character
> > +device, or use in-kernel mechanisms only, or something completely different:
> Or something self contained, so the userspace part is actually optional
> (but probably present most of the time).

With "in-kernel mechanisms" I meant self-contained. Maybe "be in-kernel
only"?

> Another slave backend I have in mind is a bus-driver tester. That
> wouldn't necessarily need a userspace part.

Yes, I envisioned that, too.

> 
> > +              e.g. sysfs        I2C slave events        I/O registers
> > +  +-----------+   v    +---------+     v     +--------+  v  +------------+
> > +  | Userspace +........+ Backend +-----------+ Driver +-----+ Controller |
> > +  +-----------+        +---------+           +--------+     +------------+
> > +                                                                | |
> > +  ----------------------------------------------------------------+--  I2C
> > +  --------------------------------------------------------------+----  Bus
> > +
> > +Note: Technically, there is also the I2C core between the backend and the
> > +driver. However, at this time of writing, the layer is transparent.
> s/this/the/ ?

Maybe.

> > +The bus driver sends an event to the backend using the following function:
> > +
> > +	ret = i2c_slave_event(client, event, &val)
> > +
> > +'client' describes the i2c slave device. 'event' is one of the special event
> > +types described hereafter. 'val' holds an u8 value for the data byte to be
> > +read/written and is thus bidirectional. The pointer to val must always be
> > +provided even if val is not used for an event. 'ret' is the return value from
> Does that mean that I have to pass a valid address, or can I use NULL,
> too?

Is NULL a valid pointer to val?

> > +the backend. Mandatory events must be provided by the bus drivers and must be
> > +checked for by backend drivers.
> Currently all commands are mandatory. Does it make sense to mark the
> events accordingly already now? Do you expect the set of events to
> grow?

Yes, support for general call address or device ids might be added
somewhen.

> 
> > +* I2C_SLAVE_READ_PROCESSED (mandatory)
> > +
> > +'val': backend returns next byte to be sent
> > +'ret': always 0
> > +
> > +The bus driver requests the next byte to be sent to another I2C master in
> > +'val'. Important: This does not mean that the previous byte has been acked or
> > +even has been put on the wires! Most hardware requests the next byte when the
> > +previous one is still to be shifted out to ensure seamless transmission. If the
> > +master stops reading after the previous byte, the next byte is never used. It
> > +probably needs to be sent again on the next I2C_SLAVE_READ_REQUEST, depending a
> > +bit on your backend.
> I didn't look into the actual implementation yet, but if I understand
> correctly a slave driver only sees I2C_SLAVE_READ_REQUESTED once and
> then only one I2C_SLAVE_READ_PROCESSED per byte, right? Does the slave

Right.

> driver gets noticed somehow if a previously requested byte doesn't make
> it on the wire? Otherwise you cannot correctly maintain e.g. the current

Some HW can do this, but not all. That would maybe be another candidate
for an optional event. Although, people should try hard to not need it.

> read position of the eeprom driver, do you? (That's a bit like one of
> the problems with buffer support you pointed out further down.)

You need to assume that if the next byte is requested, the previous byte
made it to the bus. So, you should do pre-increment in
I2C_SLAVE_READ_PROCESSED, not post-increment. I didn't want to write
this example so explicit because not all slave-backends will have a
position pointer. And besides, it might make sense to extract the code for
managing a position pointer from the slave-eeprom driver. Then, we'd
have only one trusted implementation of EEPROM-alike behaviour and
people can concentrate on reacting to reads/writes.

> > +* I2C_SLAVE_STOP (mandatory)
> > +
> > +'val': unused
> > +'ret': always 0
> > +
> > +A stop condition was received. This can happen anytime and the backend should
> > +reset its state to be able to receive new requests.
> While being obvious when you understood i2c, "reset its state" could be
> misleading. Only the transfer specific stuff should be reset. I expect
> the eeprom example to not reset the current position on STOP.

OK. Being precise is better, I agree.

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