Re: [PATCH 2/3] i2c: slave-eeprom: add eeprom simulator driver

From: Uwe Kleine-König
Date: Sun Nov 23 2014 - 15:20:27 EST

Hello Wolfram,

On Sat, Nov 22, 2014 at 07:26:30PM +0100, Wolfram Sang wrote:
> > this mail is thematically more a reply to patch 1 and maybe just serves
> > my understanding of the slave support.
> Sure. This shows how badly needed the documentation is :)
> ...
> > > + break;
> > > +
> > > + case I2C_SLAVE_STOP:
> > > + eeprom->first_write = true;
> > > + break;
> > > +
> > > + default:
> > > + break;
> > > + }
> > > +
> > > + return 0;
> > > +}
> > This is the most interesting function here because it uses the new
> > interface, the functions below are only to update and show the simulated
> > eeprom contents and driver boilerplate, right?
> Yes.
> > When the eeprom driver is probed and the adapter driver notices a read
> > request for the respective i2c address, this callback is called with
> > event=I2C_SLAVE_REQ_READ_START. Returning 0 here and provide the first
> > byte to send make the adapter ack the read request and send the data
> > provided. If something != 0 is returned a NAK is sent?
> We only send NAK on write requests (I use read/write from the master
I don't understand this. Who is "we" in this case?

> perspective). Then, we have to say if the received byte was successfully
> processed. When reading, the master has to ack the successful reception
> of the byte.
Right, I got this wrong in my question. On a read request (as seen from
the master) the master has to ack.

> > How is the next byte requested from the slave driver? I assume with two
> > additional calls to the callback, first with
> > event=I2C_SLAVE_REQ_READ_END, then event=I2C_SLAVE_REQ_READ_START once
> > more. Would it make sense to reduce this to a single call? Does the
> > driver at READ_END time already know if its write got acked? If so, how?
> No single call. I had this first, but my experiments showed that it is
> important for the EEPROM driver to only increase the internal pointer
> when the byte was ACKed. Otherwise, I was off-by-one.
Sure, the driver has to know how his read response was received by the
master. But assuming I understand your abstraction right there is some
redundancy. There are only three cases on a read request (well plus error

- master sends ACK, reads next byte
in this case the slave must provide another word
In your abstraction this implies
callback(I2C_SLAVE_REQ_READ_END); <-- this is redundant

- master sends ACK, then P or Sr
maybe callback(I2C_SLAVE_STOP)

- master sends NACK
in this case the message ends and the master has to send Sr or P.
In your case this results in:
nothing for the NACK?
maybe callback(I2C_SLAVE_STOP)

The situations where the slave has to react are:

- slave was addressed with R
input: address
output: NAK or (ACK + data byte)
- slave was addressed with #W:
input: address
output: NAK or ACK
- data sent by slave-transmitter was acked
output: next data byte (maybe unused because master sends Sr or P)
- data sent by slave-transmitter was nacked
output: void (unless we want to support IGNORE_NAK :-)
- slave received a data byte (write)
input: data
output: NAK or ACK

This looks like a better model in my eyes. In this model the slave
driver doesn't even need to be informed about P. Not entirely sure about
"data sent by slave-transmitter was nacked".

> Ideally, I2C_SLAVE_REQ_READ_END should be used when the master ACKed the
> byte, right. However, the rcar hardware doesn't have an interrupt for
> this, so I imply that the start of a new read request ends the old one.
> I probably should add a comment for that.
> > This means that for each byte the callback is called. Would it make
> > sense to make the API more flexible and allow the slave driver to return
> > a buffer? This would remove some callback overhead and might allow to
> > let the adapter driver make use of its DMA mechanism.
> For DMA, I haven't seen DMA slave support yet. Makes sense to me, we
haha, there is only a single slave driver yet and you're the author.

> wouldn't know the transfer size, since the master can send a stop
> anytime. This makes possible gains of using a buffer also speculative.
> Also, I2C is still a low-bandwith bus, so usually we have a high number
> of small transfers.
> For now, I'd skip this idea. As I said in another thread, we need more
> use cases. If the need arises, we can come up with something. I don't
> think the current design prevents such an addition?
It would change the API, but starting to get experience with byte
banging is probably OK.

Best regards

Pengutronix e.K. | Uwe Kleine-König |
Industrial Linux Solutions | |
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