Re: [RFC PATCH 0/4] hw/i2c: i2c slave mode support

From: Klaus Jensen <its@irrelevant.dk>
To: Damien Hedde <damien.hedde@greensocs.com>
Cc: "Peter Maydell" <peter.maydell@linaro.org>,
	"Arun Kumar Kashinath Agasar" <arun.kka@samsung.com>,
	"Corey Minyard" <minyard@acm.org>,
	"Corey Minyard" <cminyard@mvista.com>,
	"Andrew Jeffery" <andrew@aj.id.au>,
	"Klaus Jensen" <k.jensen@samsung.com>,
	qemu-devel@nongnu.org, "Joel Stanley" <joel@jms.id.au>,
	qemu-arm@nongnu.org, "Jeremy Kerr" <jk@ozlabs.org>,
	"Padmakar Kalghatgi" <p.kalghatgi@samsung.com>,
	"Matt Johnston" <matt@codeconstruct.com.au>,
	"Cédric Le Goater" <clg@kaod.org>
Subject: Re: [RFC PATCH 0/4] hw/i2c: i2c slave mode support
Date: Fri, 1 Apr 2022 11:05:23 +0200	[thread overview]
Message-ID: <YkbAU3Xga3txXDM0@apples> (raw)
In-Reply-To: <f3c78b1a-bc5c-928c-8986-ad447470b78f@greensocs.com>

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On Apr  1 10:58, Damien Hedde wrote:
> 
> On 4/1/22 08:29, Klaus Jensen wrote:
> > On Mar 31 15:32, Corey Minyard wrote:
> > > On Thu, Mar 31, 2022 at 06:57:33PM +0200, Klaus Jensen wrote:
> > > > From: Klaus Jensen <k.jensen@samsung.com>
> > > > 
> > > > Hi all,
> > > > 
> > > > This RFC series adds I2C "slave mode" support for the Aspeed I2C
> > > > controller as well as the necessary infrastructure in the i2c core to
> > > > support this.
> > > 
> > > I've been wondering when this would happen :).  I had put some thought
> > > into how this would work, but hadn't come up with anything good.
> > > 
> > > The big disadvantage of this is you are adding an interface that is
> > > incompatible with the current masters and slaves.  So you are using the
> > > same I2C bus, but slaves written this way cannot talk to existing
> > > masters, and masters written this way cannot talk to existing slave.
> > > You could adapt the masters to be able to work either way, and I suppose
> > > some slaves that could do it could have both an async send and a normal
> > > send.
> > 
> > Would it make sense to introduce a QOM Interface to differentiate
> > between the slave/master types?
> > 
> 
> Probably.
> 
> I expect a normal slave-only I2C device will be compatible with any master
> (having or having not this feature) in real life ?
> 

Yeah, it's just that currently in the i2c core we cannot "suspend" the
sending of the ACK.

> It would be great if the compatibility between "a I2C slave requiring the
> slave-mode from the bus" and the bus could be checked during the device
> plug.
> 

Makes sense, I'll see what I can come up with for a v2 :)

Thanks!

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