Re: [PATCH 3/4] dt-bindings: i2c-stm32: add SMBus Alert bindings

From: Alain Volmat <alain.volmat@st.com>
To: "wsa@kernel.org" <wsa@kernel.org>
Cc: "mark.rutland@arm.com" <mark.rutland@arm.com>,
	Rob Herring <robh@kernel.org>,
	Alexandre TORGUE <alexandre.torgue@st.com>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Pierre Yves MORDRET <pierre-yves.mordret@st.com>,
	"linux-i2c@vger.kernel.org" <linux-i2c@vger.kernel.org>,
	"mcoquelin.stm32@gmail.com" <mcoquelin.stm32@gmail.com>,
	Fabrice GASNIER <fabrice.gasnier@st.com>,
	"linux-stm32@st-md-mailman.stormreply.com"
	<linux-stm32@st-md-mailman.stormreply.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 3/4] dt-bindings: i2c-stm32: add SMBus Alert bindings
Date: Tue, 26 May 2020 12:31:41 +0200	[thread overview]
Message-ID: <20200526103141.GB14423@gnbcxd0016.gnb.st.com> (raw)
In-Reply-To: <20200523103601.GA3459@ninjato>

On Sat, May 23, 2020 at 10:36:01AM +0000, wsa@kernel.org wrote:
> 
> > > > +        st,smbus-alert:
> > > > +          description: Enable the SMBus Alert feature
> > > > +          $ref: /schemas/types.yaml#/definitions/flag
> > > > +
> > > 
> > > We already have smbus_alert interrupt. Can't you just check for this in 
> > > the slave nodes and enable if found?
> > 
> > My understanding reading the code (smbalert_probe within i2c-smbus.c, of_i2c_setup_smbus_alert called when
> > registering an adapter within i2c-core-smbus.c) is that smbus_alert refers to an interrupt on the
> > adapter side. That is an interrupt that would be triggered when the adapter is receiving an smbus_alert
> > message.
> > In our case (stm32f7), we do not have specific interrupt for that purpose. The interrupt triggered when
> > an SMBUS Alert is received (by the adapter) is the same interrupt as for other reasons and we check
> > within the irq handler within stm32f7 the reason before calling i2c_handle_smbus_alert if the status
> > register indicated an SMBUS Alert.
> > So my understanding is that we cannot rely on the mechanism of naming an interrupt smbus_alert.
> > Did I misunderstood something ?
> 
> I just wonder what is bad about specifying the same interrupt twice in
> the interrupt properties? You could then check in probe if "smbus_alert"
> is populated and if it matches the main irq.
> 

Here's my understanding of the current implementation.
During the adapter registration, the function of_i2c_setup_smbus_alert is called
and if a interrupt is named "smbus_alert" in the adapter node, a new device
"smbus_alert" will be created. This will leads to smbalert_probe to be called,
and a request_irq will be done with the irq value read from the adapter node.
This means that we will have both our handle (the handler of the main irq
of the stm32 i2c driver) and the smbus_alert handler on the same irq. Leading
to smbus_alert being called everytime there is a irq (most of the time not
smbus_alert related) coming from the stm32_i2c. (since this is our main irq).

So to me this approach can't work. I'd understand if the smbus_alert property
was on the client node in the same way as it is done for host-notify however
that's not the case.
This is why I was proposing to have our own st,smbus-alert property to decide
to enable or not the smbus_alert. In our case, we cannot rely on the mechanism
done by of_i2c_setup_smbus_alert since for us, smbus_alert irq is just one
case of all the other stm32 i2c irq. (this is the same irq, and we check after
by reading the interrupt status register).

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