linux-kernel - Re: [PATCH v2 25/31] i2c: fsi: Calculate clock divider from local bus frequency

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Date: Tue, 16 Apr 2024 14:20:24 -0700
From: Guenter Roeck <linux@...ck-us.net>
To: Eddie James <eajames@...ux.ibm.com>
Cc: Andi Shyti <andi.shyti@...nel.org>, devicetree@...r.kernel.org,
	conor+dt@...nel.org, alistair@...ple.id.au,
	linux-kernel@...r.kernel.org, lakshmiy@...ibmcom,
	linux-i2c@...r.kernel.org, krzysztof.kozlowski+dt@...aro.org,
	linux-fsi@...ts.ozlabs.org, robh@...nel.org
Subject: Re: [PATCH v2 25/31] i2c: fsi: Calculate clock divider from local
 bus frequency

On Tue, Apr 16, 2024 at 01:09:04PM -0500, Eddie James wrote:
> 
> On 4/15/24 17:11, Andi Shyti wrote:
> > Hi Eddie,
> > 
> > > @@ -689,6 +692,20 @@ static int fsi_i2c_probe(struct device *dev)
> > >   	mutex_init(&i2c->lock);
> > >   	i2c->fsi = to_fsi_dev(dev);
> > >   	INIT_LIST_HEAD(&i2c->ports);
> > > +	i2c->clock_div = I2C_DEFAULT_CLK_DIV;
> > > +
> > > +	lbus = fsi_device_local_bus_frequency(i2c->fsi);
> > > +	if (lbus) {
> > > +		u32 clock = I2C_DEFAULT_CLK_RATE;
> > I don't see the need for initialization.
> 
> 
> Does device_property_read_u32 set clock if the property isn't found? If not,
> it needs to be initialized here. Or I can set it in an else statement from
> device_property_read_u32.
> 
> 
> > 
> > > +
> > > +		if (!device_property_read_u32(dev, "clock-frequency", &clock)) {
> > > +			if (!clock)
> > > +				clock = I2C_DEFAULT_CLK_RATE;
> > > +		}

		if (device_property_read_u32(dev, "clock-frequency", &clock) || !clock)
			clock = I2C_DEFAULT_CLK_RATE;

> > > +
> > > +		// i2c clock rate = local bus clock / (4 * (i2c clock div + 1))
> > You forgot to remove this.
> 
> 
> I actually meant to leave that comment to explain how the clock rate is
> calculated, as the reverse calculation in the code is a little more
> confusing.
> 

Partially that is because you implemented DIV_ROUND_UP() manually.

> 
> > 
> > Andi
> > 
> > > +		i2c->clock_div = (((lbus + (clock - 1)) / clock) / 4) - 1;

				= DIV_ROUND_UP(lbus, clock) / 4 - 1

Guenter