RE: [PATCH v3 1/3] i3c: fix i2c and i3c scl rate by bus mode

[Vitor Soares]

From: Boris Brezillon <boris.brezillon@xxxxxxxxxxxxx>
Date: Wed, Jun 12, 2019 at 07:15:33

> On Tue, 11 Jun 2019 16:06:43 +0200
> Vitor Soares <Vitor.Soares@xxxxxxxxxxxx> wrote:
> 
> > Currently the I3C framework limits SCL frequency to FM speed when
> > dealing with a mixed slow bus, even if all I2C devices are FM+ capable.
> > 
> > The core was also not accounting for I3C speed limitations when
> > operating in mixed slow mode and was erroneously using FM+ speed as the
> > max I2C speed when operating in mixed fast mode.
> > 
> > Fixes: 3a379bbcea0a ("i3c: Add core I3C infrastructure")
> > Signed-off-by: Vitor Soares <vitor.soares@xxxxxxxxxxxx>
> > Cc: Boris Brezillon <bbrezillon@xxxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Cc: <linux-kernel@xxxxxxxxxxxxxxx>
> > ---
> > Changes in v3:
> >   Change dev_warn() to dev_dbg()
> > 
> > Changes in v2:
> >   Enhance commit message
> >   Add dev_warn() in case user-defined i2c rate doesn't match LVR constraint
> >   Add dev_warn() in case user-defined i3c rate lower than i2c rate
> > 
> >  drivers/i3c/master.c | 61 +++++++++++++++++++++++++++++++++++++++++-----------
> >  1 file changed, 48 insertions(+), 13 deletions(-)
> > 
> > diff --git a/drivers/i3c/master.c b/drivers/i3c/master.c
> > index 5f4bd52..f8e580e 100644
> > --- a/drivers/i3c/master.c
> > +++ b/drivers/i3c/master.c
> > @@ -91,6 +91,12 @@ void i3c_bus_normaluse_unlock(struct i3c_bus *bus)
> >  	up_read(&bus->lock);
> >  }
> >  
> > +static struct i3c_master_controller *
> > +i3c_bus_to_i3c_master(struct i3c_bus *i3cbus)
> > +{
> > +	return container_of(i3cbus, struct i3c_master_controller, bus);
> > +}
> > +
> >  static struct i3c_master_controller *dev_to_i3cmaster(struct device *dev)
> >  {
> >  	return container_of(dev, struct i3c_master_controller, dev);
> > @@ -565,20 +571,48 @@ static const struct device_type i3c_masterdev_type = {
> >  	.groups	= i3c_masterdev_groups,
> >  };
> >  
> > -int i3c_bus_set_mode(struct i3c_bus *i3cbus, enum i3c_bus_mode mode)
> > +int i3c_bus_set_mode(struct i3c_bus *i3cbus, enum i3c_bus_mode mode,
> > +		     unsigned long max_i2c_scl_rate)
> >  {
> > -	i3cbus->mode = mode;
> >  
> > -	if (!i3cbus->scl_rate.i3c)
> > -		i3cbus->scl_rate.i3c = I3C_BUS_TYP_I3C_SCL_RATE;
> > +	struct i3c_master_controller *master = i3c_bus_to_i3c_master(i3cbus);
> >  
> > -	if (!i3cbus->scl_rate.i2c) {
> > -		if (i3cbus->mode == I3C_BUS_MODE_MIXED_SLOW)
> > -			i3cbus->scl_rate.i2c = I3C_BUS_I2C_FM_SCL_RATE;
> > -		else
> > -			i3cbus->scl_rate.i2c = I3C_BUS_I2C_FM_PLUS_SCL_RATE;
> > +	i3cbus->mode = mode;
> > +
> > +	switch (i3cbus->mode) {
> > +	case I3C_BUS_MODE_PURE:
> > +		if (!i3cbus->scl_rate.i3c)
> > +			i3cbus->scl_rate.i3c = I3C_BUS_TYP_I3C_SCL_RATE;
> > +		break;
> > +	case I3C_BUS_MODE_MIXED_FAST:
> > +		if (!i3cbus->scl_rate.i3c)
> > +			i3cbus->scl_rate.i3c = I3C_BUS_TYP_I3C_SCL_RATE;
> > +		if (!i3cbus->scl_rate.i2c)
> > +			i3cbus->scl_rate.i2c = max_i2c_scl_rate;
> > +		break;
> > +	case I3C_BUS_MODE_MIXED_SLOW:
> > +		if (!i3cbus->scl_rate.i2c)
> > +			i3cbus->scl_rate.i2c = max_i2c_scl_rate;
> > +		if (!i3cbus->scl_rate.i3c ||
> > +		    i3cbus->scl_rate.i3c > i3cbus->scl_rate.i2c)
> > +			i3cbus->scl_rate.i3c = i3cbus->scl_rate.i2c;
> > +		break;
> > +	default:
> > +		return -EINVAL;
> >  	}
> >  
> > +	if (i3cbus->scl_rate.i3c < i3cbus->scl_rate.i2c)
> > +		dev_dbg(&master->dev,
> > +			"i3c-scl-hz=%ld lower than i2c-scl-hz=%ld\n",
> > +			i3cbus->scl_rate.i3c, i3cbus->scl_rate.i2c);
> > +
> > +	if (i3cbus->scl_rate.i2c != I3C_BUS_I2C_FM_SCL_RATE &&
> > +	    i3cbus->scl_rate.i2c != I3C_BUS_I2C_FM_PLUS_SCL_RATE &&
> > +	    i3cbus->mode != I3C_BUS_MODE_PURE)
> > +		dev_dbg(&master->dev,
> > +			"i2c-scl-hz=%ld not defined according MIPI I3C spec\n",
> > +			i3cbus->scl_rate.i2c);
> > +
> 
> Again, that's not what I suggested, so I'll write it down:
> 
> 	dev_dbg(&master->dev, "i2c-scl = %ld Hz i3c-scl = %ld Hz\n",
> 		i3cbus->scl_rate.i2c, i3cbus->scl_rate.i3c);
> 

I'm not ok with that change. The reasons are:
  i3cbus->scl_rate.i3c < i3cbus->scl_rate.i2c is an abnormal use case. As 
discuss early it can be cause by a wrong DT definition or just for 
testing purposes.

  i3cbus->scl_rate.i2c != I3C_BUS_I2C_FM_SCL_RATE && i3cbus->scl_rate.i2c 
!= I3C_BUS_I2C_FM_PLUS_SCL_RATE, the MIPI I3C Spec v1.0 clearly says that 
all I2C devices on the bus shall have a LVR register and thus support FM 
or FM+ modes.
By  definition a FM bus works at 400kHz and a FM+ bus 1MHz.
And for slaves, a FM device works up to 400kHz and a FM+ device works up 
to 1MHz respectively.

Apart of that, if the I2C device support you can use a custom higher or 
lower rate, yet not defined according MIPI I3C spec.

> dev_dbg() is not printed by default, so it's just fine to have a trace
> that prints the I3C and I2C rate unconditionally.

I'm ok to change the way that user is notified and I think that is here 
the problem.
Maybe the best is to change the first dev_dbg() to dev_warn() and the 
second dev_info().

> 
> >  	/*
> >  	 * I3C/I2C frequency may have been overridden, check that user-provided
> >  	 * values are not exceeding max possible frequency.
> > @@ -1966,9 +2000,6 @@ of_i3c_master_add_i2c_boardinfo(struct i3c_master_controller *master,
> >  	/* LVR is encoded in reg[2]. */
> >  	boardinfo->lvr = reg[2];
> >  
> > -	if (boardinfo->lvr & I3C_LVR_I2C_FM_MODE)
> > -		master->bus.scl_rate.i2c = I3C_BUS_I2C_FM_SCL_RATE;
> > -
> >  	list_add_tail(&boardinfo->node, &master->boardinfo.i2c);
> >  	of_node_get(node);
> >  
> > @@ -2417,6 +2448,7 @@ int i3c_master_register(struct i3c_master_controller *master,
> >  			const struct i3c_master_controller_ops *ops,
> >  			bool secondary)
> >  {
> > +	unsigned long i2c_scl_rate = I3C_BUS_I2C_FM_PLUS_SCL_RATE;
> >  	struct i3c_bus *i3cbus = i3c_master_get_bus(master);
> >  	enum i3c_bus_mode mode = I3C_BUS_MODE_PURE;
> >  	struct i2c_dev_boardinfo *i2cbi;
> > @@ -2466,9 +2498,12 @@ int i3c_master_register(struct i3c_master_controller *master,
> >  			ret = -EINVAL;
> >  			goto err_put_dev;
> >  		}
> > +
> > +		if (i2cbi->lvr & I3C_LVR_I2C_FM_MODE)
> > +			i2c_scl_rate = I3C_BUS_I2C_FM_SCL_RATE;
> >  	}
> >  
> > -	ret = i3c_bus_set_mode(i3cbus, mode);
> > +	ret = i3c_bus_set_mode(i3cbus, mode, i2c_scl_rate);
> >  	if (ret)
> >  		goto err_put_dev;
> >