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 12:37:27

> On Wed, 12 Jun 2019 11:16:34 +0000
> Vitor Soares <Vitor.Soares@xxxxxxxxxxxx> wrote:
> 
> > 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.
> 
> Is it buggy, and if it is, what are the symptoms? And I'm not talking
> about slow transfers here. Also, note that forcing the I2C/I3C rate
> through the DT already means you want to tweak the bus speed (either
> for debugging purposes or because slowing things down is needed to fix
> a HW bug).

Does it need to be buggy to inform the user of such inconsistence?

> 
> > 
> >   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.
> 
> Yet, you might want to apply a lower I2C freq, and this sounds like a
> valid case that doesn't deserve a dev_warn().

I already said that I'm ok to change the dev_warn(), you just have to 
tell me what is the best message level to use.

> 
> > 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.
> 
> *up to*, that's the important thing to keep in mind. There's no problem
> driving the SCL signal at a lower freq.

We already know that a FM or FM+ supports lower frequencies due backyard 
capabilities.

> 
> > 
> > Apart of that, if the I2C device support you can use a custom higher or 
> > lower rate, yet not defined according MIPI I3C spec.
> 
> I'm not going to have this discussion again, sorry. I think I gave
> enough arguments to explain why having an I2C SLC rate that's slower
> than what I2C devices support is fine.

It is clear to me that the I2C devices on I3C bus shall support FM or 
FM+.
If not they don't follow the MIPI I3C spec and for that reason I prefer 
to inform the user.

> 
> > 
> > > 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().
> 
> Same here. I'm fine having a dev_warn() when the rate is higher than
> what's supported by devices present on the bus (because that case is
> buggy), but not when it's lower and still in the valid range.

Please take some time to analyze it again, my concern is only to inform 
the user about inconsistencies (forced or not) with the I3C specification 
and I already agreed to change the message levels.

Best regards,
Vitor Soares