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

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Date:   Mon, 22 Apr 2019 17:54:29 +0000
From:   Vitor Soares <Vitor.Soares@...opsys.com>
To:     Boris Brezillon <boris.brezillon@...labora.com>,
        Vitor Soares <Vitor.Soares@...opsys.com>
CC:     "linux-i3c@...ts.infradead.org" <linux-i3c@...ts.infradead.org>,
        "joao.pinto@...opsys.com" <Joao.Pinto@...opsys.com>,
        Boris Brezillon <bbrezillon@...nel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/3] i3c: fix i2c and i3c scl rate by bus mode

From: Boris Brezillon <boris.brezillon@...labora.com>
Date: Mon, Apr 22, 2019 at 17:07:15

> On Mon, 22 Apr 2019 15:54:33 +0000
> Vitor Soares <Vitor.Soares@...opsys.com> wrote:
> 
> 
> > > > >     
> > > > > >  {
> > > > > >  	i3cbus->mode = mode;
> > > > > >  
> > > > > > -	if (!i3cbus->scl_rate.i3c)
> > > > > > -		i3cbus->scl_rate.i3c = I3C_BUS_TYP_I3C_SCL_RATE;
> > > > > > -
> > > > > > -	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;
> > > > > > +	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 = i2c_scl_rate;
> > > > > > +		break;
> > > > > > +	case I3C_BUS_MODE_MIXED_SLOW:
> > > > > > +		if (!i3cbus->scl_rate.i2c)
> > > > > > +			i3cbus->scl_rate.i2c = i2c_scl_rate;
> > > > > > +		i3cbus->scl_rate.i3c = i3cbus->scl_rate.i2c;    
> > > > > 
> > > > > Maybe we should do
> > > > > 
> > > > > 		if (!i3cbus->scl_rate.i3c ||
> > > > > 		    i3cbus->scl_rate.i3c > i3cbus->scl_rate.i2c)
> > > > > 			i3cbus->scl_rate.i3c = i3cbus->scl_rate.i2c;
> > > > > 					   
> > > > > Just in case the I3C rate forced by the user is lower than the max I2C
> > > > > rate.    
> > > > 
> > > > That was something that I considered but TBH it isn't a real use case.  
> > > 
> > > Add a WARN_ON() to at least catch such inconsistencies. And maybe we
> > > should add a dev_warn() when the user-defined rates do not match
> > > the mode/LVR constraints. It's easy to do a mistake when writing a dts.  
> > 
> > I think the WARN_ON() is too evasive on the screen and won't provide the 
> > information we want.
> > The dev_warn() should work perfectly here.
> > 
> > 		if (i3cbus->scl_rate.i3c < i3cbus->scl_rate.i2c)
> > 			dev_warn(&i3cbus->cur_master->dev->dev,
> > 				     "%s: i3c-scl-hz lower then i2c-scl-hz\n",  __func__);
> 
> Using dev_warn() sounds good, though I don't think you need the
> __func__ here. Also, please print the i2c/i3c rates in the message, and
> align the second line on the open parens.
> 
> > 		if (i3cbus->scl_rate.i2c != I3C_BUS_I2C_FM_SCL_RATE ||
> > 		    i3cbus->scl_rate.i2c != I3C_BUS_I2C_FM_PLUS_SCL_RATE)
> > 			dev_warn(&i3cbus->cur_master->dev->dev,
> > 				     "%s: i2c-scl-hz not defined according MIPI I3C spec\n", 
> > 				     __func__);
> 
> Is that really a problem? Having an i2c rate that is less than FM speed
> sounds like a valid case to me.

I'm addressing the spec constrains.

In the practice it can be SM or even HS, it depends on the interface.

> 
> > 
> > Maybe it make more sense to do this check on of_populate_i3c_bus(), what 
> > do you think?
> > 
> 
> No, we really want to have this check here, because we might support
> other HW description formats at some point (board-files, ACPI, ...).

Yes, you are right. I forgot that point.