linux-kernel - RE: [PATCH 1/2] i2c: designware: determine HS tHIGH and tLOW based on HW paramters

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Message-ID:
 <IA1PR14MB6224A402109C057748A215798A6A2@IA1PR14MB6224.namprd14.prod.outlook.com>
Date: Thu, 26 Sep 2024 09:11:29 +0000
From: Michael Wu <Michael.Wu@...ron.us>
To: Jarkko Nikula <jarkko.nikula@...ux.intel.com>, Andy Shevchenko
	<andriy.shevchenko@...ux.intel.com>, Mika Westerberg
	<mika.westerberg@...ux.intel.com>, Jan Dabros <jsd@...ihalf.com>,
	"linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>
CC: Andi Shyti <andi.shyti@...nel.org>, morgan chang <morgan.chang@...ron.us>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/2] i2c: designware: determine HS tHIGH and tLOW based on
 HW paramters

> On 9/25/24 11:04 AM, Michael Wu wrote:
> > In commit 35eba185fd1a ("i2c: designware: Calculate SCL timing
> > parameter for High Speed Mode") hs_hcnt and hs_hcnt are computed based
> > on
> > fixed tHIGH = 160 and tLOW = 320. However, this fixed values only applies
> > to the set of conditions of IC_CAP_LOADING = 400pF and
> > IC_FREQ_OPTIMIZATION = 1. Outside of this conditions set, if this fixed
> > values are still used, the calculated HCNT and LCNT will make the SCL
> > frequency unabled to reach 3.4 MHz.
> >
> > If hs_hcnt and hs_lcnt are calculated based on fixed tHIGH = 160 and
> > tLOW = 320, SCL frequency may not reach 3.4 MHz when IC_CAP_LOADING is
> > not
> > 400pF or IC_FREQ_OPTIMIZATION is not 1.
> >
> > Section 3.15.4.5 in DesignWare DW_apb_i2c Databook v2.03 says when
> > IC_CLK_FREQ_OPTIMIZATION = 0,
> >
> >      MIN_SCL_HIGHtime = 60 ns for 3.4 Mbps, bus loading = 100pF
> > 		     = 120 ns for 3,4 Mbps, bus loading = 400pF
> >      MIN_SCL_LOWtime = 160 ns for 3.4 Mbps, bus loading = 100pF
> > 		    = 320 ns for 3.4 Mbps, bus loading = 400pF
> >
> > and section 3.15.4.6 says when IC_CLK_FREQ_OPTIMIZATION = 1,
> >
> >      MIN_SCL_HIGHtime = 60 ns for 3.4 Mbps, bus loading = 100pF
> > 		     = 160 ns for 3.4 Mbps, bus loading = 400pF
> >      MIN_SCL_LOWtime = 120 ns for 3.4 Mbps, bus loading = 100pF
> > 		    = 320 ns for 3.4 Mbps, bus loading = 400pF
> >
> > In order to calculate more accurate hs_hcnt and hs_lcnt, two hardware
> > parameters IC_CAP_LOADING and IC_CLK_FREQ_OPTIMIZATION must be
> > considered together.
> >
> > Signed-off-by: Michael Wu <michael.wu@...ron.us>
> > ---
> >   drivers/i2c/busses/i2c-designware-common.c  | 16 ++++++++++++++
> >   drivers/i2c/busses/i2c-designware-core.h    |  8 +++++++
> >   drivers/i2c/busses/i2c-designware-master.c  | 24
> +++++++++++++++++++--
> >   drivers/i2c/busses/i2c-designware-platdrv.c |  2 ++
> >   4 files changed, 48 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/i2c/busses/i2c-designware-common.c
> b/drivers/i2c/busses/i2c-designware-common.c
> > index e8a688d04aee..f0a7d0ce6fd6 100644
> > --- a/drivers/i2c/busses/i2c-designware-common.c
> > +++ b/drivers/i2c/busses/i2c-designware-common.c
> > @@ -332,6 +332,22 @@ void i2c_dw_adjust_bus_speed(struct dw_i2c_dev
> *dev)
> >   }
> >   EXPORT_SYMBOL_GPL(i2c_dw_adjust_bus_speed);
> >
> > +void i2c_dw_parse_of(struct dw_i2c_dev *dev)
> > +{
> > +	int ret;
> > +
> > +	ret = device_property_read_u32(dev->dev, "bus-loading",
> > +				       &dev->bus_loading);
> 
> Like Andy said better name would be bus_capacitance_pf.
I'll rename it to bus_cap_pf due to the limit of not exceed 80 characters a line.
> 
> Also i2c_dw_parse_of() sounds too generic and may lead to think all and
> only device tree related parameters are parsed here.
> 
> > +	if (ret || dev->bus_loading < 400)
> > +		dev->bus_loading = 100;
> > +	else
> > +		dev->bus_loading = 400;
> > +
> 
> I think these are more understandable and robust if no parameter
> adjustments are not done here but used straight in the if statements in
> the i2c_dw_set_timings_master(). Less if statements that way and all
> checked in one place.

bus_loading is a hardware parameter, indicating the capacitance value.
Literally, it has no direct relationship with time, so I may create a new
function to parse it and other hardware parameters. Because the same
reason, it is not suitable to be adjusted in i2c_dw_set_timings_master().
Like i2c_parse_timing() declared in drivers/i2c/i2c-core-base.c,
I recommend changing its value after device_property_read_u32() if
necessary.

Thanks & Regards,
Michael