linux-kernel - RE: [PATCH v3 2/2] i2c: dwsignware: determine HS tHIGH and tLOW based on HW parameters

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Message-ID:
 <IA1PR14MB62241CADFE580EC861C591F58A702@IA1PR14MB6224.namprd14.prod.outlook.com>
Date: Wed, 2 Oct 2024 09:31:00 +0000
From: Michael Wu <Michael.Wu@...ron.us>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
CC: Andi Shyti <andi.shyti@...nel.org>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>,
	Jarkko Nikula <jarkko.nikula@...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>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, morgan chang
	<morgan.chang@...ron.us>, "mvp.kutali@...il.com" <mvp.kutali@...il.com>
Subject: RE: [PATCH v3 2/2] i2c: dwsignware: determine HS tHIGH and tLOW based
 on HW parameters

Hi,

> On Tue, Oct 01, 2024 at 04:29:34PM +0800, Michael Wu wrote:
> > In commit 35eba185fd1a ("i2c: designware: Calculate SCL timing parameter
> > for High Speed Mode") hs_hcnt and hs_lcnt are calculated based on fixed
> > tHIGH = 160 and tLOW = 120. However, the set of these fixed values only
> > applies to the combination of hardware parameters IC_CAP_LOADING =
> 400pF
> > and IC_CLK_FREQ_OPTIMIZATION = 1. Outside of this combination, if these
> > fixed tHIGH = 160 and tLOW = 120 are still used, the calculated hs_hcnt
> > and hs_lcnt may not be small enough, making it impossible for the SCL
> > frequency to reach 3.4 MHz.
> >
> > Section 3.15.4.5 in DesignWare DW_apb_i2b Databook v2.03 says that 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 that 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 amd hs_lcnt, two hardware
> > parameters IC_CAP_LOADING and IC_CLK_FREQ_OPTIMIZATION must be
> > considered together.
> 
> ...
> 
> > + * @bus_capacitance_pf: bus capacitance in picofarads
> 
> Since it seems a new version of the series is warranted, and looking into
> the current kernel source (no other users of this unit were observed),
> I think we may do correct capitalisation here for the sake of physics
> and unit system, i.e.
> 
>  * @bus_capacitance_pF: bus capacitance in picofarads

Are you serious? I have never seen capital letters used to declare a
variable name. Although checkpatch.pl does not consider this as an issue,
is this against everyone's custom or unspoken rules?

Sincerely,
Michael