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

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Message-ID: <Zvvyii7aViGCklcT@smile.fi.intel.com>
Date: Tue, 1 Oct 2024 16:00:58 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Michael Wu <michael.wu@...ron.us>
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,
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
	Morgan Chang <morgan.chang@...ron.us>, mvp.kutali@...il.com
Subject: Re: [PATCH v3 2/2] i2c: dwsignware: determine HS tHIGH and tLOW
 based on HW parameters

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


-- 
With Best Regards,
Andy Shevchenko