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

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Message-ID: <90f64539-3092-4d3b-bdf2-c6af51e32fdc@linux.intel.com>
Date: Wed, 25 Sep 2024 13:58:27 +0300
From: Jarkko Nikula <jarkko.nikula@...ux.intel.com>
To: Michael Wu <michael.wu@...ron.us>,
 Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
 Mika Westerberg <mika.westerberg@...ux.intel.com>,
 Jan Dabros <jsd@...ihalf.com>, linux-i2c@...r.kernel.org
Cc: Andi Shyti <andi.shyti@...nel.org>, Morgan Chang
 <morgan.chang@...ron.us>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] i2c: designware: determine HS tHIGH and tLOW based on
 HW paramters

Hi

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.

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.