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Date:	Mon, 8 Jun 2015 12:51:32 -0500
From:	Rob Herring <robherring2@...il.com>
To:	Sanchayan Maity <maitysanchayan@...il.com>
Cc:	Jonathan Cameron <jic23@...nel.org>,
	Shawn Guo <shawn.guo@...aro.org>,
	"kernel@...gutronix.de" <kernel@...gutronix.de>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	Fugang Duan <B38611@...escale.com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stefan Agner <stefan@...er.ch>
Subject: Re: [PATCH v1 1/2] iio: adc: Determine sampling frequencies by using
 minimum sample time

On Mon, Jun 8, 2015 at 12:42 AM, Sanchayan Maity
<maitysanchayan@...il.com> wrote:
> The driver currently does not take into account the minimum sample time
> as per the Figure 6-8 Chapter 9.1.1 12-bit ADC electrical characteristics.
> We set a static amount of cycles instead of considering the sample time as
> a given value, which depends on hardware characteristics.
>
> Determine sampling frequencies by first reading the device tree property
> node and then calculating the required Long Sample Time Adder (LSTAdder)
> value based on the ADC clock frequency and sample time value obtained
> from the device tree, this LSTAdder value is then used for calculating
> the sampling frequencies possible.
>
> Signed-off-by: Sanchayan Maity <maitysanchayan@...il.com>
> ---
>  .../devicetree/bindings/iio/adc/vf610-adc.txt      |  6 ++
>  drivers/iio/adc/vf610_adc.c                        | 74 ++++++++++++++++++++--
>  2 files changed, 76 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/devicetree/bindings/iio/adc/vf610-adc.txt b/Documentation/devicetree/bindings/iio/adc/vf610-adc.txt
> index 3eb40e2..4419126 100644
> --- a/Documentation/devicetree/bindings/iio/adc/vf610-adc.txt
> +++ b/Documentation/devicetree/bindings/iio/adc/vf610-adc.txt
> @@ -17,6 +17,11 @@ Recommended properties:
>    - Frequency in normal mode (ADLPC=0, ADHSC=0)
>    - Frequency in high-speed mode (ADLPC=0, ADHSC=1)
>    - Frequency in low-power mode (ADLPC=1, ADHSC=0)
> +  - fsl,min-sample-time: Minimum sampling time in nanoseconds. This value has

Add a "-ns' suffix.

Seems like this could be a common ADC property.

> +    to be chosen according to the conversion mode and the connected analog
> +        source resistance (R_as) and capacitance (C_as). Refer the datasheet's
> +        operating requirements. A safe default across a wide range of R_as and
> +        C_as as well as conversion modes is 1000ns.
>
>  Example:
>  adc0: adc@...3b000 {
> @@ -27,5 +32,6 @@ adc0: adc@...3b000 {
>         clock-names = "adc";
>         fsl,adck-max-frequency = <30000000>, <40000000>,
>                                 <20000000>;
> +       fsl,min-sample-time = <1000>;
>         vref-supply = <&reg_vcc_3v3_mcu>;
>  };
> diff --git a/drivers/iio/adc/vf610_adc.c b/drivers/iio/adc/vf610_adc.c
> index 480f335..bcae8fd 100644
> --- a/drivers/iio/adc/vf610_adc.c
> +++ b/drivers/iio/adc/vf610_adc.c
> @@ -68,6 +68,9 @@
>  #define VF610_ADC_CLK_DIV8             0x60
>  #define VF610_ADC_CLK_MASK             0x60
>  #define VF610_ADC_ADLSMP_LONG          0x10
> +#define VF610_ADC_ADSTS_SHORT   0x100
> +#define VF610_ADC_ADSTS_NORMAL  0x200
> +#define VF610_ADC_ADSTS_LONG    0x300
>  #define VF610_ADC_ADSTS_MASK           0x300
>  #define VF610_ADC_ADLPC_EN             0x80
>  #define VF610_ADC_ADHSC_EN             0x400
> @@ -124,6 +127,17 @@ enum conversion_mode_sel {
>         VF610_ADC_CONV_LOW_POWER,
>  };
>
> +enum lst_adder_sel {
> +       VF610_ADCK_CYCLES_3,
> +       VF610_ADCK_CYCLES_5,
> +       VF610_ADCK_CYCLES_7,
> +       VF610_ADCK_CYCLES_9,
> +       VF610_ADCK_CYCLES_13,
> +       VF610_ADCK_CYCLES_17,
> +       VF610_ADCK_CYCLES_21,
> +       VF610_ADCK_CYCLES_25,
> +};
> +
>  struct vf610_adc_feature {
>         enum clk_sel    clk_sel;
>         enum vol_ref    vol_ref;
> @@ -132,6 +146,8 @@ struct vf610_adc_feature {
>         int     clk_div;
>         int     sample_rate;
>         int     res_mode;
> +       u32 lst_adder_index;
> +       u32 default_sample_time;
>
>         bool    calibration;
>         bool    ovwren;
> @@ -155,11 +171,13 @@ struct vf610_adc {
>  };
>
>  static const u32 vf610_hw_avgs[] = { 1, 4, 8, 16, 32 };
> +static const u32 vf610_lst_adder[] = { 3, 5, 7, 9, 13, 17, 21, 25 };
>
>  static inline void vf610_adc_calculate_rates(struct vf610_adc *info)
>  {
>         struct vf610_adc_feature *adc_feature = &info->adc_feature;
>         unsigned long adck_rate, ipg_rate = clk_get_rate(info->clk);
> +       u32 adck_period, lst_addr_min;
>         int divisor, i;
>
>         adck_rate = info->max_adck_rate[adc_feature->conv_mode];
> @@ -174,6 +192,19 @@ static inline void vf610_adc_calculate_rates(struct vf610_adc *info)
>         }
>
>         /*
> +        * Determine the long sample time adder value to be used based
> +        * on the default minimum sample time provided.
> +        */
> +       adck_period = NSEC_PER_SEC / adck_rate;
> +       lst_addr_min = adc_feature->default_sample_time / adck_period;
> +       for (i = 0; i < ARRAY_SIZE(vf610_lst_adder); i++) {
> +               if (vf610_lst_adder[i] > lst_addr_min) {
> +                       adc_feature->lst_adder_index = i;
> +                       break;
> +               }
> +       }
> +
> +       /*
>          * Calculate ADC sample frequencies
>          * Sample time unit is ADCK cycles. ADCK clk source is ipg clock,
>          * which is the same as bus clock.
> @@ -182,12 +213,13 @@ static inline void vf610_adc_calculate_rates(struct vf610_adc *info)
>          * SFCAdder: fixed to 6 ADCK cycles
>          * AverageNum: 1, 4, 8, 16, 32 samples for hardware average.
>          * BCT (Base Conversion Time): fixed to 25 ADCK cycles for 12 bit mode
> -        * LSTAdder(Long Sample Time): fixed to 3 ADCK cycles
> +        * LSTAdder(Long Sample Time): 3, 5, 7, 9, 13, 17, 21, 25 ADCK cycles
>          */
>         adck_rate = ipg_rate / info->adc_feature.clk_div;
>         for (i = 0; i < ARRAY_SIZE(vf610_hw_avgs); i++)
>                 info->sample_freq_avail[i] =
> -                       adck_rate / (6 + vf610_hw_avgs[i] * (25 + 3));
> +                       adck_rate / (6 + vf610_hw_avgs[i] *
> +                        (25 + vf610_lst_adder[adc_feature->lst_adder_index]));
>  }
>
>  static inline void vf610_adc_cfg_init(struct vf610_adc *info)
> @@ -347,8 +379,40 @@ static void vf610_adc_sample_set(struct vf610_adc *info)
>                 break;
>         }
>
> -       /* Use the short sample mode */
> -       cfg_data &= ~(VF610_ADC_ADLSMP_LONG | VF610_ADC_ADSTS_MASK);
> +       /*
> +        * Set ADLSMP and ADSTS based on the Long Sample Time Adder value
> +        * determined.
> +        */
> +       switch (adc_feature->lst_adder_index) {
> +       case VF610_ADCK_CYCLES_3:
> +               break;
> +       case VF610_ADCK_CYCLES_5:
> +               cfg_data |= VF610_ADC_ADSTS_SHORT;
> +               break;
> +       case VF610_ADCK_CYCLES_7:
> +               cfg_data |= VF610_ADC_ADSTS_NORMAL;
> +               break;
> +       case VF610_ADCK_CYCLES_9:
> +               cfg_data |= VF610_ADC_ADSTS_LONG;
> +               break;
> +       case VF610_ADCK_CYCLES_13:
> +               cfg_data |= VF610_ADC_ADLSMP_LONG;
> +               break;
> +       case VF610_ADCK_CYCLES_17:
> +               cfg_data |= VF610_ADC_ADLSMP_LONG;
> +               cfg_data |= VF610_ADC_ADSTS_SHORT;
> +               break;
> +       case VF610_ADCK_CYCLES_21:
> +               cfg_data |= VF610_ADC_ADLSMP_LONG;
> +               cfg_data |= VF610_ADC_ADSTS_NORMAL;
> +               break;
> +       case VF610_ADCK_CYCLES_25:
> +               cfg_data |= VF610_ADC_ADLSMP_LONG;
> +               cfg_data |= VF610_ADC_ADSTS_NORMAL;
> +               break;
> +       default:
> +               dev_err(info->dev, "error in sample time select\n");
> +       }
>
>         /* update hardware average selection */
>         cfg_data &= ~VF610_ADC_AVGS_MASK;
> @@ -712,6 +776,8 @@ static int vf610_adc_probe(struct platform_device *pdev)
>
>         of_property_read_u32_array(pdev->dev.of_node, "fsl,adck-max-frequency",
>                         info->max_adck_rate, 3);
> +       of_property_read_u32(pdev->dev.of_node, "fsl,min-sample-time",
> +                       &info->adc_feature.default_sample_time);
>
>         platform_set_drvdata(pdev, indio_dev);
>
> --
> 2.4.2
>
--
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