linux-kernel - Re: [PATCH v2] iio: adc: aspeed: Add divider flag to fix incorrect voltage reading.

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Date:   Mon, 21 Feb 2022 05:45:16 +0000
From:   Joel Stanley <joel@....id.au>
To:     Billy Tsai <billy_tsai@...eedtech.com>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Andrew Jeffery <andrew@...id.au>,
        Colin King <colin.king@...onical.com>,
        linux-iio@...r.kernel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-aspeed <linux-aspeed@...ts.ozlabs.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Konstantin Klubnichkin <kitsok@...dex-team.ru>
Subject: Re: [PATCH v2] iio: adc: aspeed: Add divider flag to fix incorrect
 voltage reading.

On Mon, 21 Feb 2022 at 01:26, Billy Tsai <billy_tsai@...eedtech.com> wrote:
>
> The formula for the ADC sampling period in ast2400/ast2500 is:
> ADC clock period = PCLK * 2 * (ADC0C[31:17] + 1) * (ADC0C[9:0])
> When ADC0C[9:0] is set to 0 the sampling voltage will be lower than
> expected, because the hardware may not have enough time to
> charge/discharge to a stable voltage. This patch use the flag
> CLK_DIVIDER_ONE_BASED which will use the raw value read from the
> register, with the value of zero considered invalid to conform to the
> corrected formula.

(to answer my own question)

..and this is okay on the 2600, because we do not set need_prescaler =
true on that platform.

Reviewed-by: Joel Stanley <joel@....id.au>

>
> Fixes: 573803234e72 ("iio: Aspeed ADC")
> Reported-by: Konstantin Klubnichkin <kitsok@...dex-team.ru>
> Signed-off-by: Billy Tsai <billy_tsai@...eedtech.com>
> ---
>  drivers/iio/adc/aspeed_adc.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/iio/adc/aspeed_adc.c b/drivers/iio/adc/aspeed_adc.c
> index a957cad1bfab..ffae64f39221 100644
> --- a/drivers/iio/adc/aspeed_adc.c
> +++ b/drivers/iio/adc/aspeed_adc.c
> @@ -539,7 +539,9 @@ static int aspeed_adc_probe(struct platform_device *pdev)
>         data->clk_scaler = devm_clk_hw_register_divider(
>                 &pdev->dev, clk_name, clk_parent_name, scaler_flags,
>                 data->base + ASPEED_REG_CLOCK_CONTROL, 0,
> -               data->model_data->scaler_bit_width, 0, &data->clk_lock);
> +               data->model_data->scaler_bit_width,
> +               data->model_data->need_prescaler ? CLK_DIVIDER_ONE_BASED : 0,
> +               &data->clk_lock);
>         if (IS_ERR(data->clk_scaler))
>                 return PTR_ERR(data->clk_scaler);
>
> --
> 2.25.1
>