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

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Message-ID: <20220218085708.8194-1-billy_tsai@aspeedtech.com>
Date:   Fri, 18 Feb 2022 16:57:08 +0800
From:   Billy Tsai <billy_tsai@...eedtech.com>
To:     <jic23@...nel.org>, <lars@...afoo.de>, <joel@....id.au>,
        <andrew@...id.au>, <billy_tsai@...eedtech.com>,
        <colin.king@...onical.com>, <linux-iio@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-aspeed@...ts.ozlabs.org>, <linux-kernel@...r.kernel.org>
CC:     Konstantin Klubnichkin <kitsok@...dex-team.ru>
Subject: [PATCH] iio: adc: aspeed: Add divider flag to fix incorrect voltage reading.

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.

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