linux-kernel - Re: [PATCH 1/2] mfd: ti_am335x_tscadc: Fix idle timeout value

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Date:	Wed, 11 Sep 2013 08:00:58 +0100
From:	Lee Jones <lee.jones@...aro.org>
To:	Matthias Kaehlcke <matthias.list@...hlcke.net>,
	Samuel Ortiz <sameo@...ux.intel.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mfd: ti_am335x_tscadc: Fix idle timeout value

On Tue, 10 Sep 2013, Matthias Kaehlcke wrote:

> The old timeout value was based on the assumption that the minimum values are
> used for the open and sample delay and no averaging is done. In fact the ADC
> and touchscreen driver both use an open delay of 152 cycles and averaging over
> 16 samples. This patch adjusts the timeout value accordingly
> 
> Signed-off-by: Matthias Kaehlcke <matthias@...hlcke.net>
> ---
>  include/linux/mfd/ti_am335x_tscadc.h | 19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/mfd/ti_am335x_tscadc.h b/include/linux/mfd/ti_am335x_tscadc.h
> index db1791b..9e6a775 100644
> --- a/include/linux/mfd/ti_am335x_tscadc.h
> +++ b/include/linux/mfd/ti_am335x_tscadc.h
> @@ -126,13 +126,18 @@
>  #define TOTAL_CHANNELS		8
>  
>  /*
> -* ADC runs at 3MHz, and it takes
> -* 15 cycles to latch one data output.
> -* Hence the idle time for ADC to
> -* process one sample data would be
> -* around 5 micro seconds.
> -*/
> -#define IDLE_TIMEOUT 5 /* microsec */
> + * time in us for processing a single channel, calculated as follows:
> + *
> + * num cycles = open delay + (sample delay + conv time) * averaging
> + *
> + * num cycles: 152 + (1 + 13) * 16 = 376
> + *
> + * clock frequency: 26MHz / 8 = 3.25MHz
> + * clock period: 1 / 3.25MHz = 308ns
> + *
> + * processing time: 376 * 308ns = 116us
> + */
> +#define IDLE_TIMEOUT 116 /* microsec */

Nice, clear explanation. I like it.

Patch applied, thanks.

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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