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From: Alexandre Belloni <alexandre.belloni@bootlin.com>
To: "Uwe Kleine-König" <uwe@kleine-koenig.org>
Cc: linux-pwm@vger.kernel.org,
	Ludovic Desroches <ludovic.desroches@microchip.com>,
	Thierry Reding <thierry.reding@gmail.com>,
	Claudiu Beznea <claudiu.beznea@microchip.com>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 4/3] pwm: atmel: document known weaknesses of both hardware and software
Date: Fri, 16 Aug 2019 22:43:57 +0200
Message-ID: <20190816204357.GG3545@piout.net> (raw)
In-Reply-To: <20190816093748.11769-1-uwe@kleine-koenig.org>

On 16/08/2019 11:37:48+0200, Uwe Kleine-König wrote:
> Signed-off-by: Uwe Kleine-König <uwe@kleine-koenig.org>
> ---
>  drivers/pwm/pwm-atmel.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/drivers/pwm/pwm-atmel.c b/drivers/pwm/pwm-atmel.c
> index 42fe7bc043a8..1ddb93db9627 100644
> --- a/drivers/pwm/pwm-atmel.c
> +++ b/drivers/pwm/pwm-atmel.c
> @@ -7,6 +7,16 @@
>   *
>   * Reference manual for "atmel,at91sam9rl-pwm":
>   *   http://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-11032-32-bit-ARM926EJ-S-Microcontroller-SAM9G25_Datasheet.pdf
> + *
> + * Limitations:
> + * - Periods start with the inactive level.
> + * - Hardware has to be stopped in general to update settings.
> + *
> + * Software bugs/possible improvements:
> + * - When atmel_pwm_apply() is called with state->enabled=false a change in
> + *   state->polarity isn't honored.
> + * - Instead of sleeping to wait for a completed period, the interrupt
> + *   functionality could be used.

This is definitively not trivial to do right. The main reason it is not
done so is that reading PWM_ISR will clear all the bits so it is
necessary to be very careful to avoid race conditions. I'm not sure it
is worth the effort.


-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com

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Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-15 21:41 [PATCH 1/3] pwm: atmel: Add link to reference manual Uwe Kleine-König
2019-08-15 21:41 ` [PATCH 2/3] pwm: atmel: use a constant for maximum prescale value Uwe Kleine-König
2019-08-19  9:27   ` Claudiu.Beznea
2019-08-15 21:41 ` [PATCH 3/3] pwm: atmel: replace loop in prescale calculation by ad-hoc calculation Uwe Kleine-König
2019-08-19  9:27   ` Claudiu.Beznea
2019-08-16  9:37 ` [PATCH 4/3] pwm: atmel: document known weaknesses of both hardware and software Uwe Kleine-König
2019-08-16 20:43   ` Alexandre Belloni [this message]
2019-08-19  8:10     ` Uwe Kleine-König
2019-08-19  9:26   ` Claudiu.Beznea
2019-08-19 10:46     ` Uwe Kleine-König
2019-08-19 12:28       ` Claudiu.Beznea
2019-08-19 15:20         ` Uwe Kleine-König
2019-08-19  9:26 ` [PATCH 1/3] pwm: atmel: Add link to reference manual Claudiu.Beznea
2019-08-19  9:46   ` Nicolas.Ferre

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