linux-kernel - Re: [PATCHv9 0/2] Add Allwinner SoCs PWM support

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Date:	Wed, 17 Dec 2014 20:58:36 +0100
From:	Alexandre Belloni <alexandre.belloni@...e-electrons.com>
To:	Olliver Schinagl <oliver@...inagl.nl>
Cc:	Thierry Reding <thierry.reding@...il.com>,
	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Simon <longsleep@...il.com>, linux-pwm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv9 0/2] Add Allwinner SoCs PWM support

Hi,

I finally got some time to work on that again.

On 18/11/2014 at 14:47:33 +0100, Olliver Schinagl wrote :
> What I get from the datasheet is, that sun4i and sun5i are exactly the same,
> with the exception that sun5i only has 1 PWM (~exposed~). I belive that is
> easily solved with the bindings by having allwinner-sun4i and allwinner
> sun5i bindings if I'm not mistaken.
> 
> As for sun7i compared to the other ones, according to disp_lcd.c sun5i and
> sun7i should behave exactly the same. This is contradicting to the
> datasheet, where sun4i and sun5i are the same.
> 
> So what are the major differences that I can see between the 3? sun4i
> defines the PWM prescaler register value 0b1111 as being undefined, and
> sun5i and sun7i as /1? Did you verify this (I haven't I admit, i bumped into
> this while looking for your patch ;-) )? I wouldn't be supprised if it where
> a typo on allwinners end in the datasheet ... disp_lcd.c stops at 72000 for
> the last entry. We should just check sun4i, sun5i and sun7i hardware to see
> if it behaves the same with a prescaler of 0b1111, which I would not be
> totally surprised if it did.
> 
> The other difference I notice is that sun7i and sun5i use 16bit period
> register where sun4i uses a 8bit register. This is probably the only reason
> why they put a #ifdef in disp_lcd.c, calculations turn out differently. I
> don't recognize this behavior at all in your driver however. I do think they
> that there is a difference here, since they did split up the original driver
> here because of this difference.
> 

That is something I overlooked and I can't test at all, I only have a
cubietruck. Did you have some time to test on a sun4i?

But, from the only datasheet I have access to [1], page 56:
Each channel has a dedicated internal 16-bit up counter. If the counter
reaches the value stored in the channel period register, it resets. At
the beginning of a count period cycle, the PWMOUT is set to active state
and count from 0x0000

So I would say that they all have a 16bits period.


[1] http://dl.linux-sunxi.org/A10/A10%20Datasheet%20-%20v1.21%20%282012-04-06%29.pdf

-- 
Alexandre Belloni, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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