linux-kernel - Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N calculation

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Date:	Sat, 5 Sep 2015 12:44:51 -0700
From:	Doug Anderson <dianders@...omium.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	"open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>,
	alsa-devel@...a-project.org,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Philipp Zabel <p.zabel@...gutronix.de>,
	Andy Yan <andy.yan@...k-chips.com>,
	Yakir Yang <ykk@...k-chips.com>,
	Fabio Estevam <fabio.estevam@...escale.com>,
	Mark Brown <broonie@...nel.org>, Takashi Iwai <tiwai@...e.de>,
	Jaroslav Kysela <perex@...ex.cz>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Jon Nettleton <jon.nettleton@...il.com>,
	David Airlie <airlied@...ux.ie>
Subject: Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N calculation

Hi,

On Sat, Sep 5, 2015 at 7:01 AM, Russell King - ARM Linux
<linux@....linux.org.uk> wrote:
>> If you know the answer, just tell me.  If you're talking about 74.25
>> vs. 32 kHz it is further evidence of what I'm saying.  Note that
>> picking only one of the two listed CTS values again puts you in a
>> worse position for regenerating the proper audio clock then just using
>> the default N=4096.
>
> No it doesn't.
>
> 74.25MHz/1.001 * 4096 / (128 * 32000) = 74175 (rounded down)
>
> Now do the calcuation.
>
> (74.25MHz/1.001) / 74175 * 4096 = 4096045.511 => 32000.35556Hz
>         => error of 0.001111%
>
> Now for the calcuation using the proscribed figures.
>
> (74.25MHz/1.001) / 210937 * 11648 = 4096009.709 => 32000.07585Hz
>         => error of 0.000237%
>

Why would you round down???  Round to the closest.

(74250000 / 1.001 * 4096)  / (128 * 32000.)
=> 74175.82417582418
=> 74176

(74250000 / 1.001) / 74176 * 4096 / 128
=> 31999.924148327947

That's actually the same error as yours: 0.000237%

You're right.  Yours isn't worse, but it's also not any better.
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