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

From: Doug Anderson <dianders@chromium.org>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: Fabio Estevam <fabio.estevam@freescale.com>,
	alsa-devel@alsa-project.org,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"dri-devel@lists.freedesktop.org"
	<dri-devel@lists.freedesktop.org>,
	Jaroslav Kysela <perex@perex.cz>,
	"open list:ARM/Rockchip SoC..."
	<linux-rockchip@lists.infradead.org>,
	Mark Brown <broonie@kernel.org>, Yakir Yang <ykk@rock-chips.com>,
	Andy Yan <andy.yan@rock-chips.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N calculation
Date: Sat, 5 Sep 2015 12:44:51 -0700	[thread overview]
Message-ID: <CAD=FV=WsKgQRrKNmN+sFwQBCC2XiJLOg-yzg5w6w8n6qk8MjRQ@mail.gmail.com> (raw)
In-Reply-To: <20150905140136.GM21084@n2100.arm.linux.org.uk>

Hi,

On Sat, Sep 5, 2015 at 7:01 AM, Russell King - ARM Linux
<linux@arm.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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