Groups > linux.kernel > #1219627

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

From	Doug Anderson <dianders@chromium.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N calculation
Date	2015-09-05 21:50 +0200
Message-ID	<q5rDk-1Dm-11@gated-at.bofh.it> (permalink)
References	(6 earlier) <q59wJ-18i-5@gated-at.bofh.it> <q5b5v-3pQ-7@gated-at.bofh.it> <q5haW-3AR-21@gated-at.bofh.it> <q5m0X-25G-57@gated-at.bofh.it> <q5mki-2Hy-23@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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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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Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N calculation Doug Anderson <dianders@chromium.org> - 2015-09-04 20:30 +0200
  Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N calculation Doug Anderson <dianders@chromium.org> - 2015-09-04 21:50 +0200
    Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N  calculation Russell King - ARM Linux <linux@arm.linux.org.uk> - 2015-09-04 23:30 +0200
      Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N calculation Doug Anderson <dianders@chromium.org> - 2015-09-05 02:00 +0200
        Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N  calculation Russell King - ARM Linux <linux@arm.linux.org.uk> - 2015-09-05 02:30 +0200
          Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N calculation Doug Anderson <dianders@chromium.org> - 2015-09-05 04:10 +0200
            Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N  calculation Russell King - ARM Linux <linux@arm.linux.org.uk> - 2015-09-05 10:40 +0200
              Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N calculation Doug Anderson <dianders@chromium.org> - 2015-09-05 16:00 +0200
            Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N  calculation Russell King - ARM Linux <linux@arm.linux.org.uk> - 2015-09-05 10:40 +0200
              Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N calculation Doug Anderson <dianders@chromium.org> - 2015-09-05 15:50 +0200
                Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N  calculation Russell King - ARM Linux <linux@arm.linux.org.uk> - 2015-09-05 16:10 +0200
                Re: [PATCH 6/9] drm: bridge/dw_hdmi: adjust pixel clock values in N calculation Doug Anderson <dianders@chromium.org> - 2015-09-05 21:50 +0200

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