linux-kernel - Re: [PATCH v4 2/2] drm: rcar-du: calculate DPLLCR to be more small jitter

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Date:   Mon, 18 Dec 2017 08:38:19 +0000
From:   Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        David Airlie <airlied@...ux.ie>,
        Linux-Renesas <linux-renesas-soc@...r.kernel.org>,
        Linux-Kernel <linux-kernel@...r.kernel.org>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v4 2/2] drm: rcar-du: calculate DPLLCR to be more small jitter


Hi Laurent

Thank you for your feedback

> > +	 * To be small jitter,
> 
> Nitpicking, I would write this "to minimize the jitter".

(snip)

> > +			 * This code is assuming "used" from 64bit CPU only,
> > +			 * not from 32bit CPU. But both can compile correctly
> 
> Nitpicking again, I would write this "This code only runs on 64-bit 
> architectures, the unsigned long type can thus be used for 64-bit computation. 
> It will still compile without any warning on 32-bit architectures."

I will follow your English ;)

> > +			/*
> > +			 *	fvco	= fin * P *  N / M
> > +			 *	fclkout	= fin      * N / M / FDPLL
> > +			 *
> > +			 * To avoid duplicate calculation, let's use below
> > +			 *
> > +			 *	finnm	= fin * N / M
> 
> This is called fout in your diagram above, I would use the same name here.

Oops indeed. I didn't notice

> > +			unsigned long finnm = input * (n + 1) / (m + 1);
> > +			unsigned long fvco  = finnm * 2;
> > +
> > +			if (fvco < 2000 || fvco > 4096 * 1000 * 1000U)
> > +				continue;
> 
> How about
> 
> 		if (fvco < 1000 || fvco > 2048 * 1000 * 1000)
> 
> to avoid computing the intermediate fvco variable ?

I think you want to say

 		- if (fvco < 1000 || fvco > 2048 * 1000 * 1000)
 		+ if (fout < 1000 || fout > 2048 * 1000 * 1000)

Actually I notcied about this, but I thought it makes
user confuse. Thus, I kept original number.

I'm happy if compiler can adjust it automatically,
if not, I have no objection to modify it but we want to have such comment ?
Because above comment/explain mentions about "fvco", not "fout".

> If you agree with these small changes there's no need to resubmit the patch, 
> I'll modify it when applying, and
> 
> Reviewed-by: Laurent Pinchart <laurent.pinchart@...asonboard.com>

Thank you for your help


Best regards
---
Kuninori Morimoto