linux-kernel - Re: [PATCH v6 7/9] clk: mediatek: Add subsystem clocks of MT8173

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Message-ID: <20150806085354.GN18700@pengutronix.de>
Date:	Thu, 6 Aug 2015 10:53:54 +0200
From:	Sascha Hauer <s.hauer@...gutronix.de>
To:	James Liao <jamesjj.liao@...iatek.com>
Cc:	Matthias Brugger <matthias.bgg@...il.com>,
	Mike Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Heiko Stubner <heiko@...ech.de>, devicetree@...r.kernel.org,
	srv_heupstream@...iatek.com, linux-kernel@...r.kernel.org,
	Daniel Kurtz <djkurtz@...omium.org>,
	Ricky Liang <jcliang@...omium.org>,
	Rob Herring <robh+dt@...nel.org>,
	linux-mediatek@...ts.infradead.org,
	Sascha Hauer <kernel@...gutronix.de>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v6 7/9] clk: mediatek: Add subsystem clocks of MT8173

On Thu, Aug 06, 2015 at 04:23:51PM +0800, James Liao wrote:
> Hi Sascha,
> 
> On Wed, 2015-08-05 at 08:46 +0200, Sascha Hauer wrote:
> > On Tue, Aug 04, 2015 at 04:16:56PM +0800, James Liao wrote:
> > >  static const struct mtk_fixed_clk fixed_clks[] __initconst = {
> > >  	FIXED_CLK(CLK_TOP_CLKPH_MCK_O, "clkph_mck_o", "clk26m", 400 * MHZ),
> > >  	FIXED_CLK(CLK_TOP_USB_SYSPLL_125M, "usb_syspll_125m", "clk26m", 125 * MHZ),
> > > +	FIXED_CLK(CLK_TOP_DSI0_DIG, "dsi0_dig", "clk26m", 130 * MHZ),
> > > +	FIXED_CLK(CLK_TOP_DSI1_DIG, "dsi1_dig", "clk26m", 130 * MHZ),
> > > +	FIXED_CLK(CLK_TOP_LVDS_PXL, "lvds_pxl", "lvdspll", 148.5 * MHZ),
> > > +	FIXED_CLK(CLK_TOP_LVDS_CTS, "lvds_cts", "lvdspll", 51.975 * MHZ),
> > 
> > I would expect 51975 * KHZ here to avoid fractional numbers. Probably
> > gcc calculates that during compile time so this will work as expected,
> > still I'm not sure this is good style to use fractional numbers here.
> 
> As I know all constants will be calculated in compile time, so there
> should be no difference between 51.975 * MHZ and 51975 * KHz. 
> 
> > Anyway, on my system lvdspll is running at 150MHz. Are you sure there is
> > a clock derived from this running at 148.5MHz? Is it really correct to
> > use a fixed clock here or should it rather be lvdspll directly?
> 
> Here is the clock hierarchy between lvdspll and lvds_pxl:
> 
>             --------       AD_VPLL_DPIX_CK  --------   lvds_pxl  -----
>            |        |--------------------->|        |---------->|
>            |        |                      | cksys  |           |
> LVDSPLL -->| LVDSTX |                      | buffer |           | MMSYS
>            |        | AD_LVDSTX_CLKDIG_CTS | test   |  lvds_cts |
>            |        |--------------------->|        |---------->|
>             --------                        --------             -----
> 
> Some clocks and blocks are not modeled into CCF. But we prefer to enable
> lvdspll before enabling lvds_pxl. So I modeled lvds_pxl (and lvds_cts)
> as a fixed-rate clock with a source from lvdspll.
> 
> The frequency of these fixed-rate clocks (such as 148.5 MHz) are typical
> rate. In fact, we don't care about the actual rate of these clocks. We
> just care about the enable / disable sequence of them.

Please either use the real rate or 0 (along with a explaining why). Using
a frequency with four to five significant digits makes me think that the
actual rate is very important.

Sascha

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