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

[Sascha Hauer <s.hauer@pengutronix.de>]

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

-- 
Pengutronix e.K.                           |                             |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |