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

[Daniel Kurtz]

On Fri, Aug 7, 2015 at 10:20 AM, James Liao <jamesjj.liao@xxxxxxxxxxxx> wrote:
> Hi Sascha,
>
> On Thu, 2015-08-06 at 12:20 +0200, Sascha Hauer wrote:
>> On Thu, Aug 06, 2015 at 05:13:21PM +0800, Daniel Kurtz wrote:
>> > On Thu, Aug 6, 2015 at 5:00 PM, James Liao <jamesjj.liao@xxxxxxxxxxxx> wrote:
>> > > Hi Sascha,
>> > >
>> > > On Thu, 2015-08-06 at 10:53 +0200, Sascha Hauer wrote:
>> > >> On Thu, Aug 06, 2015 at 04:23:51PM +0800, James Liao wrote:
>> > >> > 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.
>> > >
>> > > Oops, your suggestion is much different from Daniel's.
>> > >
>> > > Daniel, could you help to comment about how we model these clocks?
>> >
>> > First of all, for clocks where the rate doesn't matter, it doesn't
>> > matters to what rate we set the clock.
>> >
>> > As for the color of our shed, "the designer says these are the typical
>> > rates" sounds good enough to me for a "real rate", so I prefer using
>> > the rates in James' patch.
>> >
>> > If not sure what Sascha's concern is, but if he insists on 0, I'm fine
>> > with that too.
>>
>> I only find it confusing. I'd expect either the correct rate or an
>> obviously dummy rate. Whatever we choose a comment explaining the
>> background would really help here. Otherwise we won't know later
>> whether this 148.5 MHz rate was introduced because a) The consumers
>> depend on this rate being reported, b) It really is the correct rate or
>> c) we don't care about the rate.
>
> So the proper setting should be:
>
> clk_name,          parent,    rate
> -------------------------------------
> "clkph_mck_o",     "clk26m",  0
> "usb_syspll_125m", "clk26m",  125 * MHZ
> "dsi0_dig",        "clk26m",  0
> "dsi1_dig",        "clk26m",  0
> "lvds_pxl",        "lvdspll", 0
> "lvds_cts",        "lvdspll", 0
>
> usb_syspll_125m will keep in 125 MHz event in different products.Others
> may be changed by DRAM or display settings.
>
> Daniel, do you think it's OK to model these clocks like above?

Yes, I am fine with this.

>
>
> Best regards,
>
> James
>
>
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