[PATCH] arm64: dts: mt8173: add clock_null

Hi Sascha,

On Wed, 2015-07-01 at 08:49 +0200, Sascha Hauer wrote:

On Tue, Jun 30, 2015 at 05:07:09PM +0800, James Liao wrote:

quoted

There are 4 clocks which are derived from clk_null directly in current
topckgen implementation:

	clkph_mck_o, dpi_ck, usb_syspll_125m, hdmitx_dig_cts

Our designer mentioned 2 things about external clocks:

1. These 4 clocks come from analog macro, not from PLL, nor from
external clocks directly.

Ok, this means there actually are clocks. We can't control these clock and
they have some known or unknown rate. This makes them fixed clocks. Just
specify them in the device tree and you are done. Give them reasonable
names and the rate if you know it, 0 otherwise.

The problem is not that you use fixed clocks for non software
controllable clocks of unknwown rates, but that you try to use a single
clock for all of them and name it 'dummy' or 'null'. Name it

dpi_ck {
	compatible = "fixed-clock";
	rate = <0>; /* unknown, generated by some Analog block */
};

Technically it's the same, but it sounds much more professional and like
you know what you are doing ;)

These clocks are SoC internal clocks. Is it suitable to specify them in
the device tree?

According to your and Heiko's suggestion, it looks the best way should
be specifying these clocks in the driver code without a dummy parent.
i.e.:

Original code:

FACTOR(CLK_TOP_CLKPH_MCK_O, "clkph_mck_o", "clk_null", 1, 1),
FACTOR(CLK_TOP_DPI, "dpi_ck", "clk_null", 1, 1),
FACTOR(CLK_TOP_USB_SYSPLL_125M, "usb_syspll_125m", "clk_null", 1, 1),
FACTOR(CLK_TOP_HDMITX_DIG_CTS, "hdmitx_dig_cts", "clk_null", 1, 1),

New code:

FIXED(CLK_TOP_CLKPH_MCK_O, "clkph_mck_o", UNKNOWN_RATE),
FIXED(CLK_TOP_DPI, "dpi_ck", UNKNOWN_RATE),
FIXED(CLK_TOP_USB_SYSPLL_125M, "usb_syspll_125m", UNKNOWN_RATE),
FIXED(CLK_TOP_HDMITX_DIG_CTS, "hdmitx_dig_cts", UNKNOWN_RATE),

Then we don't need to specify a dummy clock such as clk_null in device
tree.


Best regards,

James