Re: [PATCH v3 00/31] clk: mediatek: Cleanups and Improvements - Part 1

[Boris Lysov <arz65xx@gmail.com>]

Hi, I couldn't find a particular patch to reply to so I'm replying cover
letter to give some input on the PLL subsystem.

On Tue,  8 Feb 2022 20:40:03 +0800
Chen-Yu Tsai <wenst@chromium.org> wrote:
>  drivers/clk/mediatek/clk-pll.c                | 100 +++++-
>  drivers/clk/mediatek/clk-pll.h                |  57 ++++

In clk-pll.c there is an mtk_clk_register_pll function which at some point
executes this:

> init.ops = &mtk_pll_ops;

In my opinion there should be a possibility to define a custom mtk_pll_ops for a
given SoC instead of using a hardcoded one because not all Mediatek SoCs share
the same PLL startup/powerdown flow. For example, the existing mtk_pll_prepare
implementation won't work for the entire Mediatek Cortex-A9 SoC family (this
includes but not limited to mt6515, mt6517, mt6575, and mt6577). 

> static int mtk_pll_prepare(struct clk_hw *hw)
> {
> 	struct mtk_clk_pll *pll = to_mtk_clk_pll(hw);
> 	u32 r;
> 	u32 div_en_mask;
> 
> 	r = readl(pll->pwr_addr) | CON0_PWR_ON;
> 	writel(r, pll->pwr_addr);

This code sets a bit to 1 to start a PLL but the SoCs I mentioned above would
need to have that bit cleared (set to 0) [1] [2].

Another interesting thing in mtk_pll_prepare is
> 	udelay(20);
Is 20 ms a settle time for PLL? If yes then it would also be cool to specify an
arbitrary value easily as some PLLs have longer settle time [3] [4].

Worth noting the SoCs I mentioned aren't in mainline yet, and I think there are
more modern mainline-worthy Mediatek SoCs that might also need these changes in
the future.

Thanks.

[1] MT6577 HSPA Smartphone Application Processor Datasheet, pages 1212-1227
(*_CON0 registers).
[2] MT6515 GSM/EDGE Smartphone Application Processor Datasheet, pages
1202-1216 (*_CON0 registers).
[3] pages 1303-1306 of [1]
[4] MT6589 HSPA+ Smartphone Application Processor Datasheet, page 1344
(MDPLL1 & MDPLL2)