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

[Chen-Yu Tsai]

Hi,

On Wed, Feb 9, 2022 at 3:32 AM Boris Lysov <arz65xx@xxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxx> 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).

Ack. My scope is limited to SoCs used in Chromebooks. However Miles and
Chun-Jie, who are Cc-ed on the series, should know more.

That said, we can implement support for these varying parameters as we
see them, not before.

> > 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].

This is a question for whomever upstreamed the driver.
> 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.

Again, we can implement varying parameters as they appear.

Thanks
ChenYu


>
> 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)