Re: [PATCH v3 08/10] clk: mediatek: clk-mt8195-topckgen: Drop univplls from mfg mux parents

[AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>]

Il 30/09/22 10:44, Chen-Yu Tsai ha scritto:
> On Fri, Sep 30, 2022 at 4:29 PM AngeloGioacchino Del Regno
> <angelogioacchino.delregno@collabora.com> wrote:
>>
>> Il 30/09/22 07:59, MandyJH Liu (劉人僖) ha scritto:
>>> On Tue, 2022-09-27 at 12:11 +0200, AngeloGioacchino Del Regno wrote:
>>>> These PLLs are conflicting with GPU rates that can be generated by
>>>> the GPU-dedicated MFGPLL and would require a special clock handler
>>>> to be used, for very little and ignorable power consumption benefits.
>>>> Also, we're in any case unable to set the rate of these PLLs to
>>>> something else that is sensible for this task, so simply drop them:
>>>> this will make the GPU to be clocked exclusively from MFGPLL for
>>>> "fast" rates, while still achieving the right "safe" rate during
>>>> PLL frequency locking.
>>>>
>>>> Signed-off-by: AngeloGioacchino Del Regno <
>>>> angelogioacchino.delregno@collabora.com>
>>>> Reviewed-by: Chen-Yu Tsai <wenst@chromium.org>
>>>> ---
>>>>    drivers/clk/mediatek/clk-mt8195-topckgen.c | 9 ++++++---
>>>>    1 file changed, 6 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/drivers/clk/mediatek/clk-mt8195-topckgen.c
>>>> b/drivers/clk/mediatek/clk-mt8195-topckgen.c
>>>> index 4dde23bece66..8cbab5ca2e58 100644
>>>> --- a/drivers/clk/mediatek/clk-mt8195-topckgen.c
>>>> +++ b/drivers/clk/mediatek/clk-mt8195-topckgen.c
>>>> @@ -298,11 +298,14 @@ static const char * const ipu_if_parents[] = {
>>>>       "mmpll_d4"
>>>>    };
>>>>
>>>> +/*
>>>> + * MFG can be also parented to "univpll_d6" and "univpll_d7":
>>>> + * these have been removed from the parents list to let us
>>>> + * achieve GPU DVFS without any special clock handlers.
>>>> + */
>>>>    static const char * const mfg_parents[] = {
>>>>       "clk26m",
>>>> -    "mainpll_d5_d2",
>>>> -    "univpll_d6",
>>>> -    "univpll_d7"
>>>> +    "mainpll_d5_d2"
>>>>    };
>>>>
>>>>    static const char * const camtg_parents[] = {
>>> There might be a problem here. Since the univpll_d6 and univpll_d7 are
>>> available parents in hardware design and they can be selected other
>>> than kernel stage, like bootloader, the clk tree listed in clk_summary
>>> cannot show the real parent-child relationship in such case.
>>
>> I agree about that, but the clock framework will change the parent to
>> the "best parent" in that case... this was done to avoid writing complicated
>> custom clock ops just for that one.
>>
>> This issue is present only on MT8195, so it can be safely solved this way,
>> at least for now.
>>
>> Should this become a thing on another couple SoCs, it'll then make sense
>> to write custom clock ops just for the MFG.
> 
> Would CLK_SET_RATE_NO_REPARENT on the fast mux coupled with forcing
> the clk tree to a state that we like (mfgpll->fast_mux->gate) work?

I'm not sure that it would, and then this would mean that we'd have to add
assigned-clock-parents to the devicetree and the day we will introduce the
"complicated custom clock ops" for that, we'll most probably have to change
the devicetree as well... which is something that I'm a bit reluctant to do
as a kernel upgrade doesn't automatically mean that you upgrade the DT with
it to get the "new full functionality".

Introducing the new clock ops for the mfg mux is something that will happen
for sure, but if we don't get new SoCs with a similar "issue", I don't feel
confident to write them, as I fear these won't be as flexible as needed and
will eventually need a rewrite; that's why I want to wait to get the same
situation on "something new".

In my opinion, it is safe to keep this change as it is, even though I do
understand the shown concerns about the eventual unability to show the tree
relationship in case the bootloader chooses to initialize the mfg mux with
a univpll parent.

Regards,
Angelo