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Date:   Thu, 21 Jul 2022 11:43:02 +0200
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Subject: Re: [RFC PATCH 2/2] clk: mediatek: Add frequency hopping support
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To:     Edward-JW Yang <edward-jw.yang@mediatek.com>
Cc:     =?UTF-8?B?Sm9obnNvbiBXYW5nICjnjovogZbpkasp?= 
        <Johnson.Wang@mediatek.com>,
        "robh+dt@kernel.org" <robh+dt@kernel.org>,
        "krzysztof.kozlowski+dt@linaro.org" 
        <krzysztof.kozlowski+dt@linaro.org>,
        "mturquette@baylibre.com" <mturquette@baylibre.com>,
        "sboyd@kernel.org" <sboyd@kernel.org>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-clk@vger.kernel.org" <linux-clk@vger.kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-mediatek@lists.infradead.org" 
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        <Yu-Chang.Wang@mediatek.com>,
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        <Kuan-Hsin.Lee@mediatek.com>, Chen-Yu Tsai <wenst@chromium.org>
References: <20220612135414.3003-1-johnson.wang@mediatek.com>
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 <9addc9fb0c949e921f915fcf128783393214bfde.camel@mediatek.com>
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 <e25b8ea348bf8a5fe0fe8c74cdb94f97725f0282.camel@mediatek.com>
From:   AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@collabora.com>
In-Reply-To: <e25b8ea348bf8a5fe0fe8c74cdb94f97725f0282.camel@mediatek.com>
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Il 20/07/22 15:51, Edward-JW Yang ha scritto:
> Hi AngeloGioacchino,
> 
> Thanks for all the advices and examples.
> 
> On Thu, 2022-07-14 at 19:04 +0800, AngeloGioacchino Del Regno wrote:
>> Il 06/07/22 15:07, Edward-JW Yang ha scritto:
>>> On Wed, 2022-06-29 at 16:54 +0800, Chen-Yu Tsai wrote:
>>>> On Tue, Jun 28, 2022 at 6:09 PM AngeloGioacchino Del Regno
>>>> <angelogioacchino.delregno@collabora.com> wrote:
>>>>>
>>>>> Il 24/06/22 09:12, Edward-JW Yang ha scritto:
>>>>>> Hi AngeloGioacchino,
>>>>>>
>>>>>> Thanks for all the advices.
>>>>>>
>>>>>> On Mon, 2022-06-13 at 17:43 +0800, AngeloGioacchino Del Regno wrote:
>>>>>>> Il 12/06/22 15:54, Johnson Wang ha scritto:
>>>>>>>> Add frequency hopping support and spread spectrum clocking
>>>>>>>> control for MT8186.
>>>>>>>>
>>>>>>>> Signed-off-by: Edward-JW Yang <edward-jw.yang@mediatek.com>
>>>>>>>> Signed-off-by: Johnson Wang <johnson.wang@mediatek.com>
>>>>>>>
>>>>>>> Before going on with the review, there's one important consideration:
>>>>>>> the Frequency Hopping control is related to PLLs only (so, no other clock
>>>>>>> types get in the mix).
>>>>>>>
>>>>>>> Checking the code, the *main* thing that we do here is initializing the
>>>>>>> FHCTL by setting some registers, and we're performing the actual frequency
>>>>>>> hopping operation in clk-pll, which is right but, at this point, I think
>>>>>>> that the best way to proceed is to add the "FHCTL superpowers" to clk-pll
>>>>>>> itself, instead of adding multiple new files and devicetree bindings that
>>>>>>> are specific to the FHCTL itself.
>>>>>>>
>>>>>>> This would mean that the `fh-id` and `perms` params that you're setting in
>>>>>>> the devicetree get transferred to clk-mt8186 (and hardcoded there), as to
>>>>>>> extend the PLL declarations to include these two: that will also simplify
>>>>>>> the driver so that you won't have to match names here and there.
>>>>>>>
>>>>>>> Just an example:
>>>>>>>
>>>>>>>        PLL(CLK_APMIXED_CCIPLL, "ccipll", 0x0224, 0x0230, 0,
>>>>>>>
>>>>>>>            PLL_AO, 0, 22, 0x0228, 24, 0, 0, 0, 0x0228, 2, FHCTL_PERM_DBG_DUMP),
>>>>>>>
>>>>>>> Besides, there are another couple of reasons why you should do that instead,
>>>>>>> of which:
>>>>>>>      - The devicetree should be "generic enough", we shall not see the direct value
>>>>>>>        to write to the registers in there (yet, perms assigns exactly that)
>>>>>>>      - These values won't change on a per-device basis, I believe? They're SoC-related,
>>>>>>>        not board-related, right?
>>>>>>>
>>>>>>> In case they're board related (and/or related to TZ permissions), we can always add
>>>>>>> a bool property to the apmixedsys to advertise that board X needs to use an
>>>>>>> alternative permission (ex.: `mediatek,secure-fhctl`).
>>>>>>
>>>>>> I think we should remain clk-fhctl files because FHCTL is a independent HW and is
>>>>>> not a necessary component of clk-pll.
>>>>>
>>>>> I know what FHCTL is, but thank you anyway for the explanation, that's appreciated.
>>>>> In any case, this not being a *mandatory* component doesn't mean that when it is
>>>>> enabled it's not changing the way we manage the PLLs..........
>>>>>
>>>>>> Frequency hopping function from FHCTL is not used to replace original flow of
>>>>>> set_rate in clk-pll. They are two different ways to change PLL's frequency. The
>>>>>
>>>>> I disagree: when we want to use FHCTL, we effectively hand-over PLL control from
>>>>> APMIXEDSYS to the Frequency Hopping controller - and we're effectively replacing
>>>>> the set_rate() logic of clk-pll.
>>>
>>> Do you mean we need to drop the current set_rate() logic (direct register write) and
>>> use Frequency Hopping Controller instead?
>>>
>>
>> On PLLs that are supported by the Frequency Hopping controller, yes: we should
>> simply use a different .set_rate() callback in clk-pll.c, and we should return
>> a failure if the FHCTL fails to set the rate - so we should *not* fall back to
>> direct register writes, as on some platforms and in some conditions, using
>> direct register writes (which means that we skip FHCTL), may lead to unstable
>> system.
>>
>> This means that we need logic such that, in mtk_clk_register_pll(), we end up
>> having something like that:
>>
>> if (fhctl_is_enabled(pll))
>> 	init.ops = &mtk_pll_fhctl_ops;
>> else
>> 	init.ops = &mtk_pll_ops;
>>
>>> I need to mention that not all PLL support FHCTL, only those PLLs with FHCTL HW can
>>> choose to use FHCTL. Take 8186 for example, there are three PLLs don't support FHCTL
>>> HW.
>>
>> Where we declare the PLLs, for example, in clk-mt8186-apmixedsys.c, we can declare
>> that such PLL can be managed by FHCTL, for example:
>>
>> 	PLL(CLK_APMIXED_ARMPLL_LL, "armpll_ll", 0x0204, 0x0210, 0,
>>
>> 	    PLL_AO, 0, 22, 0x0208, 24, 0, 0, 0, 0x0208),
>>
>> becomes
>>
>> 	PLL(CLK_APMIXED_ARMPLL_LL, "armpll_ll", 0x0204, 0x0210, 0,
>>
>> 	    PLL_AO, 0, 22, 0x0208, 24, 0, 0, 0, 0x0208, true);
>>
>> where 'true' means "FHCTL is supported".
> 
> Does it still have an independent FHCTL driver after modifying to this? From your example,
> setup a clk_ops and add FHCTL properities into PLL(), seems FHCTL driver is merged into
> clk-pll and become part of clk-pll driver.
> 

The direct-MMIO part of FHCTL becomes part of the clk-pll driver, yes - but then
I also find it unacceptable to embed the IPI communication inside of there, so we
can have an "external" helper for that.


> We tend to have an indepentent driver and dts for FHCTL, and mutate only .set_rate()
> callback function instead of whole clk_ops. The boot-up sequence is like:
> 
> 1. clk-pll + clk dts
> 	probe  -> clk-pll original flow, nothing to change
> 
>          /* clk-pll provide multation API for set_rate */
> 	/* mutate necessary set_rate() instead of mutating all ops */
> 		def register_fhctl_set_rate(pll_name, callback)
> 			ops = find_pll_ops_by_name(pll_name)
> 			log("change set_rate to fhctl callback for $pll_name")
> 			ops->set_rate = callback
> 
> 2. FHCTL driver + fhctl dts
> 	probe
> 		options = parsing dts (board specific, hopping disalbe or ssc-rate)
> 		init FHCTL HW
> 		for PLL in dts
> 			if (ssc-rate > 0)
> 				enable_ssc(ssc-rate)
> 			if (hop-enabled)
> 				/* mutate CCF set_rate, FHCTL engaged CCF */
> 				register_fhctl_CCF(pll_name, callback)
> 

I really don't like having PLL names in devicetree: they're already defined in
clock drivers and they will change on a per-SoC basis - and we do have per-SoC
drivers...

Whatever goes to devicetree should be something that we need to vary on a
per-board/platform(project) basis, so, enablement of FHCTL per-pll (by using
handles and numeral bindings as per the example that I previously wrote),
enablement of spread spectrum and its rate... and nothing else.

>>
>> Then, we register the PLLs with something like:
>>
>> mtk_clk_register_plls(node, plls, num_plls, clk_data, fhctl_register_version);
>>
>> ...where fhctl_register_version is used to assign the right fhctl register offsets.
>> Also, it's not needed to assign all of the register offsets statically, because
>> they can be easily calculated based on the number of supported PLLs, since the
>> registers are structured like
>>
>> [FHCTL GLOBAL REGISTERS] <--- hp_en...slope1
>> [FHCTL SSC GLOBAL REGISTERS] <--- DSSC_CFG, DSSC0...x_CON
>>
>> [FHCTL PER-PLL REGISTERS] <--- CFG...MON
>> ^^^ where this is repeated X times for X PLLs.
>>
>> so, keeping the example of MT8186, we can get the per-pll register like:
>>
>> #define FHCTL_PLL_OFFSET	0x3c
>> #define FHCTL_PLL_LEN		0x14
>>
>> #define FHCTLx_CFG(pll_id)	(FHCTL_PLL_OFFSET + (pll_id * FHCTL_PLL_LEN))
>> #define FHCTLx_UPDNLMT(pll_id)	(FHCTL_PLL_OFFSET + (pll_id * FHCTL_PLL_LEN) + 0x4)
>> #define FHCTLx_DDS(pll_id)	(FHCTL_PLL_OFFSET + (pll_id * FHCTL_PLL_LEN) + 0x8)
>>
>> we don't need to put all of them in a structure and for each PLL.
> 
> We use structure instead of using macros is because the register offset may have
> difference between ICs. If we use macro, we need to maintain different versions of macros.
> Using structure to store these register offsets is more flexible.
> 

I understand. What I don't like about your specific approach is the amount of
register offsets that we store in that structure, looks like it's a bit too many.

I've seen that there's a common pattern at least by checking downstream 5.10 and
MT8186/95 layouts, so I still think that using these macros will be beneficial.

We can always add parameters to the structure in a later commit: in my opinion,
that will help to engineer a better, shorter, cleaner solution for calculating
these registers anyway... but I will leave this choice to you, anyway, since you
know about way more SoCs than I do.

>>
>>> So, we need both APMIXEDSYS and Frequency Hopping Controller in set_rate() logic to
>>> handle this two types of PLL.
>>>
>>
>> As already said, we preventively know which PLLs support FHCTL and which does not,
>> so we can use a different .set_rate() callback.
> 
> Ok, we can use a different .set_rate() callback when fhctl driver probing.
> 
>>
>>>>>
>>>>>> current set_rate method in clk-pll changes PLL register setting directly. Another
>>>>>> way uses FHCTL to change PLL rate.
>>>>>
>>>>> ...and of course, if we change that, we're effectively mutating the functionality
>>>>> of the MediaTek clk-pll driver and please understand that seeing a clear mutation
>>>>> in that driver is a bit more human-readable.
>>>>>
>>>>> Besides, this makes me think about one question: is there any instance in which,
>>>>> when FHCTL rate setting fails, we fall back to direct register writes?
>>>>>
>>>>> I don't think that this is feasible because we have a register in FHCTL that
>>>>> effectively hands over control to it, so direct register writes should not work
>>>>> when the PLL is not under APMIXEDSYS control, but I'm asking just to be extremely
>>>>> sure that my understanding is right.
>>>
>>> It won't fall back to direct register writes when FHCTL rate setting fails. But, PLL
>>> control mode will switch back to APMIXEDSYS after frequency hopping completed.
>>>
>>> There are two cases that we need to fall back to direct register writes:
>>>     1. PLL support FHCTL but it doesn't want to use FHCTL.
>>>     2. PLL doesn't support FHCTL HW.
>>>
>>
>> For case N.1, if this is board-specific, we have to resort to devicetree properties
>> that will enable/disable FHCTL on specific PLLs.
>>
>> mediatek,fhctl-disable = <CLK_APMIXED_MSDCPLL>, <CLK_APMIXED_NNAPLL>;
>>
>> mediatek,ssc-enable = <CLK_APMIXED_MFGPLL>, <CLK_APMIXED_TVDPLL>;
>>
>> These are just examples - I don't currently know if it's a better idea to have an
>> allowlist or a blocklist as devicetree properties, as that depends on the expected
>> number of PLLs for which we en/dis fhctl or just ssc (if we generally want fhctl
>> enabled on all but one PLLs, we should use fhctl-disable, otherwise, fhctl-enable).
> 
> We also have a properity "ssc-rate" for setting up the ssc rate in percentage. The "ssc-
> rate" properity is under fhctl dts node and can be setup on each fhctl-PLL.
> 

Right. For that, we could have a default sensible percentage when SSC is enabled
but no rate is set in devicetree, or we can perhaps consider SSC enabled when any
meaningful SSC rate is set... For example:

mediatek,ssc-enable = <CLK_APMIXED_MFGPLL>, <CLK_APMIXED_TVDPLL>;
mediatek,ssc-percent = <5>, <5>;

... or something like:

mediatek,ssc = <CLK_APMIXED_MFGPLL 5>, <CLK_APMIXED_TVDPLL 5>;

...but I'd like to have some feedback on that from somebody else, as I don't know
if that would be acceptable in devicetree, or if there's any cleaner, niftier
solution.

>>
>>>>>
>>>>>> We will set some PLL's frequency be controlled
>>>>>> by clk-pll and some are controlled by FHCTL.
>>>>>
>>>>> Another question: is this also changing on a per-board basis?
>>>>>
>>>>> (note: the pll names in the example are random and not specific to anything)
>>>>>
>>>>> Example: board A wants FHCTL on MMPLL, TVDPLL, MPLL, but *shall not* hand over
>>>>>                     NNAPLL, MFGPLL
>>>>>             board B wants FHCTL on NNAPLL, TVDPLL but *shall not* hand over MMPLL
>>>>>
>>>>> Granted that the two A, B boards are using the same SoC, can that ever happen?
>>>
>>> This could happen if A, B boards have different desense issue.
>>>
>>
>> Ok, so it's definitely board specific. Devicetree is the way to go for this.
>>
>>>>>
>>>>>> And use `perms` param to decide
>>>>>> whether a PLL is using FHCTL to change its frequency.
>>>>>
>>>>> The perms param seems to be about:
>>>>>     * Enabling debug (but you're not providing any way to actually use debugging
>>>>>       features, so what's the point?)
>>>
>>> Debugging feature is not used yet, we can removed it.
>>>
>>
>> If the debugging features of the FHCTL driver will be like what I can see on
>> the downstream MT6893 5.10 kernel, that's not really applicable to upstream.
>>
>> In that case, please remove the debug.
> 
> Ok, we will remove it.
> 
>>
>>>>>     * Handing over PLL control to FHCTL for hopping (can be as well done with
>>>>>       simply using a different .set_rate() callback instead of a flag)
>>>
>>> There has some PLL that have FHCTL but don't want to use FHCTL. The flag is used in
>>> this case.
>>>
>>
>> Use the flag to set the right .set_rate() callback, set at probe time, instead of
>> checking that flag at every set_rate() call.
> 
> We will setup .set_rate() callback when doing fhctl-pll init.
> 
>>
>>>>>     * Enabling/disabling Spread Spectrum Clocking (and I think that this is a
>>>>>       legit use for flags, but if it's just one flag, you can as well use a
>>>>>       bool and manage this with a devicetree param like "enable-ssc")
>>>>>
>>>>> That said, I think that the current way of enabling the FHCTL is more complicated
>>>>> than how it should really be.
>>>
>>> Here needs an option to decide whether to enable FHCTL-hopping or FHCTL-ssc since
>>> these two are per-board basis.
>>>
>>> We cannot force all PLL hand over to FHCTL for hopping casue not all PLLs support
>>> FHCTL and not all PLLs have need of using FHCTL-hopping.
>>>
>>
>> Board specific -> devicetree
>>
>> SoC specific -> hardcode, no devicetree.
>>
>>>>>
>>>>>>
>>>>>> FHCTL has another function called SSC(spread spectrum clocking) which is used to
>>>>>> solve PLL de-sense problem. De-sense problem is board-related so we introduce a
>>>>>> `ssc-rate` param in the devicetree to decide whether SSC is enabled and how many
>>>>>> rate should be set. Mixing SSC function into clk-pll may cause clk-pll more
>>>>>> complex.
>>>>>>
>>>>>
>>>>> Thing is, I don't get why you think that adding SSC to clk-pll would complicate it
>>>>> so much... it's really just a few register writes and nothing else, so I really
>>>>> don't see where the problem is, here.
>>>>>
>>>>> Another issue is that this driver may be largely incomplete, so perhaps I can't
>>>>> really see the complications you're talking about? Is this the case?
>>>>>
>>>>> Regarding keeping the FHCTL code in separated files, that's fine, but I would still
>>>>> integrate it tightly in clk-pll and its registration flow, because - yes, this is
>>>>> for sure not mandatory, but the main parameters are constant, they never change for
>>>>> a specific PLL, as they're register offsets, bits and masks (which, again, will
>>>>> never change as long as we're using the same SoC).
>>>
>>> The driver may need to supoport microP by future HW design, standalone file clk-
>>> fhctl.c helps to trigger init flow of such as ap-init-flow, microP-init-flow .....,
>>> and those different init-flow also need to run some communication API with microP.
>>> Those communication APIs are not suitable to merge into clk-pll.
>>>
>>
>> Let's use clk-fhctl as an helper then, we can make sure to call the init flow for
>> the microP in the SoC-specific clock drivers, I think that's not a problem?
>>
>> clk_mtfuturesoc_someip_probe()
>> {
>> 	.... register clocks ....
>>
>> 	freqhopping_microp_init();
>>
>> 	return ret;
>> }
>>
>> If there's hardware out there that supports such feature and a downstream kernel to
>> look at, please tell me which one, so that I will be able to check it out and
>> perhaps understand how this flow works.
>>
>> P.S.: I guess it's not fhctl-sspm?
> 
> You could find clk-fhctl-mcupm.c and clk-fhctl-gpueb.c on the downstream MT6893 5.10
> kernel. Those codes require the PLL hardware specification to determine which PLL
> group(eg. PLL TOP group, GPUEB group) runs on which microP and has responsibilty to
> communicate with the microP.
> 
> If we implement these things into clk-pll driver, clk-pll driver not only needs to control
> PLL frequency but also needs to deal with microP IPI. It makes clk-pll driver have others
> works that is not belong to PLL operation. That's why we tend to have a standalone driver
> for FHCTL.
> 

Ok having something to analyze made this entire thing a bit more clear in my mind,
thanks for the pointers.

Analyzing clk-fhctl-mcupm and clk-fhctl-gpueb makes me see that there's a lot of
common code between the two: x_hopping_v1(), x_ssc_enable_v1(), x_ssc_disable_v1()
(where x = {gpueb,mcupm}) are really the same functions, duplicated and renamed
and nothing else.
The only difference is the get_xxxx_ipidev(), which is avoidable by assigning
mboxes = <...something...> in devicetree (gpueb mailbox, or mcupm mailbox).

Even the `FH_DEVCTL_CMD_ID` enumeration uses the same values!

To unroll that riddle, I would at that point add a new MediaTek specific clock
driver (like clk-pll) and call it `clk-ipi.c`, because that's what it does in
the end: whatever we do, goes through a mailbox instead of a direct MMIO write.

That clk-fhctl-ipi would contain a probe function that gets the mailbox handle,
then we would add something like `clk_fhctl_set_rate()` function, export it in
the `clk-mtk.h` or in a new `clk-fhctl.h` header, then assign the right callback
in either the SoC's clock driver (by registering a different clock type, which,
in this case, would be clk-fhctl-ipi instead of clk-pll), or in clk-pll itself...

In the end, I'm effectively proposing to:

1. Merge the direct-MMIO handling of FHCTL in clk-pll;
2. Create a new driver (and clock type, eventually) for the IPI handling of FHCTL.

Regards,
Angelo