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Received: from KL1PR03MB7149.apcprd03.prod.outlook.com (2603:1096:820:ca::7) by TYZPR03MB8252.apcprd03.prod.outlook.com (2603:1096:405:1d::9) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.21.71.11; Tue, 26 May 2026 09:58:22 +0000 Received: from KL1PR03MB7149.apcprd03.prod.outlook.com ([fe80::2f06:12a:fff0:6506]) by KL1PR03MB7149.apcprd03.prod.outlook.com ([fe80::2f06:12a:fff0:6506%5]) with mapi id 15.21.0071.005; Tue, 26 May 2026 09:58:21 +0000 Message-ID: <3fda1592-f7d0-4e86-8615-602804673414@amlogic.com> Date: Tue, 26 May 2026 17:58:19 +0800 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 07/10] clk: amlogic: Support POWER_OF_TWO for PLL pre-divider To: Jerome Brunet Cc: Jian Hu via B4 Relay , Michael Turquette , Stephen Boyd , Rob Herring , Krzysztof Kozlowski , Conor Dooley , Neil Armstrong , Xianwei Zhao , Kevin Hilman , Martin Blumenstingl , linux-kernel@vger.kernel.org, linux-clk@vger.kernel.org, devicetree@vger.kernel.org, linux-amlogic@lists.infradead.org, linux-arm-kernel@lists.infradead.org References: <20260511-b4-a9_clk-v1-0-41cb4071b7c9@amlogic.com> <20260511-b4-a9_clk-v1-7-41cb4071b7c9@amlogic.com> <1jy0hm6n7e.fsf@starbuckisacylon.baylibre.com> <8d89b669-e72e-4663-9596-999a12922d32@amlogic.com> <1jqzn65y9l.fsf@starbuckisacylon.baylibre.com> From: Jian Hu In-Reply-To: <1jqzn65y9l.fsf@starbuckisacylon.baylibre.com> Content-Type: text/plain; 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This would be a relatively large >> change. > Yes > >> >> Moreover, for all Amlogic chips, the n divider is an indispensable part of >> the DCO clock. > There is hardly a justification here > >> The difference between SoC generations is as follows: >> Previous SoCs PLL: n = 1, 2, 3, 4... (linear divider) >> A9 SoC PLL: n = 2^0, 2^1, 2^2, 2^3, 2^4... (power-of-two >> divider) > Yes that was fairly obvious > >> Therefore, splitting out the n divider from the DCO clock might not be a >> good design choice. > I'm not sure I agree and you've only stated your point of view without > providing any technical justification here. > > From the datasheets of the different SoC we have, the documented > limitation is always the DCO output rate range. Nothing related to n (or > m, or the mult-range for that matter). This is a legacy problem, we > started with monolithic driver and slowly simplified it. > > As far as I can see now, reworking the PLL driver to be a simple > multiplier driver with range output rate constraint could actually be > simpler than the current code. I would also make simpler to accomodate > differences such as the one presented here. > > Unless you can provide technical reasons why going in this direction > would be incorrect, that's where I'd prefer to go. > >> [...] >> >> Best regards, >> >> Jian > -- > Jerome I agree that having an independent N divider would simplify the PLL rate calculation. A separate pre-divider for N is technically possible, but there are some hardware constraints that need to be considered: N = 1 is the preferred operating mode except a few fixed-frequency PLLs. Larger N values reduce the PLL phase detector frequency, which may negatively impact jitter performance and overall PLL stability. Because of this, we cannot guarantee stable system operation when arbitrary larger N values are used. Some PLLs require non-1 N values to generate specific fixed output frequencies because the target rate cannot be achieved with N = 1 while keeping the PLL while keeping the PLL within its valid operating range. So N is designed to have other values ​​to satisfy this requirement. For example, the AXG PCIe PLL uses N = 3 to generate the required 100 MHz output frequency, since the target frequency cannot be achieved with N = 1. Additionally, is the refactored pre-divider N implemented as a separate patchset, independent from the A9 PLL changes? Best regards, Jian