linux-kernel - Re: [PATCH 1/5] clk: qcom: apss-ipq-pll: drop 'alpha_en

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Message-ID: <45461b57-cb5a-43a5-8b9c-09ae059805a9@gmail.com>
Date: Fri, 25 Oct 2024 22:05:04 +0200
From: Gabor Juhos <j4g8y7@...il.com>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc: Bjorn Andersson <andersson@...nel.org>,
 Michael Turquette <mturquette@...libre.com>, Stephen Boyd
 <sboyd@...nel.org>, linux-arm-msm@...r.kernel.org,
 linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] clk: qcom: apss-ipq-pll: drop 'alpha_en_mask' from
 IPQ5018 PLL config

2024. 10. 25. 8:24 keltezéssel, Dmitry Baryshkov írta:
> On Mon, Oct 21, 2024 at 10:21:57PM +0200, Gabor Juhos wrote:
>> Since neither 'alpha' nor 'alpha_hi' is defined in the configuration,
>> those will be initialized with zero values  implicitly. By using zero
>> alpha values, the output rate of the PLL will be the same whether
>> alpha mode is enabled or not.
>>
>> Remove the superfluous initialization of the 'alpha_en_mask' member
>> to make it clear that enabling alpha mode is not required to get the
>> desired output rate.
>>
>> No functional changes, the initial rate of the PLL is the same both
>> before and after the patch.
> 
> After going through DISPCC changes, I think the whole series is
> incorrect: these PLL can change the rate (e.g. to facilitate CPU
> frequency changes). Normally PLL ops do not check the alpha_en bit when
> changing the rate, so the driver might try to set the PLL to the rate
> which requires alpha value, while the alpha_en bit isn't set.

Both clk_alpha_pll_stromer_set_rate() which is used for IPQ5018 (patch 1), and
clk_alpha_pll_stromer_plus_set_rate() used for IPQ5332 (patch 2) sets the
ALPHA_EN bit unconditionally.

For the PLLs affected by the remaining patches, clk_alpha_pll_set_rate() is used
which also unconditionally sets the ALPHA_EN bit via __clk_alpha_pll_set_rate().

I have created the patches after analysing the side effects of [1]. Due to the
bug described in that change, the clk_alpha_pll_configure() function in the
current kernel never sets the ALPHA_EN bit in the USER_CTL register. This means
that setting 'alpha_en_mask' in the configurations has no effect actually.

So, if we assume that the affected PLLs are working correctly now, it is not
because the 'alpha_en_mask' is specifed in the configuration but due to the fact
that the set_rate op sets the ALPHA_EN bit.

At least, I came to this after the analysis.

[1]
https://lore.kernel.org/r/20241021-fix-alpha-mode-config-v1-1-f32c254e02bc@gmail.com

Regards,
Gabor