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

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Message-ID: <64vqb56a5gvpy5mut47n34nlqmbtfctvyljgylwuapgp53un5y@mj7k4awd3fay>
Date: Sat, 26 Oct 2024 21:55:58 +0300
From: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To: Gabor Juhos <j4g8y7@...il.com>
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

On Fri, Oct 25, 2024 at 10:05:04PM +0200, Gabor Juhos wrote:
> 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.

Ack. Please mention in the commit message that it's safe to drop the
alpha_en bit, because it will get reset by the set_rate function.

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

-- 
With best wishes
Dmitry