linux-kernel - Re: [PATCH] clk: imx: pll14xx: fix recalc

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Message-ID: <3693d66b-801e-dc31-15ee-e010f2577655@lenbrook.com>
Date:   Tue, 14 Feb 2023 09:51:24 -0500
From:   Kevin Groeneveld <kgroeneveld@...brook.com>
To:     Stephen Boyd <sboyd@...nel.org>, Abel Vesa <abelvesa@...nel.org>,
        Fabio Estevam <festevam@...il.com>,
        Michael Turquette <mturquette@...libre.com>,
        NXP Linux Team <linux-imx@....com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Shawn Guo <shawnguo@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-clk@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clk: imx: pll14xx: fix recalc_rate for negative kdiv

On 2023-02-10 18:24, Stephen Boyd wrote:
> Quoting Kevin Groeneveld (2022-12-10 12:38:35)
>> kdiv is a signed 16 bit value in the DEV_CTL1 register. Commit
>> 53990cf9d5b4 ("clk: imx: pll14xx: consolidate rate calculation") changed
>> the kdiv variable from a short int to just int. When the value read from
>> the DIV_CTL1 register is assigned directly to an int the sign of the value
>> is lost resulting in incorrect results when the value is negative. Adding
> 
> Can the field be negative?

Yes it can be. That is how I found the issue as I had a negative value 
in imx_pll14xx_rate_table which used to work but broke with the above 
mentioned commit.

The i.MX8MM reference manual states:
 >Formula for Fraction PLLOUT:
 >* FOUT=((m+k/65536) x FIN) /(p x 2s)
 >* Where, 1 ≤ p ≤ 63, 64 ≤ m ≤ 1023, 0 ≤ s ≤ 6, -32768 ≤ k ≤ 32767
 >* p, m, and s are unsigned integers. k is a two's complement integer.

The comments in the imx_pll14xx_calc_settings function also mention the 
same range for kdiv. And this function also uses negative values. For 
example:

 >/* Then see if we can get the desired rate by only adjusting kdiv 
(glitch free) */
 >rate_min = pll14xx_calc_rate(pll, mdiv, pdiv, sdiv, KDIV_MIN, prate);
 >rate_max = pll14xx_calc_rate(pll, mdiv, pdiv, sdiv, KDIV_MAX, prate);

Where KDIV_MIN is defined as SHRT_MIN which is negative.


Kevin