linux-kernel - Re: [PATCH 1/3] clk: amlogic: Fix out-of-range PLL frequency setting

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Message-ID: <7be1f72e-00b1-4e86-88b2-260200b10480@amlogic.com>
Date: Wed, 22 Oct 2025 21:50:47 +0800
From: Chuan Liu <chuan.liu@...ogic.com>
To: Jerome Brunet <jbrunet@...libre.com>,
 Chuan Liu via B4 Relay <devnull+chuan.liu.amlogic.com@...nel.org>
Cc: Neil Armstrong <neil.armstrong@...aro.org>,
 Michael Turquette <mturquette@...libre.com>, Stephen Boyd
 <sboyd@...nel.org>, Kevin Hilman <khilman@...libre.com>,
 Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
 linux-amlogic@...ts.infradead.org, linux-clk@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] clk: amlogic: Fix out-of-range PLL frequency setting

Hi Jerome,
Thanks for your review.


On 10/22/2025 7:57 PM, Jerome Brunet wrote:
> [ EXTERNAL EMAIL ]
> 
> On Wed 22 Oct 2025 at 14:58, Chuan Liu via B4 Relay <devnull+chuan.liu.amlogic.com@...nel.org> wrote:
> 
>> From: Chuan Liu <chuan.liu@...ogic.com>
>>
>> meson_clk_get_pll_range_index incorrectly determines the maximum value
>> of 'm'.
> 
> This explanation is little light !
> 
> How did the problem show up ? Under which condition ? How did you come
> this conclusion ?

In actual use cases, we haven't encountered any issues caused by this,
because we ensure that range->max <= (1 << pll->m.width) when
configuring the range.

If the calculated 'm' falls into the range:
- range->max < m < (1 << pll->m.width)
An incorrect 'm' value may be selected here. Therefore, comparing
against range->max is more appropriate in this case.

> 
> Other people having problems might benefit from the explanation
> 
>>
>> Fixes: 8eed1db1adec6 ("clk: meson: pll: update driver for the g12a")
>> Signed-off-by: Chuan Liu <chuan.liu@...ogic.com>
>> ---
>>   drivers/clk/meson/clk-pll.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/clk/meson/clk-pll.c b/drivers/clk/meson/clk-pll.c
>> index 1ea6579a760f..b07e1eb19d12 100644
>> --- a/drivers/clk/meson/clk-pll.c
>> +++ b/drivers/clk/meson/clk-pll.c
>> @@ -191,7 +191,7 @@ static int meson_clk_get_pll_range_index(unsigned long rate,
>>        *m = meson_clk_get_pll_range_m(rate, parent_rate, *n, pll);
>>
>>        /* the pre-divider gives a multiplier too big - stop */
>> -     if (*m >= (1 << pll->m.width))
>> +     if (*m > pll->range->max)
> 
> Making sure m does not exceed the maximum value is valid too.
> You should check both conditions then

Ok, fix it in the next version.

> 
>>                return -EINVAL;
>>
>>        return 0;
> 
> --
> Jerome