linux-kernel - Re: [PATCH] clk: fractional-divider: check parent rate only for general approximation

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Message-ID: <d507fe24-c2ba-0c38-4985-7f8f4bf89958@katsuster.net>
Date:   Thu, 31 Jan 2019 08:44:05 +0900
From:   Katsuhiro Suzuki <katsuhiro@...suster.net>
To:     Stephen Boyd <sboyd@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        linux-clk@...r.kernel.org
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] clk: fractional-divider: check parent rate only for
 general approximation

Hello Stephen,

Thank you for your comment.
I'll add detail description and send it.

Best Regards,
Katsuhiro Suzuki

On 2019/01/30 6:55, Stephen Boyd wrote:
> Quoting Katsuhiro Suzuki (2019-01-07 05:21:24)
>> Custom approximation of fractional-divider may not need parent clock
>> rate checking. For example Rockchip SoCs work fine using grand parent
>> clock rate evne if target rate is greater than parent.
>>
>> This patch removes parent clock rate check from custom approximation.
>>
>> Signed-off-by: Katsuhiro Suzuki <katsuhiro@...suster.net>
>> ---
>>   drivers/clk/clk-fractional-divider.c | 10 +++++++---
>>   1 file changed, 7 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/clk/clk-fractional-divider.c b/drivers/clk/clk-fractional-divider.c
>> index 545dceec0bbf..b0fc5509e0ff 100644
>> --- a/drivers/clk/clk-fractional-divider.c
>> +++ b/drivers/clk/clk-fractional-divider.c
>> @@ -79,13 +79,17 @@ static long clk_fd_round_rate(struct clk_hw *hw, unsigned long rate,
>>          unsigned long m, n;
>>          u64 ret;
>>   
>> -       if (!rate || rate >= *parent_rate)
>> +       if (!rate)
> 
> Is your fraction multiplying the rate up? If it's a divider it shouldn't
> be increasing the rate more than whatever the parent is supplying so
> this looks odd. How does the grandparent matter here? Maybe you can show
> the example in the commit text so we can all understand what's going on
> here.
> 
>>                  return *parent_rate;
>>   
> 
>