linux-kernel - Re: [PATCH v3] clk: mvebu: Prevent division by zero in clk_double_div_recalc

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Message-ID: <a79dda0a-258d-4567-b473-44aabe81b649@lunn.ch>
Date: Tue, 8 Oct 2024 23:58:00 +0200
From: Andrew Lunn <andrew@...n.ch>
To: Stephen Boyd <sboyd@...nel.org>
Cc: Alexandra Diupina <adiupina@...ralinux.ru>,
	Gregory Clement <gregory.clement@...tlin.com>,
	Sebastian Hesselbarth <sebastian.hesselbarth@...il.com>,
	Michael Turquette <mturquette@...libre.com>,
	linux-arm-kernel@...ts.infradead.org, linux-clk@...r.kernel.org,
	linux-kernel@...r.kernel.org, lvc-project@...uxtesting.org
Subject: Re: [PATCH v3] clk: mvebu: Prevent division by zero in
 clk_double_div_recalc_rate()

On Mon, Oct 07, 2024 at 03:56:29PM -0700, Stephen Boyd wrote:
> Quoting Alexandra Diupina (2024-09-24 06:14:44)
> > >> diff --git a/drivers/clk/mvebu/armada-37xx-periph.c b/drivers/clk/mvebu/armada-37xx-periph.c
> > >> index 8701a58a5804..b32c6d4d7ee5 100644
> > >> --- a/drivers/clk/mvebu/armada-37xx-periph.c
> > >> +++ b/drivers/clk/mvebu/armada-37xx-periph.c
> > >> @@ -343,7 +343,12 @@ static unsigned long clk_double_div_recalc_rate(struct clk_hw *hw,
> > >>          div = get_div(double_div->reg1, double_div->shift1);
> > >>          div *= get_div(double_div->reg2, double_div->shift2);
> > >>   
> > >> -       return DIV_ROUND_UP_ULL((u64)parent_rate, div);
> > >> +       if (!div) {
> > >> +               pr_err("Can't recalculate the rate of clock %s\n", hw->init->name);
> > > hw->init is set to NULL after registration (see clk_register() code). If
> > > div is 0 what does the hardware do?
> > 
> > Thanks for noticing the error. Yes, hw->init is set to zero,
> > I will replace that code with clk_hw_get_name(hw).
> > If the value of div is 0, should I return 0 as stated in the
> > comment for .recalc_rate (in struct clk_ops) or should I
> > return parent_rate as in some other similar rate recalculation
> > functions (in some other drivers)?
> 
> It depends on what the hardware does. Does the hardware pass on the
> parent rate if the divider is zero? If so, then return parent_rate. Or
> does it turn off completely? If so, return zero.

I don't think anybody knows what the hardware does in this
condition. I also suspect it has never happened, or if it has, nobody
has complained.

I would say, let is divide by 0, so there is an obvious kernel stack
trace and hopefully a report of the issue. It can then be investigated
in a way we can then find out what the hardware actually is doing.

   Andrew