linux-kernel - Re: [PATCH v1 2/2] clk: divider: Properly handle rates exceeding UINT

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Date:   Fri, 26 May 2023 01:01:24 +0200
From:   'Sebastian Reichel' <sebastian.reichel@...labora.com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        "linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel@...labora.com" <kernel@...labora.com>
Subject: Re: [PATCH v1 2/2] clk: divider: Properly handle rates exceeding
 UINT_MAX

Hi,

On Mon, May 22, 2023 at 08:18:53AM +0000, David Laight wrote:
> From: Sebastian Reichel
> > Sent: 19 May 2023 20:05
> > 
> > Requesting rates exceeding UINT_MAX (so roughly 4.3 GHz) results
> > in very small rate being chosen instead of very high ones, since
> > DIV_ROUND_UP_ULL takes a 32 bit integer as second argument.
> > 
> > Correct this by using DIV64_U64_ROUND_UP instead, which takes proper
> > 64 bit values for dividend and divisor.
> 
> This doesn't look right on 32-bit architectures.
> While you really don't want to be doing full 64bit divides
> there is also the problem that any input values over 4.3Ghz
> have already been masked.
> In the values can be over 4.3GHz then the function arguments
> need to be 64bit - not long.

The common clock framework uses 'unsigned long' for clock rates
everywhere, so it's limited to ~4.3 GHz on 32-bit. I guess it does
not really matter - at least I don't expect new 32-bit platforms
with such high clock rates. Considering Intel and AMD already reach
5 GHz boost rates nowadays this is definetly not true for 64-bit.

The current function does (u64 / u32), so it's wrong on 32-bit
(rate can never be bigger than u32, so the u64 is useless) and
also wrong on 64-bit architectures (second argument may be bigger
than u32).

I will prepare a v2 using DIV_ROUND_UP(), which should improve the
performance on 32-bit and fix the bug on 64-bit architectures.

-- Sebastian

> 	David
> 
> > Note, that this is usually not an issue. ULONG_MAX sets the lower
> > 32 bits and thus effectively requests UINT_MAX. On most platforms
> > that is good enough. To trigger a real bug one of the following
> > conditions must be met:
> > 
> >  * A parent clock with more than 8.5 GHz is available
> >  * Instead of ULONG_MAX a specific frequency like 4.3 GHz is
> >    requested. That would end up becoming 5 MHz instead :)
> > 
> > Signed-off-by: Sebastian Reichel <sebastian.reichel@...labora.com>
> > ---
> >  drivers/clk/clk-divider.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/clk/clk-divider.c b/drivers/clk/clk-divider.c
> > index a2c2b5203b0a..dddaaf0f9d25 100644
> > --- a/drivers/clk/clk-divider.c
> > +++ b/drivers/clk/clk-divider.c
> > @@ -220,7 +220,7 @@ static int _div_round_up(const struct clk_div_table *table,
> >  			 unsigned long parent_rate, unsigned long rate,
> >  			 unsigned long flags)
> >  {
> > -	int div = DIV_ROUND_UP_ULL((u64)parent_rate, rate);
> > +	int div = DIV64_U64_ROUND_UP(parent_rate, rate);
> > 
> >  	if (flags & CLK_DIVIDER_POWER_OF_TWO)
> >  		div = __roundup_pow_of_two(div);
> > @@ -237,7 +237,7 @@ static int _div_round_closest(const struct clk_div_table *table,
> >  	int up, down;
> >  	unsigned long up_rate, down_rate;
> > 
> > -	up = DIV_ROUND_UP_ULL((u64)parent_rate, rate);
> > +	up = DIV64_U64_ROUND_UP(parent_rate, rate);
> >  	down = parent_rate / rate;
> > 
> >  	if (flags & CLK_DIVIDER_POWER_OF_TWO) {
> > @@ -473,7 +473,7 @@ int divider_get_val(unsigned long rate, unsigned long parent_rate,
> >  {
> >  	unsigned int div, value;
> > 
> > -	div = DIV_ROUND_UP_ULL((u64)parent_rate, rate);
> > +	div = DIV64_U64_ROUND_UP(parent_rate, rate);
> > 
> >  	if (!_is_valid_div(table, div, flags))
> >  		return -EINVAL;
> > --
> > 2.39.2
> 
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
> 

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