linux-kernel - Re: [PATCH v1 1/1] clk: fractional-divider: Move mask calculations out of lock

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Date: Sat, 9 Mar 2024 18:27:56 +0200
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: Christophe JAILLET <christophe.jaillet@...adoo.fr>
Cc: linux-clk@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Michael Turquette <mturquette@...libre.com>, Stephen Boyd <sboyd@...nel.org>, 
	Heiko Stuebner <heiko@...ech.de>
Subject: Re: [PATCH v1 1/1] clk: fractional-divider: Move mask calculations
 out of lock

On Sat, Mar 9, 2024 at 9:19 AM Christophe JAILLET
<christophe.jaillet@...adoo.fr> wrote:
> Le 03/03/2024 à 13:14, Andy Shevchenko a écrit :

..

> > @@ -195,14 +195,14 @@ static int clk_fd_set_rate(struct clk_hw *hw, unsigned long rate,
> >               n--;
> >       }
> >
> > +     mmask = GENMASK(fd->mwidth - 1, 0) << fd->mshift;
> > +     nmask = GENMASK(fd->nwidth - 1, 0) << fd->nshift;

> if this is a hot path, you could maybe even compute:

It's not. set_rate() may be called only on disabled (and unprepared?)
clocks, which makes it already a too slow operation.

>     mask = ~(GENMASK(fd->mwidth - 1, 0) << fd->mshift |
>              GENMASK(fd->nwidth - 1, 0) << fd->nshift)
>
> unless gcc is smart enough to do it by itself.
>
> >       if (fd->lock)
> >               spin_lock_irqsave(fd->lock, flags);
> >       else
> >               __acquire(fd->lock);
> >
> > -     mmask = GENMASK(fd->mwidth - 1, 0) << fd->mshift;
> > -     nmask = GENMASK(fd->nwidth - 1, 0) << fd->nshift;
> > -
> >       val = clk_fd_readl(fd);
> >       val &= ~(mmask | nmask);
>
>     val &= mask;
>
> >       val |= (m << fd->mshift) | (n << fd->nshift);
>
> and pre-compute "(m << fd->mshift) | (n << fd->nshift)" outside of the
> lock too.

All of these sound to me as premature optimisations. I only wanted to
get back to the status quo.

-- 
With Best Regards,
Andy Shevchenko