linux-kernel - Re: [PATCH 8/9] iio: imu: st_lsm6dsx: add event configurability on a per axis basis

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Message-ID: <82bf13fd5ada664d9e4fdbc3ee453204e55d318b.camel@baylibre.com>
Date: Tue, 18 Nov 2025 12:01:57 +0100
From: Francesco Lavra <flavra@...libre.com>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
Cc: Lorenzo Bianconi <lorenzo@...nel.org>, Jonathan Cameron
 <jic23@...nel.org>,  David Lechner <dlechner@...libre.com>, Nuno
 Sá <nuno.sa@...log.com>,  linux-iio@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/9] iio: imu: st_lsm6dsx: add event configurability on
 a per axis basis

On Tue, 2025-11-18 at 11:44 +0100, Andy Shevchenko wrote:
> On Mon, Nov 17, 2025 at 08:23:35PM +0100, Francesco Lavra wrote:
> > On Thu, 2025-10-30 at 15:56 +0200, Andy Shevchenko wrote:
> > > On Thu, Oct 30, 2025 at 12:23:19PM +0100, Francesco Lavra wrote:
> > > > On Thu, 2025-10-30 at 10:24 +0200, Andy Shevchenko wrote:
> > > > > On Thu, Oct 30, 2025 at 08:27:51AM +0100, Francesco Lavra wrote:
> 
> ...
> 
> > > > > > +       old_enable = hw->enable_event[event];
> > > > > > +       new_enable = state ? (old_enable | BIT(axis)) :
> > > > > > (old_enable
> > > > > > &
> > > > > > ~BIT(axis));
> > > > > > +       if (!!old_enable == !!new_enable)
> > > > > 
> > > > > This is an interesting check. So, old_enable and new_enable are
> > > > > _not_
> > > > > booleans, right?
> > > > > So, this means the check test if _any_ of the bit was set and
> > > > > kept
> > > > > set or
> > > > > none were set
> > > > > and non is going to be set. Correct? I think a short comment
> > > > > would be
> > > > > good to have.
> > > > 
> > > > old_enable and new_enable are bit masks, but we are only interested
> > > > in
> > > > whether any bit is set, to catch the cases where the bit mask goes
> > > > from
> > > > zero to non-zero and vice versa. Will add a comment.
> > > 
> > > If it's a true bitmask (assuming unsigned long type) then all this
> > > can be
> > > done
> > > via bitmap API calls. Otherwise you can also compare a Hamming
> > > weights of
> > > them
> > > (probably that gives even the same size of the object file, but !!
> > > instructions
> > >  will be changed to hweight() calls (still a single assembly instr on
> > > modern
> > >  architectures).
> > 
> > These are u8 variables, so we can't use the bitmap API.
> 
> OK. But hweight8() can still be used.
> 
> > And I don't
> > understand the reason for using hweight(), given that the !! operators
> > would still be needed.
> 
> No, you won't need !! with that.

I still don't understand. Are you proposing to replace `if (!!old_enable ==
!!new_enable)` with `if (hweight8(old_enable) == hweight8(new_enable))`?
That won't work, because we only need to check whether the Hamming weight
goes from zero to non-zero and vice versa.