[PATCH 02/13] alist: Allow inclusion from OS headers

[Simon Glass]

Hi Tom,

On Thu, 28 May 2026 at 20:18, Tom Rini <trini at konsulko.com> wrote:
>
> On Thu, May 28, 2026 at 12:12:09PM -0500, Simon Glass wrote:
> > Hi Tom,
> >
> > On Wed, 27 May 2026 at 21:43, Tom Rini <trini at konsulko.com> wrote:
> > >
> > > On Wed, May 27, 2026 at 08:36:45PM +0200, Heinrich Schuchardt wrote:
> > > > On 5/27/26 18:10, Simon Glass wrote:
> > > > > Sandbox needs to include system headers in some files, but also wants
> > > > > to use alist. Adjust the headers to permit this.
> > > > >
> > > > > Signed-off-by: Simon Glass <sjg at chromium.org>
> > > > > ---
> > > > >
> > > > >   include/alist.h | 16 +++++++++++-----
> > > > >   lib/alist.c     |  1 +
> > > > >   2 files changed, 12 insertions(+), 5 deletions(-)
> > > > >
> > > > > diff --git a/include/alist.h b/include/alist.h
> > > > > index b00d9ea97d6..69d7cdb722f 100644
> > > > > --- a/include/alist.h
> > > > > +++ b/include/alist.h
> > > > > @@ -10,8 +10,14 @@
> > > > >   #define __ALIST_H
> > > > >   #include <stdbool.h>
> > > > > -#include <linux/bitops.h>
> > > > > +
> > > > > +#ifdef USE_HOSTCC
> > > > > +#include <sys/types.h>
> > > > > +#include <stdint.h>
> > > > > +#else
> > > > >   #include <linux/types.h>
> > > > > +#endif
> > > > > +#define BIT(nr)                    (1UL << (nr))
> > > >
> > > > BIT(nr) is already defined in include/linux/bitops.h.
> > > >
> > > > We must not assume that no module includes both alist.h and linux/bitops.h.
> > > > E.g. include/expo.h and include/lmb.h include both alist.h and
> > > > linux/bitops.h. We should avoid redefinitions.
> > > >
> > > > The following is an abuse of the enum type:
> > > >
> > > > enum alist_flags {
> > > >         ALISTF_FAIL     = BIT(0),
> > > > };
> > > >
> > > > As this is the only use of BIT(), we could simply use the value "1" here.
> > > >
> > > > #define ALISTF_FAIL (1)
> > > >
> > > > But as ALISTF_FAIL is the only bit in flags used, the best solution would be
> > > > replacing flags by a boolean called fail.
> > >
> > > I see this as another reminder about how frustrating it was that alist
> > > was introduced under the guise of "must have this for an x86 feature"
> > > but instead is a solution in need of a problem to solve, and whenever
> > > it's used for something else, yet another problem with it is shown.
> >
> > The alist was motivated by its original commit. It allows easy
> > iteration, uses less memory (no next/prev pointers in every element),
> > uses fewer allocations than linked lists (one malloc() for the whole
> > list instead of one for each node) and has much better cache locality
> > as a result (i.e. improved performance).
> >
> > I asked my friendly AI to compare them:
> >
> >   - Reach for alist when you want a growable, index-addressable, cache-friendly
> >   collection of small value objects you mostly append to and iterate (e.g.
> >   bootflow/bootdev scan results, parsed tables).
> >   - Reach for list_head when elements need O(1) removal/splice from arbitrary
> >   positions, must keep stable identity while linked, or already exist as
> >   independently-allocated objects (most driver-model / subsystem object lists).
> >
> >   One nuance: alist_for_each is pure pointer arithmetic over the buffer, whereas
> >    list_for_each_entry uses container_of to recover the enclosing struct from
> >   the embedded node — a direct consequence of array-of-values vs.
> >   intrusive-links.
>
> This changes absolutely nothing about what I said the last time you
> wanted to use alist more widely, and in turn what I said about it a year
> ago, or whenever that was. I'm not going to go ask an unfriendly AI to
> say what's wrong with alist, I'm just going to say that I already said
> it.

I understand that, but it does actually have value. It is less
error-prone (although the API could use some improvements) and the
performance improvements are worth having too. I have found it very
useful in bootstd and wish I had started with it.

Not that I expect to change your mind, but this is a technical
discussion which could be covered in a future community call. I would
like to understand your objection beyond just 'Linux doesn't have it'.

Regards,
Simon