Re: [PATCH 17/18] tty: serial: samsung: shrink port feature flags to u8

[Jiri Slaby]

On 19. 01. 24, 9:56, Tudor Ambarus wrote:
> On 1/16/24 19:03, Sam Protsenko wrote:
> > On Wed, Jan 10, 2024 at 4:25 AM Tudor Ambarus <tudor.ambarus@xxxxxxxxxx> wrote:
> > > There's a single flag defined as of now. Shrink the feature flags to u8
> > > and aim for a better memory footprint for ``struct s3c24xx_uart_info``.
> > >
> > > Signed-off-by: Tudor Ambarus <tudor.ambarus@xxxxxxxxxx>
> > > ---
> > >   drivers/tty/serial/samsung_tty.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/tty/serial/samsung_tty.c b/drivers/tty/serial/samsung_tty.c
> > > index 5df2bcebf9fb..598d9fe7a492 100644
> > > --- a/drivers/tty/serial/samsung_tty.c
> > > +++ b/drivers/tty/serial/samsung_tty.c
> > > @@ -90,7 +90,7 @@ struct s3c24xx_uart_info {
> > >
> > >          /* uart port features */
> > >
> > > -       unsigned int            has_divslot:1;
> > > +       u8                      has_divslot:1;
> >
> > But that's already a bit field. Why does it matter which type it is?
>
> If using unsigned int the bitfied is combined with the previous u8
> fields, whereas if using u8 the bitfield will be independently defined.
> So no benefit in terms of memory footprint, it's just a cosmetic change
> to align the bitfield with the previous u8 fields. Allowing u32 for just
> a bit can be misleading as one would ask itself where are the other
> bits. Between a u32 bitfield and a bool a u8 bitfield seems like a good
> compromise.

Why? What's wrong with bool? bitfields have terrible semantics wrt 
atomic writes for example.

--
js
suse labs