On Thu, 3 Nov 2022, Greg KH wrote:
> On Wed, Oct 19, 2022 at 01:55:03PM +0300, Ilpo Järvinen wrote:
> > The struct tty_buffer has flags which is only used for storing TTYB_NORMAL.
> > There is also a few quite confusing operations for checking the presense
> > of TTYB_NORMAL. Simplify things by converting flags to bool.
> >
> > Despite the name remaining the same, the meaning of "flags" is altered
> > slightly by this change. Previously it referred to flags of the buffer
> > (only TTYB_NORMAL being used as a flag). After this change, flags tell
> > whether the buffer contains/should be allocated with flags array along
> > with character data array. It is much more suitable name that
> > TTYB_NORMAL was for this purpose, thus the name remains.
> >
> > Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> > ---
> >
> > v2:
> > - Make it more obvious why flags is not renamed (both in kerneldoc
> > comment and commit message).
> >
> > drivers/tty/tty_buffer.c | 28 ++++++++++++++--------------
> > include/linux/tty_buffer.h | 5 +----
> > include/linux/tty_flip.h | 4 ++--
> > 3 files changed, 17 insertions(+), 20 deletions(-)
> >
> > diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
> > index 5e287dedce01..b408d830fcbc 100644
> > --- a/drivers/tty/tty_buffer.c
> > +++ b/drivers/tty/tty_buffer.c
> > @@ -107,7 +107,7 @@ static void tty_buffer_reset(struct tty_buffer *p, size_t size)
> > p->commit = 0;
> > p->lookahead = 0;
> > p->read = 0;
> > - p->flags = 0;
> > + p->flags = true;
> > }
> >
> > /**
> > @@ -249,7 +249,7 @@ void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
> > * __tty_buffer_request_room - grow tty buffer if needed
> > * @port: tty port
> > * @size: size desired
> > - * @flags: buffer flags if new buffer allocated (default = 0)
> > + * @flags: buffer has to store flags along character data
> > *
> > * Make at least @size bytes of linear space available for the tty buffer.
> > *
> > @@ -260,19 +260,19 @@ void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
> > * Returns: the size we managed to find.
> > */
> > static int __tty_buffer_request_room(struct tty_port *port, size_t size,
> > - int flags)
> > + bool flags)
> > {
> > struct tty_bufhead *buf = &port->buf;
> > struct tty_buffer *b, *n;
> > int left, change;
> >
> > b = buf->tail;
> > - if (b->flags & TTYB_NORMAL)
> > + if (!b->flags)
> > left = 2 * b->size - b->used;
> > else
> > left = b->size - b->used;
> >
> > - change = (b->flags & TTYB_NORMAL) && (~flags & TTYB_NORMAL);
> > + change = !b->flags && flags;
> > if (change || left < size) {
> > /* This is the slow path - looking for new buffers to use */
> > n = tty_buffer_alloc(port, size);
> > @@ -300,7 +300,7 @@ static int __tty_buffer_request_room(struct tty_port *port, size_t size,
> >
> > int tty_buffer_request_room(struct tty_port *port, size_t size)
> > {
> > - return __tty_buffer_request_room(port, size, 0);
> > + return __tty_buffer_request_room(port, size, true);
>
> Did this logic just get inverted?
>
> Maybe it's the jet-lag, but this feels like it's not correct anymore.
As you can see, the old way is sooo confusing :-). I'll admit I stumbled
myself with this same default thing first. It's even more confusing than
the other places.
This check is true when flag bytes are present / required to be present:
(~flags & TTYB_NORMAL)
It's very very confusing way to check such condition due to layered
reverse logic.
With old code, the per character flag bytes won't be there in the buffer
if TTYB_NORMAL is present. Thus, the old default of 0 means
__tty_buffer_request_room will allocate room for those flag bytes.
If you think about it carefully, the old code passed 0. Therefore, ~0 &
TTYB_NORMAL is going to be true. After my change true is passed and true
matches to the original code.
So the logic was not inverted. I just cleared those layered reverse logic
traps the original had which makes my patch look it's inverting things.
I really appreciate you took your time to find out this little detail
from it! This is far from a simple change because of how trappy the old
way of doing things is.
> Maybe a commet up above where you calculate "left" would make more sense
> as to what is going on?
Do you mean you want me to add a comment there? I don't see any
pre-existing comments that you could be pointing me to.
Should I resubmit it since you probably dropped the patch?
While doing this cleanup, I realized there would likely be room for
some improvements which would avoid allocing a new tty_buffer. I'll
probably look into those at some point.
--
i.
