linux-kernel - Re: [PATCH v2 1/2] tty: Convert tty

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Date:   Wed, 9 Nov 2022 13:02:43 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc:     linux-serial <linux-serial@...r.kernel.org>,
        Jiri Slaby <jirislaby@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] tty: Convert tty_buffer flags to bool

On Thu, Nov 03, 2022 at 12:11:26PM +0200, Ilpo Järvinen wrote:
> 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@...ux.intel.com>
> > > ---
> > > 
> > > 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?

No need, I took is as-is now, thanks.

greg k-h