Re: [PATCH] powercap: Adjust printing the constraint name with new line

[Lukasz Luba]

Hi Rafael,

On 11/10/20 7:19 PM, Rafael J. Wysocki wrote:
> On Mon, Nov 9, 2020 at 6:25 PM Lukasz Luba <lukasz.luba@xxxxxxx> wrote:
> > The constraint name has limit of size 30, which sometimes might be hit.
> > When this happens the new line might be lost. Prevent this and set the
> > new line when the name string is too long."
> >
> > Signed-off-by: Lukasz Luba <lukasz.luba@xxxxxxx>
> > ---
> >   drivers/powercap/powercap_sys.c | 4 ++++
> >   1 file changed, 4 insertions(+)
> >
> > diff --git a/drivers/powercap/powercap_sys.c b/drivers/powercap/powercap_sys.c
> > index f808c5fa9838..575f9fdb810e 100644
> > --- a/drivers/powercap/powercap_sys.c
> > +++ b/drivers/powercap/powercap_sys.c
> > @@ -174,6 +174,10 @@ static ssize_t show_constraint_name(struct device *dev,
> >                                                                  "%s\n", name);
> >                          buf[POWERCAP_CONSTRAINT_NAME_LEN] = '\0';
> >                          len = strlen(buf);
> > +
> > +                       /* When the 'name' is too long, don't lose new line */
> > +                       if (strlen(name) >= POWERCAP_CONSTRAINT_NAME_LEN)
> > +                               buf[POWERCAP_CONSTRAINT_NAME_LEN - 1] = '\n';
>
> Wouldn't it be better to pass POWERCAP_CONSTRAINT_NAME_LEN - 1 to
> snprintf() above?

My apologies for delay, I was on holidays.

The snprintf() overwrites the '\n' in that case also. I've experimented
with it a bit more and tried to come with a bit 'cleaner' solution.

What we are looking is probably: "%.*s\n" semantic.
Another solution would be:
------------------------------8<---------------------------
                if (name) {
-                       snprintf(buf, POWERCAP_CONSTRAINT_NAME_LEN,
-                                                               "%s\n", 
name);
-                       buf[POWERCAP_CONSTRAINT_NAME_LEN] = '\0';
+                       sprintf(buf, "%.*s\n", 
POWERCAP_CONSTRAINT_NAME_LEN -1,
+                               name);
                        len = strlen(buf);
                }

----------------------------->8----------------------------

I've check this and it behaves very well.

It looks cleaner and it is a used pattern in the kernel.
What do you think? Is it good for v2?

Regards,
Lukasz