Re: [PATCH v2] powerpc/64s: Clarify copy_and_flush() cache sync loop comment

[Amit Machhiwal <amachhiw@linux.ibm.com>]

On 2026/07/09 11:40 AM, Nikhil Kumar Singh wrote:
> Hi Mukesh,
> 
> Ideally, checkpatch should take care of this, I see no checkpatch errors.

That is because the default $max_line_length is set to 100 columns in
checkpatch.pl scripts. The kernel coding style document recommends 80,
but checkpatch enforces 100 unless you pass --max-line-length=80
explicitly.

  my $max_line_length = 100;

You will see the warnings being reported when you run the script as:

  ❯ scripts/checkpatch.pl ./v2_20260706_nikhilks_powerpc_64s_clarify_copy_and_flush_cache_sync_loop_comment.mbx --max-line-length=80
  WARNING: line length of 82 exceeds 80 columns
  #44: FILE: arch/powerpc/kernel/head_64.S:716:
  +4:	li	r0,8			/* r0 is the number of 8-byte words	*/
  [...]
  
  total: 0 errors, 12 warnings, 26 lines checked

I agree with Mukesh here and I think the comment should be fixed to fit
within 80 column width limit as staying within 80 columns is certainly
still preferred.

Thanks,
Amit

> 
> ```
> ./scripts/checkpatch.pl
> v2-0001-powerpc-64s-Clarify-copy_and_flush-cache-sync-loo.patch
> total: 0 errors, 0 warnings, 26 lines checked
> 
> v2-0001-powerpc-64s-Clarify-copy_and_flush-cache-sync-loo.patch has no
> obvious style problems and is ready for submission.
> ```
> 
> Thanks
> ~ Nikhil
> 
> On 07/07/26 12:48, Nikhil Kumar Singh wrote:
> > Hi Mukesh,
> > 
> > Thanks for reviewing the patch and for the suggestion.
> > I'd prefer to keep the current alignment as I find it a bit more
> > readable.
> > 
> > Thanks,
> > ~ Nikhil
> > 
> > On 07/07/26 11:08, Mukesh Kumar Chaurasiya wrote:
> > > On Mon, Jul 06, 2026 at 01:57:08PM +0530, Nikhil Kumar Singh wrote:
> > > > The value loaded into r0 in copy_and_flush() represents the number of
> > > > 8-byte words processed between cache synchronization operations.
> > > > 
> > > > The existing comment refers to cache line size, which can make
> > > > it appear
> > > > that the value is a cache line size in bytes rather than a loop count.
> > > > Clarify the comment to explain that the loop processes 8 words
> > > > (64 bytes)
> > > > per cache synchronization iteration, and that increasing the
> > > > value would
> > > > skip cache maintenance for intermediate cache lines.
> > > > 
> > > > This is a comment-only change with no functional impact.
> > > > 
> > > > Signed-off-by: Nikhil Kumar Singh <nikhilks@linux.ibm.com>
> > > > ---
> > > >   arch/powerpc/kernel/head_64.S | 20 ++++++++++++--------
> > > >   1 file changed, 12 insertions(+), 8 deletions(-)
> > > > 
> > > > diff --git a/arch/powerpc/kernel/head_64.S
> > > > b/arch/powerpc/kernel/head_64.S
> > > > index 63432a33ec49..a54f6f979173 100644
> > > > --- a/arch/powerpc/kernel/head_64.S
> > > > +++ b/arch/powerpc/kernel/head_64.S
> > > > @@ -713,14 +713,18 @@ p_end: .8byte _end - copy_to_here
> > > >   _GLOBAL(copy_and_flush)
> > > >       addi    r5,r5,-8
> > > >       addi    r6,r6,-8
> > > > -4:    li    r0,8            /* Use the smallest common    */
> > > > -                    /* denominator cache line    */
> > > > -                    /* size.  This results in    */
> > > > -                    /* extra cache line flushes    */
> > > > -                    /* but operation is correct.    */
> > > > -                    /* Can't get cache line size    */
> > > > -                    /* from NACA as it is being    */
> > > > -                    /* moved too.            */
> > > > +4:    li    r0,8            /* r0 is the number of 8-byte words    */
> > > > +                    /* to copy per cache sync iteration. */
> > > > +                    /* 8 words * 8 bytes = 64 bytes. 64B is    */
> > > > +                    /* the current default cache line size.    */
> > > > +                    /* This is a loop count, not a byte    */
> > > > +                    /* count. Increasing it may skip    */
> > > > +                    /* dcbst/icbi for lines in between and */
> > > > +                    /* leave stale instructions in icache. */
> > > > +                    /* This results in extra cache line    */
> > > > +                    /* flushes but operation is correct. */
> > > > +                    /* Can't get cache line size from NACA */
> > > > +                    /* as it is being moved too.        */
> > > The comment looks fine. But the alignement of the comment is still
> > > different from earlier. Keep the alignment same as earlier, it looks a
> > > little nicer.
> > > 
> > > Regards,
> > > Mukesh
> > > >         mtctr    r0            /* put # words/line in ctr    */
> > > >   3:    addi    r6,r6,8            /* copy a cache line */
> > > > -- 
> > > > 2.43.5
> > > >