Re: [PATCH] drm/fbdev-generic: optimize out a redundant assignment clause

[Thomas Zimmermann <tzimmermann@xxxxxxx>]

Hi

Am 30.03.23 um 06:17 schrieb Lucas De Marchi:
> On Wed, Mar 29, 2023 at 11:04:17AM +0200, Thomas Zimmermann wrote:
> > (cc'ing Lucas)
> >
> > Hi
> >
> > Am 25.03.23 um 08:46 schrieb Sui Jingfeng:
> > >  The assignment already done in drm_client_buffer_vmap(),
> > >  just trival clean, no functional change.
> > >
> > > Signed-off-by: Sui Jingfeng <15330273260@xxxxxx>
> > > ---
> > >  drivers/gpu/drm/drm_fbdev_generic.c | 5 ++---
> > >  1 file changed, 2 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/drm_fbdev_generic.c 
> > > b/drivers/gpu/drm/drm_fbdev_generic.c
> > > index 4d6325e91565..1da48e71c7f1 100644
> > > --- a/drivers/gpu/drm/drm_fbdev_generic.c
> > > +++ b/drivers/gpu/drm/drm_fbdev_generic.c
> > > @@ -282,7 +282,7 @@ static int drm_fbdev_damage_blit(struct 
> > > drm_fb_helper *fb_helper,
> > >                   struct drm_clip_rect *clip)
> > >  {
> > >      struct drm_client_buffer *buffer = fb_helper->buffer;
> > > -    struct iosys_map map, dst;
> > > +    struct iosys_map map;
> > >      int ret;
> > >      /*
> > > @@ -302,8 +302,7 @@ static int drm_fbdev_damage_blit(struct 
> > > drm_fb_helper *fb_helper,
> > >      if (ret)
> > >          goto out;
> > > -    dst = map;
> > > -    drm_fbdev_damage_blit_real(fb_helper, clip, &dst);
> > > +    drm_fbdev_damage_blit_real(fb_helper, clip, &map);
> >
> > I see what you're doing and it's probably correct in this case.
> >
> > But there's a larger issue with this iosys interfaces. Sometimes the 
> > address has to be modified (see calls of iosys_map_incr()). That can 
> > prevent incorrect uses of the mapping in other places, especially in 
> > unmap code.
>
> using a initializer for the cases it's needed IMO would make these kind
> of problems go away, because then the intent is explicit
>
> > I think it would make sense to consider a separate structure for the 
> > I/O location. The buffer as a whole would still be represented by 
> > struct iosys_map.  And that new structure, let's call it struct 
> > iosys_ptr, would point to an actual location within the buffer's
>
> sounds fine to me, but I'd have to take a deeper look later (or when
> someone writes the patch).  It seems we'd replicate almost the entire
> API to just accomodate the 2 structs.  And the different types will lead
> to confusion when one or the other should be used

I think we can split the current interface onto two categories: mapping 
and I/O. The former would use iosys_map and the latter would use 
iosys_ptr. And we'd need a helper that turns gets a ptr for a given map.

If I find the tine, I'll probably type up a patch.

Best regards
Thomas

> thanks
> Lucas De Marchi
>
> > memory range. A few locations and helpers would need changes, but 
> > there are not so many callers that it's an issue.  This would also 
> > allow for a few debugging tests that ensure that iosys_ptr always 
> > operates within the bounds of an iosys_map.
> >
> > I've long considered this idea, but there was no pressure to work on 
> > it. Maybe now.
> >
> > Best regards
> > Thomas
> >
> > >      drm_client_buffer_vunmap(buffer);
> >
> > --
> > Thomas Zimmermann
> > Graphics Driver Developer
> > SUSE Software Solutions Germany GmbH
> > Maxfeldstr. 5, 90409 Nürnberg, Germany
> > (HRB 36809, AG Nürnberg)
> > Geschäftsführer: Ivo Totev

--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Ivo Totev

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