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

[Lucas De Marchi]

On Thu, Mar 30, 2023 at 08:57:36AM +0200, Thomas Zimmermann wrote:
> 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.

yeah, a split would make it better rather than just make iosys_ptr
replace the current cases where we copy the struct. In this case most
places passing a buffer around in the same driver would migrate to
iosys_ptr.

thanks
Lucas De Marchi

> 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