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

[Sui Jingfeng]

On 2023/3/29 17:04, 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.
Yes, I just realized that.

iosys_map_incr() change the internal state of a opaque structure, this 
is somewhat evil.

if it is non-opaque, then this is abstract failure.

You have to worry about that if it is changed by a accident call 
iosys_map_incr() from other place.

The map should be const, I guess most programmer expect  the map be a const.

making it const please, copy on demand, modify the copy only, leave the 
original mapping untouched.

Hope this could eliminate the embarrassing.

Sorry for missing the point.

> 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 
> 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.
I have also get some idea from you idea.
> Best regards
> Thomas
>
> >         drm_client_buffer_vunmap(buffer);