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

[Thomas Zimmermann <tzimmermann@xxxxxxx>]

Am 29.03.23 um 11:04 schrieb Thomas Zimmermann:
> (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 

'prevent correct uses'

> unmap code.
>
> 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.
>
> 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

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