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

[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

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
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