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

[Sui Jingfeng]

On 2023/3/30 15:26, Thomas Zimmermann wrote:
> Hi
>
> Am 30.03.23 um 09:17 schrieb Sui Jingfeng:
> > Hi,
> >
> > On 2023/3/30 14:57, 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.
> >   Here i fix a typo, 'tine' -> 'time'
> >
> > As far as i can see, they are two major type of memory in the system.
> >
> > System memory or VRAM,  for the gpu with dedicate video ram, VRAM is 
> > belong to the IO memory category.
> >
> > But there are system choose carveout part of system ram as video 
> > ram(i915?,  for example).
> >
> > the name iosys_map and iosys_ptr have no difference at the first 
> > sight, tell me which one is for mapping system ram
> >
> > and which one is for mapping vram?
>
> As you say correctly, graphics buffers and be in various locations. 
> They can even move between I/O and system memory. The idea behind 
> iosys_map ("I/O and/or system mapping") is that it's a single 
> interface that can handle both.
I somewhat miss the point, sound like const pointer(const void* const p) 
V.S. plain pointer (void *)

I understand what you meant then.


> Best regards
> Thomas
>
> > > 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