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

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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.

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.

Ok. that's fine then.

Best regards
Thomas

        drm_client_buffer_vunmap(buffer);




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