Re: [PATCH 6/6] drm/gud: Use the shadow plane helper

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Hi

Am 22.11.22 um 21:58 schrieb Noralf Trønnes via B4 Submission Endpoint:
From: Noralf Trønnes <noralf@xxxxxxxxxxx>

Use the shadow plane helper to take care of preparing the framebuffer for
CPU access. The synchronous flushing is now done inline without the use of
a worker. The async path now uses a shadow buffer to hold framebuffer
changes and it doesn't read the framebuffer behind userspace's back
anymore.

Signed-off-by: Noralf Trønnes <noralf@xxxxxxxxxxx>
---
  drivers/gpu/drm/gud/gud_drv.c      |  1 +
  drivers/gpu/drm/gud/gud_internal.h |  1 +
  drivers/gpu/drm/gud/gud_pipe.c     | 69 ++++++++++++++++++++++++--------------
  3 files changed, 46 insertions(+), 25 deletions(-)

diff --git a/drivers/gpu/drm/gud/gud_drv.c b/drivers/gpu/drm/gud/gud_drv.c
index d57dab104358..5aac7cda0505 100644
--- a/drivers/gpu/drm/gud/gud_drv.c
+++ b/drivers/gpu/drm/gud/gud_drv.c
@@ -365,6 +365,7 @@ static void gud_debugfs_init(struct drm_minor *minor)
  static const struct drm_simple_display_pipe_funcs gud_pipe_funcs = {
  	.check      = gud_pipe_check,
  	.update	    = gud_pipe_update,
+	DRM_GEM_SIMPLE_DISPLAY_PIPE_SHADOW_PLANE_FUNCS
  };
static const struct drm_mode_config_funcs gud_mode_config_funcs = {
diff --git a/drivers/gpu/drm/gud/gud_internal.h b/drivers/gpu/drm/gud/gud_internal.h
index e351a1f1420d..0d148a6f27aa 100644
--- a/drivers/gpu/drm/gud/gud_internal.h
+++ b/drivers/gpu/drm/gud/gud_internal.h
@@ -43,6 +43,7 @@ struct gud_device {
  	struct drm_framebuffer *fb;
  	struct drm_rect damage;
  	bool prev_flush_failed;
+	void *shadow_buf;
  };
static inline struct gud_device *to_gud_device(struct drm_device *drm)
diff --git a/drivers/gpu/drm/gud/gud_pipe.c b/drivers/gpu/drm/gud/gud_pipe.c
index dfada6eedc58..7686325f7ee7 100644
--- a/drivers/gpu/drm/gud/gud_pipe.c
+++ b/drivers/gpu/drm/gud/gud_pipe.c
@@ -358,10 +358,10 @@ static void gud_flush_damage(struct gud_device *gdrm, struct drm_framebuffer *fb
  void gud_flush_work(struct work_struct *work)
  {
  	struct gud_device *gdrm = container_of(work, struct gud_device, work);
-	struct iosys_map gem_map = { }, fb_map = { };
+	struct iosys_map shadow_map;
  	struct drm_framebuffer *fb;
  	struct drm_rect damage;
-	int idx, ret;
+	int idx;
if (!drm_dev_enter(&gdrm->drm, &idx))
  		return;
@@ -369,6 +369,7 @@ void gud_flush_work(struct work_struct *work)
  	mutex_lock(&gdrm->damage_lock);
  	fb = gdrm->fb;
  	gdrm->fb = NULL;
+	iosys_map_set_vaddr(&shadow_map, gdrm->shadow_buf);
  	damage = gdrm->damage;
  	gud_clear_damage(gdrm);
  	mutex_unlock(&gdrm->damage_lock);
@@ -376,33 +377,33 @@ void gud_flush_work(struct work_struct *work)
  	if (!fb)
  		goto out;
- ret = drm_gem_fb_vmap(fb, &gem_map, &fb_map);
-	if (ret)
-		goto fb_put;
+	gud_flush_damage(gdrm, fb, &shadow_map, true, &damage);
- ret = drm_gem_fb_begin_cpu_access(fb, DMA_FROM_DEVICE);
-	if (ret)
-		goto vunmap;
-
-	/* Imported buffers are assumed to be WriteCombined with uncached reads */
-	gud_flush_damage(gdrm, fb, &fb_map, !fb->obj[0]->import_attach, &damage);
-
-	drm_gem_fb_end_cpu_access(fb, DMA_FROM_DEVICE);
-vunmap:
-	drm_gem_fb_vunmap(fb, &gem_map);
-fb_put:
  	drm_framebuffer_put(fb);
  out:
  	drm_dev_exit(idx);
  }
-static void gud_fb_queue_damage(struct gud_device *gdrm, struct drm_framebuffer *fb,
-				struct drm_rect *damage)
+static int gud_fb_queue_damage(struct gud_device *gdrm, struct drm_framebuffer *fb,
+			       const struct iosys_map *map, struct drm_rect *damage)
  {
  	struct drm_framebuffer *old_fb = NULL;
+	struct iosys_map shadow_map;
mutex_lock(&gdrm->damage_lock); + if (!gdrm->shadow_buf) {
+		gdrm->shadow_buf = vzalloc(fb->pitches[0] * fb->height);
+		if (!gdrm->shadow_buf) {
+			mutex_unlock(&gdrm->damage_lock);
+			return -ENOMEM;
+		}
+	}
+
+	iosys_map_set_vaddr(&shadow_map, gdrm->shadow_buf);
+	iosys_map_incr(&shadow_map, drm_fb_clip_offset(fb->pitches[0], fb->format, damage));
+	drm_fb_memcpy(&shadow_map, fb->pitches, map, fb, damage);
+

It's all good (sans the begin_cpu_access issue) in terms of code, but the memcpy here can have quite an impact on performace.

So why not just nuke the async code entirely? It's a userspace problem and we don't have many(/any?) other drivers with such a workaround. Udl, our other usb-based driver, works well enough without.

Best regards
Thomas

  	if (fb != gdrm->fb) {
  		old_fb = gdrm->fb;
  		drm_framebuffer_get(fb);
@@ -420,6 +421,26 @@ static void gud_fb_queue_damage(struct gud_device *gdrm, struct drm_framebuffer
if (old_fb)
  		drm_framebuffer_put(old_fb);
+
+	return 0;
+}
+
+static void gud_fb_handle_damage(struct gud_device *gdrm, struct drm_framebuffer *fb,
+				 const struct iosys_map *map, struct drm_rect *damage)
+{
+	int ret;
+
+	if (gdrm->flags & GUD_DISPLAY_FLAG_FULL_UPDATE)
+		drm_rect_init(damage, 0, 0, fb->width, fb->height);
+
+	if (gud_async_flush) {
+		ret = gud_fb_queue_damage(gdrm, fb, map, damage);
+		if (ret != -ENOMEM)
+			return;
+	}
+
+	/* Imported buffers are assumed to be WriteCombined with uncached reads */
+	gud_flush_damage(gdrm, fb, map, !fb->obj[0]->import_attach, damage);
  }
int gud_pipe_check(struct drm_simple_display_pipe *pipe,
@@ -544,6 +565,7 @@ void gud_pipe_update(struct drm_simple_display_pipe *pipe,
  	struct drm_device *drm = pipe->crtc.dev;
  	struct gud_device *gdrm = to_gud_device(drm);
  	struct drm_plane_state *state = pipe->plane.state;
+	struct drm_shadow_plane_state *shadow_plane_state = to_drm_shadow_plane_state(state);
  	struct drm_framebuffer *fb = state->fb;
  	struct drm_crtc *crtc = &pipe->crtc;
  	struct drm_rect damage;
@@ -557,6 +579,8 @@ void gud_pipe_update(struct drm_simple_display_pipe *pipe,
  			gdrm->fb = NULL;
  		}
  		gud_clear_damage(gdrm);
+		vfree(gdrm->shadow_buf);
+		gdrm->shadow_buf = NULL;
  		mutex_unlock(&gdrm->damage_lock);
  	}
@@ -572,13 +596,8 @@ void gud_pipe_update(struct drm_simple_display_pipe *pipe,
  	if (crtc->state->active_changed)
  		gud_usb_set_u8(gdrm, GUD_REQ_SET_DISPLAY_ENABLE, crtc->state->active);
- if (drm_atomic_helper_damage_merged(old_state, state, &damage)) {
-		if (gdrm->flags & GUD_DISPLAY_FLAG_FULL_UPDATE)
-			drm_rect_init(&damage, 0, 0, fb->width, fb->height);
-		gud_fb_queue_damage(gdrm, fb, &damage);
-		if (!gud_async_flush)
-			flush_work(&gdrm->work);
-	}
+	if (drm_atomic_helper_damage_merged(old_state, state, &damage))
+		gud_fb_handle_damage(gdrm, fb, &shadow_plane_state->data[0], &damage);
if (!crtc->state->enable)
  		gud_usb_set_u8(gdrm, GUD_REQ_SET_CONTROLLER_ENABLE, 0);


--
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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