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drm/fb-helper: Synchronize dirty worker with vblank

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Before updating the display from the console's shadow buffer, the dirty
worker now waits for a vblank. This allows several screen updates to pile
up and acts as a rate limiter. If a DRM master is present, it could
interfere with the vblank. Don't wait in this case.

v4:
	* share code with WAITFORVSYNC ioctl (Emil)
	* use lock guard
v3:
	* add back helper->lock
	* acquire DRM master status while waiting for vblank
v2:
	* don't hold helper->lock while waiting for vblank

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://lore.kernel.org/r/20250829091447.46719-1-tzimmermann@suse.de
This commit is contained in:
Thomas Zimmermann
2025-08-29 11:13:45 +02:00
parent cf207ea2c3
commit d8c4bddcd8
3 changed files with 51 additions and 24 deletions
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44
drivers/gpu/drm/drm_client_modeset.c
View File

@@ -1293,6 +1293,50 @@ int drm_client_modeset_dpms(struct drm_client_dev *client, int mode)
}
EXPORT_SYMBOL(drm_client_modeset_dpms);
/**
* drm_client_modeset_wait_for_vblank() - Wait for the next VBLANK to occur
* @client: DRM client
* @crtc_index: The ndex of the CRTC to wait on
*
* Block the caller until the given CRTC has seen a VBLANK. Do nothing
* if the CRTC is disabled. If there's another DRM master present, fail
* with -EBUSY.
*
* Returns:
* 0 on success, or negative error code otherwise.
*/
int drm_client_modeset_wait_for_vblank(struct drm_client_dev *client, unsigned int crtc_index)
{
struct drm_device *dev = client->dev;
struct drm_crtc *crtc;
int ret;
/*
* Rate-limit update frequency to vblank. If there's a DRM master
* present, it could interfere while we're waiting for the vblank
* event. Don't wait in this case.
*/
if (!drm_master_internal_acquire(dev))
return -EBUSY;
crtc = client->modesets[crtc_index].crtc;
/*
* Only wait for a vblank event if the CRTC is enabled, otherwise
* just don't do anything, not even report an error.
*/
ret = drm_crtc_vblank_get(crtc);
if (!ret) {
drm_crtc_wait_one_vblank(crtc);
drm_crtc_vblank_put(crtc);
}
drm_master_internal_release(dev);
return 0;
}
EXPORT_SYMBOL(drm_client_modeset_wait_for_vblank);
#ifdef CONFIG_DRM_KUNIT_TEST
#include "tests/drm_client_modeset_test.c"
#endif

30
drivers/gpu/drm/drm_fb_helper.c
View File

@@ -368,6 +368,10 @@ static void drm_fb_helper_fb_dirty(struct drm_fb_helper *helper)
unsigned long flags;
int ret;
mutex_lock(&helper->lock);
drm_client_modeset_wait_for_vblank(&helper->client, 0);
mutex_unlock(&helper->lock);
if (drm_WARN_ON_ONCE(dev, !helper->funcs->fb_dirty))
return;
@@ -1068,15 +1072,9 @@ int drm_fb_helper_ioctl(struct fb_info *info, unsigned int cmd,
unsigned long arg)
{
struct drm_fb_helper *fb_helper = info->par;
struct drm_device *dev = fb_helper->dev;
struct drm_crtc *crtc;
int ret = 0;
mutex_lock(&fb_helper->lock);
if (!drm_master_internal_acquire(dev)) {
ret = -EBUSY;
goto unlock;
}
guard(mutex)(&fb_helper->lock);
switch (cmd) {
case FBIO_WAITFORVSYNC:
@@ -1096,28 +1094,12 @@ int drm_fb_helper_ioctl(struct fb_info *info, unsigned int cmd,
* make. If we're not smart enough here, one should
* just consider switch the userspace to KMS.
*/
crtc = fb_helper->client.modesets[0].crtc;
/*
* Only wait for a vblank event if the CRTC is
* enabled, otherwise just don't do anythintg,
* not even report an error.
*/
ret = drm_crtc_vblank_get(crtc);
if (!ret) {
drm_crtc_wait_one_vblank(crtc);
drm_crtc_vblank_put(crtc);
}
ret = 0;
ret = drm_client_modeset_wait_for_vblank(&fb_helper->client, 0);
break;
default:
ret = -ENOTTY;
}
drm_master_internal_release(dev);
unlock:
mutex_unlock(&fb_helper->lock);
return ret;
}
EXPORT_SYMBOL(drm_fb_helper_ioctl);

1
include/drm/drm_client.h
View File

@@ -220,6 +220,7 @@ int drm_client_modeset_check(struct drm_client_dev *client);
int drm_client_modeset_commit_locked(struct drm_client_dev *client);
int drm_client_modeset_commit(struct drm_client_dev *client);
int drm_client_modeset_dpms(struct drm_client_dev *client, int mode);
int drm_client_modeset_wait_for_vblank(struct drm_client_dev *client, unsigned int crtc_index);
/**
* drm_client_for_each_modeset() - Iterate over client modesets