Hi
Am 09.09.25 um 05:29 schrieb Michael Kelley:
From: Michael Kelley Sent: Thursday, September 4, 2025 10:36 PM
From: Thomas Zimmermann <tzimmerm...@suse.de> Sent: Thursday, September 4, 2025
7:56 AM
Compositors often depend on vblanks to limit their display-update
rate. Without, they see vblank events ASAP, which breaks the rate-
limit feature. This creates high CPU overhead. It is especially a
problem with virtual devices with fast framebuffer access.
The series moves vkms' vblank timer to DRM and converts the hyperv
DRM driver. An earlier version of this series contains examples of
other updated drivers. In principle, any DRM driver without vblank
hardware can use the timer.
I've tested this patch set in a Hyper-V guest against the linux-next20250829
kernel. All looks good. Results and perf are the same as reported here [4].
So far I haven't seen the "vblank timer overrun" error, which is consistent
with the changes you made since my earlier testing. I'll keep running this
test kernel for a while to see if anything anomalous occurs.
As I continued to run with this patch set, I got a single occurrence of this
WARN_ON. I can't associate it with any particular action as I didn't notice
it until well after it occurred.
I've investigated. The stack trace comes from the kernel console's
display update. It runs concurrently to the vblank timeout. What likely
happens here is that the update code reads two values and in between,
the vblank timeout updates them. So the update then compares an old and
a new value; leading to an incorrect result with triggers the warning.
I'll include a fix in the series' next iteration. But I also think that
it's not critical. DRM's vblank helpers can usually deal with such problems.
Best regards
Thomas
[213730.719364] ------------[ cut here ]------------
[213730.719423] hyperv_drm 5620e0c7-8062-4dce-aeb7-520c7ef76171: [drm]
drm_WARN_ON(!ktime_compare(*vblank_time, vblank->time))
[213730.719522] WARNING: drivers/gpu/drm/drm_vblank.c:2309 at
drm_crtc_vblank_get_vblank_timeout+0x90/0xb0 [drm], CPU#4: kworker/4:0/7172
[213730.719871] Modules linked in: nls_iso8859_1(E) dm_multipath(E)
scsi_dh_rdac(E) scsi_dh_emc(E) scsi_dh_alua(E) binfmt_misc(E) intel_rapl_msr(E)
intel_rapl_common(E) rapl(E) hyperv_fb(E) cfbfillrect(E) cfbimgblt(E)
fb_io_fops(E) serio_raw(E) cfbcopyarea(E) hv_balloon(E) joydev(E) mac_hid(E)
sch_fq_codel(E) msr(E) ramoops(E) reed_solomon(E) efi_pstore(E) autofs4(E)
btrfs(E) blake2b_generic(E) raid10(E) raid456(E) async_raid6_recov(E)
async_memcpy(E) async_pq(E) async_xor(E) async_tx(E) xor(E) raid6_pq(E)
raid1(E) raid0(E) hyperv_drm(E) drm_client_lib(E) drm_shmem_helper(E)
syscopyarea(E) sysfillrect(E) sysimgblt(E) fb_sys_fops(E) drm_kms_helper(E)
drm(E) drm_panel_orientation_quirks(E) fb(E) hid_generic(E) hid_hyperv(E)
lcd(E) hid(E) hv_storvsc(E) ledtrig_backlight(E) hyperv_keyboard(E)
hv_netvsc(E) hv_utils(E) scsi_transport_fc(E) ghash_clmulni_intel(E)
hv_vmbus(E) aesni_intel(E)
[213730.720514] CPU: 4 UID: 0 PID: 7172 Comm: kworker/4:0 Kdump: loaded
Tainted: G E 6.17.0-rc3-next-20250829+ #3 PREEMPT(voluntary)
[213730.723220] Tainted: [E]=UNSIGNED_MODULE
[213730.724452] Hardware name: Microsoft Corporation Virtual Machine/Virtual
Machine, BIOS Hyper-V UEFI Release v4.1 11/21/2024
[213730.724993] Workqueue: events drm_fb_helper_damage_work [drm_kms_helper]
[213730.725491] RIP: 0010:drm_crtc_vblank_get_vblank_timeout+0x90/0xb0 [drm]
[213730.726082] Code: 8b 7f 08 4c 8b 67 50 4d 85 e4 74 33 e8 99 b6 7f c7 48 c7 c1 60
e8 93 c0 4c 89 e2 48 c7 c7 b5 25 94 c0 48 89 c6 e8 00 06 e3 c6 <0f> 0b eb c0 e8
07 f6 f1 c6 48 89 03 5b 41 5c 5d c3 cc cc cc cc 4c
[213730.726506] RSP: 0018:ffffbba54e0efc00 EFLAGS: 00010282
[213730.726692] RAX: 0000000000000000 RBX: ffffbba54e0efc78 RCX:
0000000000000027
[213730.726899] RDX: ffff954f07d1cec8 RSI: 0000000000000001 RDI:
ffff954f07d1cec0
[213730.727094] RBP: ffffbba54e0efc10 R08: 0000000000000000 R09:
ffffbba54e0efa70
[213730.727280] R10: ffffbba54e0efa68 R11: ffffffff88d4c748 R12:
ffff954e010a4cc0
[213730.727456] R13: 0000000000000000 R14: ffff954e20070d80 R15:
ffff954e251002c8
[213730.727636] FS: 0000000000000000(0000) GS:ffff954f7e938000(0000)
knlGS:0000000000000000
[213730.727834] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[213730.728009] CR2: 00007fe11629d010 CR3: 000000011f843004 CR4:
0000000000b70ef0
[213730.728186] Call Trace:
[213730.728359] <TASK>
[213730.728511]
drm_crtc_vblank_helper_get_vblank_timestamp_from_timer+0x15/0x20
[drm_kms_helper]
[213730.728674] drm_crtc_get_last_vbltimestamp+0x55/0x90 [drm]
[213730.728864] drm_crtc_next_vblank_start+0x4e/0x90 [drm]
[213730.729043] drm_atomic_helper_wait_for_fences+0x7c/0x1e0 [drm_kms_helper]
[213730.729196] drm_atomic_helper_commit+0xa1/0x160 [drm_kms_helper]
[213730.729335] drm_atomic_commit+0xb0/0xe0 [drm]
[213730.729481] ? __pfx___drm_printfn_info+0x10/0x10 [drm]
[213730.729643] drm_atomic_helper_dirtyfb+0x1aa/0x280 [drm_kms_helper]
[213730.729800] drm_fbdev_shmem_helper_fb_dirty+0x4c/0xb0 [drm_shmem_helper]
[213730.729939] drm_fb_helper_damage_work+0x8d/0x170 [drm_kms_helper]
[213730.730071] process_one_work+0x19c/0x3f0
[213730.730204] worker_thread+0x2c3/0x3d0
[213730.730332] kthread+0x116/0x230
[213730.730459] ? __pfx_worker_thread+0x10/0x10
[213730.730580] ? _raw_spin_unlock_irq+0x12/0x40
[213730.730744] ? __pfx_kthread+0x10/0x10
[213730.730898] ret_from_fork+0xec/0x130
[213730.731027] ? __pfx_kthread+0x10/0x10
[213730.731152] ret_from_fork_asm+0x1a/0x30
[213730.731277] </TASK>
[213730.731396] ---[ end trace 0000000000000000 ]---
For Patches 1, 2, and 4 of the series on a Hyper-V guest,
Tested-by: Michael Kelley <mhkli...@outlook.com>
[4] https://lore.kernel.org/dri-devel/20250523161522.409504-1-
mhkli...@outlook.com/T/#m2e288dddaf7b3c025bbbf88da4b4c39e7aa950a7
The series has been motivated by a recent discussion about hypervdrm [1]
and other long-standing bug reports. [2][3]
v3:
- fix deadlock (Ville, Lyude)
v2:
- rework interfaces
[1] https://lore.kernel.org/dri-devel/20250523161522.409504-1-
mhkli...@outlook.com/T/#ma2ebb52b60bfb0325879349377738fadcd7cb7ef
[2] https://bugzilla.suse.com/show_bug.cgi?id=1189174
[3] https://invent.kde.org/plasma/kwin/-/merge_requests/1229#note_284606
Thomas Zimmermann (4):
drm/vblank: Add vblank timer
drm/vblank: Add CRTC helpers for simple use cases
drm/vkms: Convert to DRM's vblank timer
drm/hypervdrm: Use vblank timer
Documentation/gpu/drm-kms-helpers.rst | 12 ++
drivers/gpu/drm/Makefile | 3 +-
drivers/gpu/drm/drm_vblank.c | 161 +++++++++++++++++-
drivers/gpu/drm/drm_vblank_helper.c | 176 ++++++++++++++++++++
drivers/gpu/drm/hyperv/hyperv_drm_modeset.c | 11 ++
drivers/gpu/drm/vkms/vkms_crtc.c | 83 +--------
drivers/gpu/drm/vkms/vkms_drv.h | 2 -
include/drm/drm_modeset_helper_vtables.h | 12 ++
include/drm/drm_vblank.h | 32 ++++
include/drm/drm_vblank_helper.h | 56 +++++++
10 files changed, 467 insertions(+), 81 deletions(-)
create mode 100644 drivers/gpu/drm/drm_vblank_helper.c
create mode 100644 include/drm/drm_vblank_helper.h
--
2.50.1
--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)
