Re: [PATCH v2] drm/loongson: stop to set get_vblank_timestamp for LS7A1000 CRTC

[Icenowy Zheng]

在 2026-03-23一的 11:40 +0800，Jianmin Lv写道：
> 
> 
> On 2026/3/21 下午5:29, Icenowy Zheng wrote:
> > 在 2026-03-21六的 17:28 +0800，Icenowy Zheng写道：
> > > As there's no known hardware capability about querying vblank on
> > > the
> > > LS7A1000 display controller, setting get_vblank_timestamp will
> > > mislead the
> > > kernel about the support of DC-backed high precision vblank
> > > query.
> > > 
> > > Drop this function pointer in the CRTC function table for
> > > LS7A1000.
> > > 
> > > This solves a kernel warning when booting Linux 7.0-rc3 on a
> > > Loongson-3A4000+LS7A1000 Haier Boyue G51 laptop (with injected
> > > EDID
> > > for
> > > replicating the display timing set by the firmware).
> > > 
> The drm_crtc_vblank_helper_get_vblank_timestamp will call 
> get_scanout_position hook to caculate timestamp, and the driver has 
> get_scanout_position to support it (LS7A1000 has related register).
> So 
> the get_vblank_timestamp hook is ok for LS7A1000, and we'd better
> look 
> into the warning information.

The warning information is copied here:
```
[    0.334960] ------------[ cut here ]------------
[    0.334960] WARNING: drivers/gpu/drm/drm_vblank.c:747 at
drm_crtc_vblank_helper_get_vblank_timestamp_internal+0x3c8/0x438,
CPU#0: kworker/0:1/11
[    0.334960] loongson 0000:00:06.1: [drm]
drm_WARN_ON_ONCE(drm_drv_uses_atomic_modeset(dev))
[    0.334960] Modules linked in:
[    0.334960] CPU: 0 UID: 0 PID: 11 Comm: kworker/0:1 Not tainted
7.0.0-rc4-3a4000-laptop-test+ #31 PREEMPT 
[    0.334960] Hardware name: Haier Boyue G51/Boyue G51, BIOS
K3L3A4000.HR10.200909.T 09/09/2020
[    0.334960] Workqueue: events drm_fb_helper_damage_work
[    0.334960] Stack : ffffffff819dec03 0000000000000000
0000000000000018 9800000100223938
[    0.334960]         9800000100223938 9800000100223a68
0000000000000000 0000000000000000
[    0.334960]         b8e8b3f2ec6b8200 98000000092bffa8
9800000100223768 0000000000000000
[    0.334960]         ffffffff81632b08 0000000000000030
ffffffff813c9530 0000000000200b20
[    0.334960]         fffffffffffffffc 0000000000000000
0000000000000000 ffffffff816c1d50
[    0.334960]         ffffffff81890000 00000000000002eb
0000000000000000 ffffffff80c1b180
[    0.334960]         9800000100ff8500 0000000000000000
9800000005c965a8 0000000000000000
[    0.334960]         ffffffff81a70000 9800000100220000
9800000100223930 0000000000000000
[    0.334960]         ffffffff802274dc 0000000000000000
0000000000000000 0000000000000000
[    0.334960]         0000000000000000 0000000000000000
ffffffff802274fc b8e8b3f2ec6b8200
[    0.334960]         ...
[    0.334960] Call Trace:
[    0.334960] [<ffffffff802274fc>] show_stack+0x64/0x158
[    0.334960] [<ffffffff8021bac4>] dump_stack_lvl+0x70/0xb0
[    0.334960] [<ffffffff8026a988>] __warn+0xa8/0x120
[    0.334960] [<ffffffff8026ab60>] warn_slowpath_fmt+0x160/0x1f8
[    0.334960] [<ffffffff80be5668>]
drm_crtc_vblank_helper_get_vblank_timestamp_internal+0x3c8/0x438
[    0.334960] [<ffffffff80be48d8>]
drm_crtc_get_last_vbltimestamp+0x68/0xb0
[    0.334960] [<ffffffff80be5e14>] drm_update_vblank_count+0xac/0x418
[    0.334960] [<ffffffff80be6270>] drm_vblank_enable+0xf0/0x288
[    0.334960] [<ffffffff80be7010>] drm_vblank_get+0xf8/0x160
[    0.334960] [<ffffffff80beb2e0>]
drm_client_modeset_wait_for_vblank+0x50/0xa0
[    0.334960] [<ffffffff80c0a144>]
drm_fb_helper_damage_work+0x8c/0x1d8
[    0.334960] [<ffffffff8028fac8>] process_one_work+0x198/0x348
[    0.334960] [<ffffffff802906c0>] worker_thread+0x238/0x390
[    0.334960] [<ffffffff8029c0d8>] kthread+0x160/0x198
[    0.334960] [<ffffffff8021fd8c>] ret_from_kernel_thread+0x14/0x1c

[    0.334960] ---[ end trace 0000000000000000 ]---
```

> 
> > > Signed-off-by: Icenowy Zheng <zhengxingda@xxxxxxxxxxx>
> > 
> > Oh forgot to attach when crafting v2:
> > 
> > ```
> > Acked-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
> > ```
> > 
> > > ---
> > > Changes in v2:
> > > - Re-formatted Loongson product model numbers per request from
> > > Huacai.
> > > 
> > >   drivers/gpu/drm/loongson/lsdc_crtc.c | 1 -
> > >   1 file changed, 1 deletion(-)
> > > 
> > > diff --git a/drivers/gpu/drm/loongson/lsdc_crtc.c
> > > b/drivers/gpu/drm/loongson/lsdc_crtc.c
> > > index 587fbe285e9ef..b3af8e0cdb15f 100644
> > > --- a/drivers/gpu/drm/loongson/lsdc_crtc.c
> > > +++ b/drivers/gpu/drm/loongson/lsdc_crtc.c
> > > @@ -721,7 +721,6 @@ static const struct drm_crtc_funcs
> > > ls7a1000_crtc_funcs = {
> > >   	.late_register = lsdc_crtc_late_register,
> > >   	.enable_vblank = lsdc_crtc_enable_vblank,
> > >   	.disable_vblank = lsdc_crtc_disable_vblank,
> > > -	.get_vblank_timestamp =
> > > drm_crtc_vblank_helper_get_vblank_timestamp,
> > >   	.atomic_print_state = lsdc_crtc_atomic_print_state,
> > >   };
> > >