On Sat, 23 Apr 2022 at 03:12, Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx> wrote:
>
>
>
> On 4/22/2022 5:07 PM, Dmitry Baryshkov wrote:
> > On 23/04/2022 02:45, Kuogee Hsieh wrote:
> >> Current DP driver implementation has adding safe mode done at
> >> dp_hpd_plug_handle() which is expected to be executed under event
> >> thread context.
> >>
> >> However there is possible circular locking happen (see blow stack trace)
> >> after edp driver call dp_hpd_plug_handle() from dp_bridge_enable() which
> >> is executed under drm_thread context.
> >>
> >> To break this circular locking, this patch have safe mode added at
> >> dp_connector_get_mode() which is executed under drm thread context.
> >> Therefore no lock acquired required for &dev->mode_config.mutex while
> >> adding fail safe mode since it has been hold by drm thread already.
> >>
> >> ======================================================
> >> WARNING: possible circular locking dependency detected
> >> 5.15.35-lockdep #6 Tainted: G W
> >> ------------------------------------------------------
> >> frecon/429 is trying to acquire lock:
> >> ffffff808dc3c4e8 (&dev->mode_config.mutex){+.+.}-{3:3}, at:
> >> dp_panel_add_fail_safe_mode+0x4c/0xa0
> >>
> >> but task is already holding lock:
> >> ffffff808dc441e0 (&kms->commit_lock[i]){+.+.}-{3:3}, at:
> >> lock_crtcs+0xb4/0x124
> >>
> >> which lock already depends on the new lock.
> >>
> >> the existing dependency chain (in reverse order) is:
> >>
> >> -> #3 (&kms->commit_lock[i]){+.+.}-{3:3}:
> >> __mutex_lock_common+0x174/0x1a64
> >> mutex_lock_nested+0x98/0xac
> >> lock_crtcs+0xb4/0x124
> >> msm_atomic_commit_tail+0x330/0x748
> >> commit_tail+0x19c/0x278
> >> drm_atomic_helper_commit+0x1dc/0x1f0
> >> drm_atomic_commit+0xc0/0xd8
> >> drm_atomic_helper_set_config+0xb4/0x134
> >> drm_mode_setcrtc+0x688/0x1248
> >> drm_ioctl_kernel+0x1e4/0x338
> >> drm_ioctl+0x3a4/0x684
> >> __arm64_sys_ioctl+0x118/0x154
> >> invoke_syscall+0x78/0x224
> >> el0_svc_common+0x178/0x200
> >> do_el0_svc+0x94/0x13c
> >> el0_svc+0x5c/0xec
> >> el0t_64_sync_handler+0x78/0x108
> >> el0t_64_sync+0x1a4/0x1a8
> >>
> >> -> #2 (crtc_ww_class_mutex){+.+.}-{3:3}:
> >> __mutex_lock_common+0x174/0x1a64
> >> ww_mutex_lock+0xb8/0x278
> >> modeset_lock+0x304/0x4ac
> >> drm_modeset_lock+0x4c/0x7c
> >> drmm_mode_config_init+0x4a8/0xc50
> >> msm_drm_init+0x274/0xac0
> >> msm_drm_bind+0x20/0x2c
> >> try_to_bring_up_master+0x3dc/0x470
> >> __component_add+0x18c/0x3c0
> >> component_add+0x1c/0x28
> >> dp_display_probe+0x954/0xa98
> >> platform_probe+0x124/0x15c
> >> really_probe+0x1b0/0x5f8
> >> __driver_probe_device+0x174/0x20c
> >> driver_probe_device+0x70/0x134
> >> __device_attach_driver+0x130/0x1d0
> >> bus_for_each_drv+0xfc/0x14c
> >> __device_attach+0x1bc/0x2bc
> >> device_initial_probe+0x1c/0x28
> >> bus_probe_device+0x94/0x178
> >> deferred_probe_work_func+0x1a4/0x1f0
> >> process_one_work+0x5d4/0x9dc
> >> worker_thread+0x898/0xccc
> >> kthread+0x2d4/0x3d4
> >> ret_from_fork+0x10/0x20
> >>
> >> -> #1 (crtc_ww_class_acquire){+.+.}-{0:0}:
> >> ww_acquire_init+0x1c4/0x2c8
> >> drm_modeset_acquire_init+0x44/0xc8
> >> drm_helper_probe_single_connector_modes+0xb0/0x12dc
> >> drm_mode_getconnector+0x5dc/0xfe8
> >> drm_ioctl_kernel+0x1e4/0x338
> >> drm_ioctl+0x3a4/0x684
> >> __arm64_sys_ioctl+0x118/0x154
> >> invoke_syscall+0x78/0x224
> >> el0_svc_common+0x178/0x200
> >> do_el0_svc+0x94/0x13c
> >> el0_svc+0x5c/0xec
> >> el0t_64_sync_handler+0x78/0x108
> >> el0t_64_sync+0x1a4/0x1a8
> >>
> >> -> #0 (&dev->mode_config.mutex){+.+.}-{3:3}:
> >> __lock_acquire+0x2650/0x672c
> >> lock_acquire+0x1b4/0x4ac
> >> __mutex_lock_common+0x174/0x1a64
> >> mutex_lock_nested+0x98/0xac
> >> dp_panel_add_fail_safe_mode+0x4c/0xa0
> >> dp_hpd_plug_handle+0x1f0/0x280
> >> dp_bridge_enable+0x94/0x2b8
> >> drm_atomic_bridge_chain_enable+0x11c/0x168
> >> drm_atomic_helper_commit_modeset_enables+0x500/0x740
> >> msm_atomic_commit_tail+0x3e4/0x748
> >> commit_tail+0x19c/0x278
> >> drm_atomic_helper_commit+0x1dc/0x1f0
> >> drm_atomic_commit+0xc0/0xd8
> >> drm_atomic_helper_set_config+0xb4/0x134
> >> drm_mode_setcrtc+0x688/0x1248
> >> drm_ioctl_kernel+0x1e4/0x338
> >> drm_ioctl+0x3a4/0x684
> >> __arm64_sys_ioctl+0x118/0x154
> >> invoke_syscall+0x78/0x224
> >> el0_svc_common+0x178/0x200
> >> do_el0_svc+0x94/0x13c
> >> el0_svc+0x5c/0xec
> >> el0t_64_sync_handler+0x78/0x108
> >> el0t_64_sync+0x1a4/0x1a8
> >>
> >> Signed-off-by: Kuogee Hsieh <quic_khsieh@xxxxxxxxxxx>
> >> ---
> >> drivers/gpu/drm/msm/dp/dp_display.c | 6 ------
> >> drivers/gpu/drm/msm/dp/dp_panel.c | 23 +++++++++++++----------
> >> 2 files changed, 13 insertions(+), 16 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/msm/dp/dp_display.c
> >> b/drivers/gpu/drm/msm/dp/dp_display.c
> >> index 92cd50f..01453db 100644
> >> --- a/drivers/gpu/drm/msm/dp/dp_display.c
> >> +++ b/drivers/gpu/drm/msm/dp/dp_display.c
> >> @@ -555,12 +555,6 @@ static int dp_hpd_plug_handle(struct
> >> dp_display_private *dp, u32 data)
> >> mutex_unlock(&dp->event_mutex);
> >> - /*
> >> - * add fail safe mode outside event_mutex scope
> >> - * to avoid potiential circular lock with drm thread
> >> - */
> >> - dp_panel_add_fail_safe_mode(dp->dp_display.connector);
> >> -
> >> /* uevent will complete connection part */
> >> return 0;
> >> };
> >> diff --git a/drivers/gpu/drm/msm/dp/dp_panel.c
> >> b/drivers/gpu/drm/msm/dp/dp_panel.c
> >> index 1aa9aa8c..23fee42 100644
> >> --- a/drivers/gpu/drm/msm/dp/dp_panel.c
> >> +++ b/drivers/gpu/drm/msm/dp/dp_panel.c
> >> @@ -151,15 +151,6 @@ static int dp_panel_update_modes(struct
> >> drm_connector *connector,
> >> return rc;
> >> }
> >> -void dp_panel_add_fail_safe_mode(struct drm_connector *connector)
> >> -{
> >> - /* fail safe edid */
> >> - mutex_lock(&connector->dev->mode_config.mutex);
> >> - if (drm_add_modes_noedid(connector, 640, 480))
> >> - drm_set_preferred_mode(connector, 640, 480);
> >> - mutex_unlock(&connector->dev->mode_config.mutex);
> >> -}
> >> -
> >> int dp_panel_read_sink_caps(struct dp_panel *dp_panel,
> >> struct drm_connector *connector)
> >> {
> >> @@ -216,7 +207,11 @@ int dp_panel_read_sink_caps(struct dp_panel
> >> *dp_panel,
> >> goto end;
> >> }
> >> - dp_panel_add_fail_safe_mode(connector);
> >> + /* fail safe edid */
> >> + mutex_lock(&connector->dev->mode_config.mutex);
> >> + if (drm_add_modes_noedid(connector, 640, 480))
> >> + drm_set_preferred_mode(connector, 640, 480);
> >> + mutex_unlock(&connector->dev->mode_config.mutex);
> >> }
> >> if (panel->aux_cfg_update_done) {
> >> @@ -266,6 +261,14 @@ int dp_panel_get_modes(struct dp_panel *dp_panel,
> >> return -EINVAL;
> >> }
> >> + /*
> >> + * add fail safe mode (640x480) here
> >> + * since we are executed in drm_thread context,
> >> + * no mode_config.mutex acquired required
> >> + */
> >> + if (drm_add_modes_noedid(connector, 640, 480))
> >> + drm_set_preferred_mode(connector, 640, 480);
> >> +
> >> if (dp_panel->edid)
> >> return dp_panel_update_modes(connector, dp_panel->edid);
> > Also, wouldn't calling get_modes() several times make cause adding more
> > and more 640x480 modes to the modes list?
> >
>
> Shouldnt DRM be blocking that here? Call should trickle down here only
> if count_modes was 0
>
> if (out_resp->count_modes == 0) {
> if (is_current_master)
> connector->funcs->fill_modes(connector,
> dev->mode_config.max_width,
> dev->mode_config.max_height);
> else
> drm_dbg_kms(dev, "User-space requested a forced probe on
> [CONNECTOR:%d:%s] but is not the DRM master, demoting to read-only probe",
> connector->base.id, connector->name);
> }
>
count_modes is set by userspace:
/*
* This ioctl is called twice, once to determine how much space is
* needed, and the 2nd time to fill it.
*/
So, nothing prevents userspace from passing zero count_mode more than once.
However drm_helper_probe_single_connector_modes() will set old modes
to MODE_STALE and then will call get_modes().
Then drm_mode_prune_invalid() will prune stale modes. So, this should be fine.
A more generic question is why do we need to add the mode in two places?
--
With best wishes
Dmitry
