drm/amdgpu: Don't hang in amdgpu_flip_work_func on disabled crtc.

[Dan Carpenter]

Hello Mario Kleiner,

The patch e1d09dc0ccc6: "drm/amdgpu: Don't hang in
amdgpu_flip_work_func on disabled crtc." from Feb 19, 2016, leads to
the following static checker warning:

drivers/gpu/drm/amd/amdgpu/amdgpu_display.c:127 amdgpu_flip_work_func()	warn: should this be 'repcnt == -1'
drivers/gpu/drm/amd/amdgpu/amdgpu_display.c:136 amdgpu_flip_work_func() error: double unlock 'spin_lock:&crtc->dev->event_lock'
drivers/gpu/drm/amd/amdgpu/amdgpu_display.c:136 amdgpu_flip_work_func() error: double unlock 'irqsave:flags'


drivers/gpu/drm/amd/amdgpu/amdgpu_display.c
    64  static void amdgpu_flip_work_func(struct work_struct *__work)
    65  {
    66          struct amdgpu_flip_work *work =
    67                  container_of(__work, struct amdgpu_flip_work, flip_work);
    68          struct amdgpu_device *adev = work->adev;
    69          struct amdgpu_crtc *amdgpuCrtc = adev->mode_info.crtcs[work->crtc_id];
    70  
    71          struct drm_crtc *crtc = &amdgpuCrtc->base;
    72          unsigned long flags;
    73          unsigned i, repcnt = 4;
    74          int vpos, hpos, stat, min_udelay = 0;
    75          struct drm_vblank_crtc *vblank = &crtc->dev->vblank[work->crtc_id];
    76  
    77          if (amdgpu_flip_handle_fence(work, &work->excl))
    78                  return;
    79  
    80          for (i = 0; i < work->shared_count; ++i)
    81                  if (amdgpu_flip_handle_fence(work, &work->shared[i]))
    82                          return;
    83  
    84          /* We borrow the event spin lock for protecting flip_status */
    85          spin_lock_irqsave(&crtc->dev->event_lock, flags);
    86  
    87          /* If this happens to execute within the "virtually extended" vblank
    88           * interval before the start of the real vblank interval then it needs
    89           * to delay programming the mmio flip until the real vblank is entered.
    90           * This prevents completing a flip too early due to the way we fudge
    91           * our vblank counter and vblank timestamps in order to work around the
    92           * problem that the hw fires vblank interrupts before actual start of
    93           * vblank (when line buffer refilling is done for a frame). It
    94           * complements the fudging logic in amdgpu_get_crtc_scanoutpos() for
    95           * timestamping and amdgpu_get_vblank_counter_kms() for vblank counts.
    96           *
    97           * In practice this won't execute very often unless on very fast
    98           * machines because the time window for this to happen is very small.
    99           */
   100          while (amdgpuCrtc->enabled && repcnt--) {
                                              ^^^^^^^^
Exists the loop with spin_lock held and repcnt == -1.


   101                  /* GET_DISTANCE_TO_VBLANKSTART returns distance to real vblank
   102                   * start in hpos, and to the "fudged earlier" vblank start in
   103                   * vpos.
   104                   */
   105                  stat = amdgpu_get_crtc_scanoutpos(adev->ddev, work->crtc_id,
   106                                                    GET_DISTANCE_TO_VBLANKSTART,
   107                                                    &vpos, &hpos, NULL, NULL,
   108                                                    &crtc->hwmode);
   109  
   110                  if ((stat & (DRM_SCANOUTPOS_VALID | DRM_SCANOUTPOS_ACCURATE)) !=
   111                      (DRM_SCANOUTPOS_VALID | DRM_SCANOUTPOS_ACCURATE) ||
   112                      !(vpos >= 0 && hpos <= 0))
   113                          break;
   114  
   115                  /* Sleep at least until estimated real start of hw vblank */
   116                  spin_unlock_irqrestore(&crtc->dev->event_lock, flags);
   117                  min_udelay = (-hpos + 1) * max(vblank->linedur_ns / 1000, 5);
   118                  if (min_udelay > vblank->framedur_ns / 2000) {
   119                          /* Don't wait ridiculously long - something is wrong */
   120                          repcnt = 0;

Exit with spin_lock released and repcnt == 0.

   121                          break;
   122                  }
   123                  usleep_range(min_udelay, 2 * min_udelay);
   124                  spin_lock_irqsave(&crtc->dev->event_lock, flags);
   125          };
   126  
   127          if (!repcnt)
                     ^^^^^^
Assumes exit with zero.

   128                  DRM_DEBUG_DRIVER("Delay problem on crtc %d: min_udelay %d, "
   129                                   "framedur %d, linedur %d, stat %d, vpos %d, "
   130                                   "hpos %d\n", work->crtc_id, min_udelay,
   131                                   vblank->framedur_ns / 1000,
   132                                   vblank->linedur_ns / 1000, stat, vpos, hpos);
   133  
   134          /* set the flip status */
   135          amdgpuCrtc->pflip_status = AMDGPU_FLIP_SUBMITTED;
   136          spin_unlock_irqrestore(&crtc->dev->event_lock, flags);

Assumes lock held.

   137  
   138          /* Do the flip (mmio) */
   139          adev->mode_info.funcs->page_flip(adev, work->crtc_id, work->base);
   140  }

regards,
dan carpenter