[PATCH v4] drm/vmwgfx: Treat zero SVGA_REG_CURSOR_MAX_DIMENSION as unset

[fausten <yunfeng.li.nb@gmail.com>]

The cursor plane code validates cursor dimensions against
SVGA_REG_CURSOR_MAX_DIMENSION before every cursor update, and
rejects the update with -EINVAL if the cursor is larger than the
reported maximum.

However, this register is only available on hardware versions 18
and above, which introduced cursor MOBs. Older hardware (e.g. a
VMware Fusion guest with SVGA version 2) returns 0 for it.
In that case every cursor update is rejected,
and the log is spammed with:

  [drm] Cursor dimensions (64, 64) exceed device max 0

The visible symptom is that the hardware cursor never appears at
all on VMware Fusion guests (SVGA version 2), making the mouse
pointer invisible even though the input devices work fine.

Treat a reported maximum of 0 as "not implemented" and skip the
dimension check in that case, restoring the pre-existing behaviour
of accepting the cursor.

Tested on VMware Fusion with an SVGA version 2 device where the
cursor previously did not show up.

Fixes: d5ed8749168a ("drm/vmwgfx: enforce cursor size limits for MOB cursors")
Signed-off-by: Yunfeng Li <yunfeng.li.nb@gmail.com>
---
 drivers/gpu/drm/vmwgfx/vmwgfx_cursor_plane.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

---
Changes in v3:
- Use an explicit cursor_max_dim > 0 comparison (Maaz).
- Fix the comments: the register requires cursor MOBs which exist
  only on hw version 18 and above; it is not Fusion-specific
  (Maaz).

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_cursor_plane.c b/drivers/gpu/drm/vmwgfx/vmwgfx_cursor_plane.c
index d1e7df500..3d4660684 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_cursor_plane.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_cursor_plane.c
@@ -237,8 +237,10 @@ static int vmw_cursor_mob_get(struct vmw_cursor_plane *vcp,
 	mob_max_size = vmw_read(dev_priv, SVGA_REG_MOB_MAX_SIZE);
 	cursor_max_dim = vmw_read(dev_priv, SVGA_REG_CURSOR_MAX_DIMENSION);
 
-	if (size > mob_max_size || vps->base.crtc_w > cursor_max_dim ||
-	    vps->base.crtc_h > cursor_max_dim)
+	/* Cursor MOBs, and thus this register, exist only on hw version 18+. */
+	if (cursor_max_dim > 0 &&
+	    (size > mob_max_size || vps->base.crtc_w > cursor_max_dim ||
+	     vps->base.crtc_h > cursor_max_dim))
 		return -EINVAL;
 
 	if (vps->cursor.mob) {
@@ -748,8 +750,10 @@ int vmw_cursor_plane_atomic_check(struct drm_plane *plane,
 		u32 cursor_max_dim =
 			vmw_read(vmw, SVGA_REG_CURSOR_MAX_DIMENSION);
 
-		if (new_state->crtc_w > cursor_max_dim ||
-		    new_state->crtc_h > cursor_max_dim) {
+		/* Cursor MOBs, and thus this register, exist only on hw version 18+. */
+		if (cursor_max_dim > 0 &&
+		    (new_state->crtc_w > cursor_max_dim ||
+		     new_state->crtc_h > cursor_max_dim)) {
 			drm_warn(&vmw->drm,
 				 "Cursor dimensions (%d, %d) exceed device max %u\n",
 				 new_state->crtc_w, new_state->crtc_h,
-- 
2.33.0