[PATCH] drm/i915/dmc: fix an unlikely NULL pointer deference at probe

[Jani Nikula <jani.nikula@intel.com>]

intel_dmc_update_dc6_allowed_count() oopses when DMC hasn't been
initialized, and dmc is thus NULL.

That would be the case when the call path is
intel_power_domains_init_hw() -> {skl,bxt,icl}_display_core_init() ->
gen9_set_dc_state() -> intel_dmc_update_dc6_allowed_count(), as
intel_power_domains_init_hw() is called *before* intel_dmc_init().

However, gen9_set_dc_state() calls intel_dmc_update_dc6_allowed_count()
conditionally, depending on the current and target DC states. At probe,
the target is disabled, but if DC6 is enabled, the function is called,
and an oops follows. Apparently it's quite unlikely that DC6 is enabled
at probe, as we haven't seen this failure mode before.

Add NULL checks and switch the dmc->display references to just display.

Fixes: 88c1f9a4d36d ("drm/i915/dmc: Create debugfs entry for dc6 counter")
Cc: Mohammed Thasleem <mohammed.thasleem@intel.com>
Cc: Imre Deak <imre.deak@intel.com>
Cc: <stable@vger.kernel.org> # v6.16+
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

---

Rare case, but this may also throw off the rc6 counting in debugfs when
it does happen.
---
 drivers/gpu/drm/i915/display/intel_dmc.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dmc.c b/drivers/gpu/drm/i915/display/intel_dmc.c
index 2fb6fec6dc99..169bbbc91f6d 100644
--- a/drivers/gpu/drm/i915/display/intel_dmc.c
+++ b/drivers/gpu/drm/i915/display/intel_dmc.c
@@ -1570,10 +1570,10 @@ void intel_dmc_update_dc6_allowed_count(struct intel_display *display,
 	struct intel_dmc *dmc = display_to_dmc(display);
 	u32 dc5_cur_count;
 
-	if (DISPLAY_VER(dmc->display) < 14)
+	if (!dmc || DISPLAY_VER(display) < 14)
 		return;
 
-	dc5_cur_count = intel_de_read(dmc->display, DG1_DMC_DEBUG_DC5_COUNT);
+	dc5_cur_count = intel_de_read(display, DG1_DMC_DEBUG_DC5_COUNT);
 
 	if (!start_tracking)
 		dmc->dc6_allowed.count += dc5_cur_count - dmc->dc6_allowed.dc5_start;
@@ -1587,7 +1587,7 @@ static bool intel_dmc_get_dc6_allowed_count(struct intel_display *display, u32 *
 	struct intel_dmc *dmc = display_to_dmc(display);
 	bool dc6_enabled;
 
-	if (DISPLAY_VER(display) < 14)
+	if (!dmc || DISPLAY_VER(display) < 14)
 		return false;
 
 	mutex_lock(&power_domains->lock);
-- 
2.47.3