[PATCH v2] drm/i915/dmc: fix an unlikely NULL pointer deference at probe

[Imre Deak <imre.deak@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.

It is also strange to have DC6 enabled at boot, since that would require
the DMC firmware (loaded by BIOS); the BIOS loading the DMC firmware and
the driver stopping / reprogramming the firmware is a poorly specified
sequence and as such unlikely an intentional BIOS behaviour. It's more
likely that BIOS is leaving an unintentionally enabled DC6 HW state
behind (without actually loading the required DMC firmware for this).

The tracking of the DC6 allowed counter only works if starting /
stopping the counter depends on the _SW_ DC6 state vs. the current _HW_
DC6 state (since stopping the counter requires the DC5 counter captured
when the counter was started). Thus, using the HW DC6 state is incorrect
and it also leads to the above oops. Fix both issues by using the SW DC6
state for the tracking.

This is v2 of the fix originally sent by Jani, updated based on the
first References: discussion below.

Link: https://lore.kernel.org/all/3626411dc9e556452c432d0919821b76d9991217@intel.com
Link: https://lore.kernel.org/all/20260228130946.50919-2-ltao@redhat.com
Fixes: 88c1f9a4d36d ("drm/i915/dmc: Create debugfs entry for dc6 counter")
Cc: Mohammed Thasleem <mohammed.thasleem@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Tao Liu <ltao@redhat.com>
Cc: <stable@vger.kernel.org> # v6.16+
Signed-off-by: Imre Deak <imre.deak@intel.com>
---
 drivers/gpu/drm/i915/display/intel_display_power_well.c | 2 +-
 drivers/gpu/drm/i915/display/intel_dmc.c                | 3 +--
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_display_power_well.c b/drivers/gpu/drm/i915/display/intel_display_power_well.c
index 9c8d29839cafc..969b2c421d308 100644
--- a/drivers/gpu/drm/i915/display/intel_display_power_well.c
+++ b/drivers/gpu/drm/i915/display/intel_display_power_well.c
@@ -852,7 +852,7 @@ void gen9_set_dc_state(struct intel_display *display, u32 state)
 			power_domains->dc_state, val & mask);
 
 	enable_dc6 = state & DC_STATE_EN_UPTO_DC6;
-	dc6_was_enabled = val & DC_STATE_EN_UPTO_DC6;
+	dc6_was_enabled = power_domains->dc_state & DC_STATE_EN_UPTO_DC6;
 	if (!dc6_was_enabled && enable_dc6)
 		intel_dmc_update_dc6_allowed_count(display, true);
 
diff --git a/drivers/gpu/drm/i915/display/intel_dmc.c b/drivers/gpu/drm/i915/display/intel_dmc.c
index c3b411259a0c5..90ba932d940ac 100644
--- a/drivers/gpu/drm/i915/display/intel_dmc.c
+++ b/drivers/gpu/drm/i915/display/intel_dmc.c
@@ -1598,8 +1598,7 @@ static bool intel_dmc_get_dc6_allowed_count(struct intel_display *display, u32 *
 		return false;
 
 	mutex_lock(&power_domains->lock);
-	dc6_enabled = intel_de_read(display, DC_STATE_EN) &
-		      DC_STATE_EN_UPTO_DC6;
+	dc6_enabled = power_domains->dc_state & DC_STATE_EN_UPTO_DC6;
 	if (dc6_enabled)
 		intel_dmc_update_dc6_allowed_count(display, false);
 
-- 
2.49.1