From: Imre Deak Date: Mon, 9 Mar 2026 16:48:03 +0000 (+0200) Subject: drm/i915/dmc: Fix an unlikely NULL pointer deference at probe X-Git-Tag: v7.0-rc5~14^2~3^2~3 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ac57eb3b7d2ad649025b5a0fa207315f755ac4f6;p=thirdparty%2Fkernel%2Flinux.git drm/i915/dmc: Fix an unlikely NULL pointer deference at probe 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 Link: 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 Cc: Jani Nikula Cc: Tao Liu Cc: # v6.16+ Tested-by: Tao Liu Reviewed-by: Jani Nikula Signed-off-by: Imre Deak Link: https://patch.msgid.link/20260309164803.1918158-1-imre.deak@intel.com (cherry picked from commit 2344b93af8eb5da5d496b4e0529d35f0f559eaf0) Signed-off-by: Joonas Lahtinen --- 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 db185a859133..fba9fa41f827 100644 --- a/drivers/gpu/drm/i915/display/intel_display_power_well.c +++ b/drivers/gpu/drm/i915/display/intel_display_power_well.c @@ -806,7 +806,7 @@ 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 1006b060c3f3..0b15cb764b1d 100644 --- a/drivers/gpu/drm/i915/display/intel_dmc.c +++ b/drivers/gpu/drm/i915/display/intel_dmc.c @@ -1599,8 +1599,7 @@ 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);