On Mon, 02 Mar 2026, Imre Deak <[email protected]> wrote: > 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/[email protected] > Link: https://lore.kernel.org/all/[email protected] > Fixes: 88c1f9a4d36d ("drm/i915/dmc: Create debugfs entry for dc6 counter") > Cc: Mohammed Thasleem <[email protected]> > Cc: Jani Nikula <[email protected]> > Cc: Tao Liu <[email protected]> > Cc: <[email protected]> # v6.16+ > Signed-off-by: Imre Deak <[email protected]>
Reviewed-by: Jani Nikula <[email protected]> However, I still think the whole gen9_set_dc_state() is a bit fragile wrt DMC loaded or not. Pretty much everything else wraps the relevant parts within intel_dmc_has_payload(), and it's obvious what's going on. The comment for the function primarily talks about DMC but there's not even a mention of the possibility DMC is not loaded. I also think intel_dmc_update_dc6_allowed_count() is fragile in oopsing when DMC is not loaded, and I still think that should be fixed too. The patch at hand looks like it fixes the root cause, but I still think the parts around it could use some more robustness, if only to make it evident to the reader what the possible conditions are. BR, Jani. > --- > 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); -- Jani Nikula, Intel
