Re: [PATCH] drm/i915: Avoid divbyzero in modesetting

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 22/02/16 13:09, Maarten Lankhorst wrote:
> Hey,
> 
> Op 22-02-16 om 12:52 schreef Tvrtko Ursulin:
>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>
>> Not sure if intel_wm_config->num_pipes_active is supposed to
>> ever be zero when intel_update_watermarks gets called. But
>> since it can be triggered in early platform bringup perhaps
>> we want to guard against it rather than divide by zero.
>>
>> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>> Cc: Matt Roper <matthew.d.roper@intel.com>
>> Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
>> Cc: Daniel Vetter <daniel@ffwll.ch>
>> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
>> ---
>>   drivers/gpu/drm/i915/intel_pm.c | 9 +++++++--
>>   1 file changed, 7 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/i915/intel_pm.c b/drivers/gpu/drm/i915/intel_pm.c
>> index feb57598727a..2b7998889617 100644
>> --- a/drivers/gpu/drm/i915/intel_pm.c
>> +++ b/drivers/gpu/drm/i915/intel_pm.c
>> @@ -2831,8 +2831,13 @@ skl_ddb_get_pipe_allocation_limits(struct drm_device *dev,
>>   		nth_active_pipe++;
>>   	}
>>   
>> -	pipe_size = ddb_size / config->num_pipes_active;
>> -	alloc->start = nth_active_pipe * ddb_size / config->num_pipes_active;
>> +	if (WARN_ON(!config->num_pipes_active)) {
>> +		pipe_size = 0;
>> +		alloc->start = 0;
>> +	} else {
>> +		pipe_size = ddb_size / config->num_pipes_active;
>> +		alloc->start = nth_active_pipe * ddb_size / config->num_pipes_active;
>> +	}
>>   	alloc->end = alloc->start + pipe_size;
>>   }
>>   
> How can this happen? It seems in that case cstate->base.active would be false for the current pipe,
> and the code should bail early already..

Don't know, but does it harm to guard against it? Helps early
platform bringup a bit.

 [drm:intel_modeset_readout_hw_state] [CRTC:21] hw state readout: enabled
 [drm:intel_modeset_readout_hw_state] [CRTC:26] hw state readout: disabled
 [drm:intel_modeset_readout_hw_state] [CRTC:31] hw state readout: disabled
 [drm:intel_modeset_readout_hw_state] PORT PLL A hw state readout: crtc_mask 0x00000001, on 0
 [drm:intel_modeset_readout_hw_state] PORT PLL B hw state readout: crtc_mask 0x00000000, on 0
 [drm:intel_modeset_readout_hw_state] PORT PLL C hw state readout: crtc_mask 0x00000000, on 0
 [drm:intel_modeset_readout_hw_state] [ENCODER:33:TMDS-33] hw state readout: disabled, pipe A
 [drm:intel_modeset_readout_hw_state] [ENCODER:35:DP MST-35] hw state readout: disabled, pipe A
 [drm:intel_modeset_readout_hw_state] [ENCODER:36:DP MST-36] hw state readout: disabled, pipe B
 [drm:intel_modeset_readout_hw_state] [ENCODER:37:DP MST-37] hw state readout: disabled, pipe C
 [drm:intel_modeset_readout_hw_state] [ENCODER:42:TMDS-42] hw state readout: disabled, pipe A
 [drm:intel_modeset_readout_hw_state] [ENCODER:44:DP MST-44] hw state readout: disabled, pipe A
 [drm:intel_modeset_readout_hw_state] [ENCODER:45:DP MST-45] hw state readout: disabled, pipe B
 [drm:intel_modeset_readout_hw_state] [ENCODER:46:DP MST-46] hw state readout: disabled, pipe C
 [drm:intel_modeset_readout_hw_state] [CONNECTOR:34:DP-1] hw state readout: disabled
 [drm:intel_modeset_readout_hw_state] [CONNECTOR:40:HDMI-A-1] hw state readout: disabled
 [drm:intel_modeset_readout_hw_state] [CONNECTOR:43:DP-2] hw state readout: disabled
 [drm:intel_modeset_readout_hw_state] [CONNECTOR:47:HDMI-A-2] hw state readout: disabled
 [drm:drm_calc_timestamping_constants [drm]] *ERROR* crtc 21: Can't calculate constants, dotclock = 0!
 [drm:i915_get_vblank_timestamp] crtc 0 is disabled
 [drm:i915_get_vblank_timestamp] crtc 0 is disabled
 [drm:i915_get_vblank_timestamp] crtc 0 is disabled
 [drm:i915_get_vblank_timestamp] crtc 0 is disabled
 [drm:i915_get_vblank_timestamp] crtc 0 is disabled
 [drm:i915_get_vblank_timestamp] crtc 0 is disabled
 [drm:intel_disable_pipe] disabling pipe A

...

[   10.062881]  [<ffffffffa016add2>] skl_update_pipe_wm+0x102/0x8c0 [i915]
[   10.062942]  [<ffffffffa016b96f>] skl_update_wm+0xff/0x5f0 [i915]
[   10.063027]  [<ffffffffa016ce6e>] intel_update_watermarks+0x1e/0x30 [i915]
[   10.063087]  [<ffffffffa01d3ee2>] intel_crtc_disable_noatomic+0xd2/0x150 [i915]
[   10.063148]  [<ffffffffa01dd3d2>] intel_modeset_setup_hw_state+0xdd2/0xde0 [i915]
[   10.063181]  [<ffffffffa01dfd83>] intel_modeset_init+0x15a3/0x1950 [i915]
[   10.063181]  [<ffffffffa02160b6>] i915_driver_load+0x13c6/0x1720 [i915]

Regards,

Tvrtko
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