[PATCH 4/4] drm/dp_mst: Fix PBN divider calculation for UHBR rates

[kernel test robot]

Hi Imre,

kernel test robot noticed the following build errors:

[auto build test ERROR on drm-tip/drm-tip]

url:    https://github.com/intel-lab-lkp/linux/commits/Imre-Deak/drm-i915-dp-Fix-UHBR-link-M-N-values/20231114-043135
base:   git://anongit.freedesktop.org/drm/drm-tip drm-tip
patch link:    https://lore.kernel.org/r/20231113201110.510724-4-imre.deak%40intel.com
patch subject: [PATCH 4/4] drm/dp_mst: Fix PBN divider calculation for UHBR rates
config: i386-randconfig-002-20231114 (https://download.01.org/0day-ci/archive/20231114/202311140621.sw31vG8M-lkp@intel.com/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20231114/202311140621.sw31vG8M-lkp@intel.com/reproduce)

If you fix the issue in a separate patch/commit (i.e. not just a new version of
the same patch/commit), kindly add following tags
| Reported-by: kernel test robot <lkp at intel.com>
| Closes: https://lore.kernel.org/oe-kbuild-all/202311140621.sw31vG8M-lkp@intel.com/

All errors (new ones prefixed by >>):

   ld: drivers/gpu/drm/display/drm_dp_mst_topology.o: in function `drm_dp_get_vc_payload_bw':
>> drivers/gpu/drm/display/drm_dp_mst_topology.c:3598: undefined reference to `__udivdi3'


vim +3598 drivers/gpu/drm/display/drm_dp_mst_topology.c

  3570	
  3571	/**
  3572	 * drm_dp_get_vc_payload_bw - get the VC payload BW for an MST link
  3573	 * @mgr: The &drm_dp_mst_topology_mgr to use
  3574	 * @link_rate: link rate in 10kbits/s units
  3575	 * @link_lane_count: lane count
  3576	 *
  3577	 * Calculate the total bandwidth of a MultiStream Transport link. The returned
  3578	 * value is in units of PBNs/(timeslots/1 MTP). This value can be used to
  3579	 * convert the number of PBNs required for a given stream to the number of
  3580	 * timeslots this stream requires in each MTP.
  3581	 */
  3582	int drm_dp_get_vc_payload_bw(const struct drm_dp_mst_topology_mgr *mgr,
  3583				     int link_rate, int link_lane_count)
  3584	{
  3585		int ret;
  3586	
  3587		if (link_rate == 0 || link_lane_count == 0)
  3588			drm_dbg_kms(mgr->dev, "invalid link rate/lane count: (%d / %d)\n",
  3589				    link_rate, link_lane_count);
  3590	
  3591		/* See DP v2.0 2.6.4.2, 2.7.6.3 VCPayload_Bandwidth_for_OneTimeSlotPer_MTP_Allocation */
  3592		/*
  3593		 * TODO: Return the value with a higher precision, allowing a better
  3594		 * slots per MTP allocation granularity. With the current returned
  3595		 * value +1 slot/MTP can get allocated on UHBR links.
  3596		 */
  3597		ret = mul_u32_u32(link_rate * link_lane_count,
> 3598				  drm_dp_bw_channel_coding_efficiency(drm_dp_is_uhbr_rate(link_rate))) /
  3599		      (1000000ULL * 8 * 5400);
  3600	
  3601		return ret;
  3602	}
  3603	EXPORT_SYMBOL(drm_dp_get_vc_payload_bw);
  3604	

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