[Intel-gfx] [PATCH 3/4] drm/i915/dp_mst: Fix PBN / MTP_TU size calculation for UHBR rates

[Murthy, Arun R]

> -----Original Message-----
> From: Intel-gfx <intel-gfx-bounces at lists.freedesktop.org> On Behalf Of Imre
> Deak
> Sent: Wednesday, November 15, 2023 7:08 PM
> To: intel-gfx at lists.freedesktop.org
> Subject: Re: [Intel-gfx] [PATCH 3/4] drm/i915/dp_mst: Fix PBN / MTP_TU size
> calculation for UHBR rates
> 
> On Mon, Nov 13, 2023 at 10:11:09PM +0200, Imre Deak wrote:
> > Atm the allocated MST PBN value is calculated from the TU size (number
> > of allocated MTP slots) as
> >
> >   PBN = TU * pbn_div
> >
> > pbn_div being the link BW for each MTP slot. For DP 1.4 link rates
> > this worked, as pbn_div there is guraranteed to be an integer number,
> > however on UHBR this isn't the case. To get a PBN, TU pair where TU is
> > a properly rounded-up value covering all the BW corresponding to PBN,
> > calculate first PBN and from PBN the TU value.
> >
> > Signed-off-by: Imre Deak <imre.deak at intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_dp_mst.c | 10 ++++++++--
> >  1 file changed, 8 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > index b943dbf394a22..a32ab0b4fc9d7 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_mst.c
> > @@ -170,6 +170,7 @@ static int
> > intel_dp_mst_find_vcpi_slots_for_bpp(struct intel_encoder *encoder,
> >
> >  	for (bpp = max_bpp; bpp >= min_bpp; bpp -= step) {
> >  		struct intel_link_m_n remote_m_n;
> > +		int alloc_tu;
> >  		int link_bpp;
> >
> >  		drm_dbg_kms(&i915->drm, "Trying bpp %d\n", bpp); @@ -
> 200,9 +201,14
> > @@ static int intel_dp_mst_find_vcpi_slots_for_bpp(struct intel_encoder
> *encoder,
> >  		 * crtc_state->dp_m_n.tu), provided that the driver doesn't
> >  		 * enable SSC on the corresponding link.
> >  		 */
> > +		crtc_state->pbn =
> DIV_ROUND_UP_ULL(mul_u32_u32(mst_state->pbn_div * 64,
> > +
> remote_m_n.data_m),
> > +						   remote_m_n.data_n);
> 
> I realized this may allocate fewer PBNs than required, since the actual pbn_div
> value is not an integer. Also PBN can be calculated in a more direct way from
> the effective pixel data rate, so I'd like to do that instead.
> 
> I'll send a new version with the above changes.
> 
Also spec says about a constant value of 64 for TU size.

Thanks and Regards,
Arun R Murthy
-------------------
> > +
> > +		alloc_tu = DIV_ROUND_UP_ULL(crtc_state->pbn, mst_state-
> >pbn_div);
> 
> 
> > +		drm_WARN_ON(&i915->drm, alloc_tu < remote_m_n.tu);
> >  		drm_WARN_ON(&i915->drm, remote_m_n.tu < crtc_state-
> >dp_m_n.tu);
> > -		crtc_state->dp_m_n.tu = remote_m_n.tu;
> > -		crtc_state->pbn = remote_m_n.tu * mst_state->pbn_div;
> > +		crtc_state->dp_m_n.tu = alloc_tu;
> >
> >  		slots = drm_dp_atomic_find_time_slots(state, &intel_dp-
> >mst_mgr,
> >  						      connector->port,
> > --
> > 2.39.2
> >