On Fri, Jan 26, 2024 at 01:36:02PM +0200, Ville Syrjälä wrote:
> On Tue, Jan 23, 2024 at 12:28:32PM +0200, Imre Deak wrote:
> > Copy intel_dp_max_data_rate() to DRM core. It will be needed by a
> > follow-up DP tunnel patch, checking the maximum rate the DPRX (sink)
> > supports. Accordingly use the drm_dp_max_dprx_data_rate() name for
> > clarity. This patchset will also switch calling the new DRM function
> > in i915 instead of intel_dp_max_data_rate().
> >
> > Signed-off-by: Imre Deak <imre.deak@xxxxxxxxx>
> > ---
> > drivers/gpu/drm/display/drm_dp_helper.c | 58 +++++++++++++++++++++++++
> > include/drm/display/drm_dp_helper.h | 2 +
> > 2 files changed, 60 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> > index b1ca3a1100dab..24911243d4d3a 100644
> > --- a/drivers/gpu/drm/display/drm_dp_helper.c
> > +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> > @@ -4058,3 +4058,61 @@ int drm_dp_bw_channel_coding_efficiency(bool is_uhbr)
> > return 800000;
> > }
> > EXPORT_SYMBOL(drm_dp_bw_channel_coding_efficiency);
> > +
> > +/*
> > + * Given a link rate and lanes, get the data bandwidth.
> > + *
> > + * Data bandwidth is the actual payload rate, which depends on the data
> > + * bandwidth efficiency and the link rate.
> > + *
> > + * For 8b/10b channel encoding, SST and non-FEC, the data bandwidth efficiency
> > + * is 80%. For example, for a 1.62 Gbps link, 1.62*10^9 bps * 0.80 * (1/8) =
> > + * 162000 kBps. With 8-bit symbols, we have 162000 kHz symbol clock. Just by
> > + * coincidence, the port clock in kHz matches the data bandwidth in kBps, and
> > + * they equal the link bit rate in Gbps multiplied by 100000. (Note that this no
> > + * longer holds for data bandwidth as soon as FEC or MST is taken into account!)
> > + *
> > + * For 128b/132b channel encoding, the data bandwidth efficiency is 96.71%. For
> > + * example, for a 10 Gbps link, 10*10^9 bps * 0.9671 * (1/8) = 1208875
> > + * kBps. With 32-bit symbols, we have 312500 kHz symbol clock. The value 1000000
> > + * does not match the symbol clock, the port clock (not even if you think in
> > + * terms of a byte clock), nor the data bandwidth. It only matches the link bit
> > + * rate in units of 10000 bps.
> > + *
> > + * Note that protocol layers above the DPRX link level considered here can
> > + * further limit the maximum data rate. Such layers are the MST topology (with
> > + * limits on the link between the source and first branch device as well as on
> > + * the whole MST path until the DPRX link) and (Thunderbolt) DP tunnels -
> > + * which in turn can encapsulate an MST link with its own limit - with each
> > + * SST or MST encapsulated tunnel sharing the BW of a tunnel group.
> > + *
> > + * TODO: Add support for querying the max data rate with the above limits as
> > + * well.
> > + *
> > + * Returns the maximum data rate in kBps units.
> > + */
> > +int drm_dp_max_dprx_data_rate(int max_link_rate, int max_lanes)
> > +{
> > + int ch_coding_efficiency =
> > + drm_dp_bw_channel_coding_efficiency(drm_dp_is_uhbr_rate(max_link_rate));
> > + int max_link_rate_kbps = max_link_rate * 10;
>
> That x10 value seems rather pointless.
I suppose the point was to make the units clearer, but it could be
clarified instead in max_link_rates' documentation, which is missing
atm.
> > +
> > + /*
> > + * UHBR rates always use 128b/132b channel encoding, and have
> > + * 97.71% data bandwidth efficiency. Consider max_link_rate the
> > + * link bit rate in units of 10000 bps.
> > + */
> > + /*
> > + * Lower than UHBR rates always use 8b/10b channel encoding, and have
> > + * 80% data bandwidth efficiency for SST non-FEC. However, this turns
> > + * out to be a nop by coincidence:
> > + *
> > + * int max_link_rate_kbps = max_link_rate * 10;
> > + * max_link_rate_kbps = DIV_ROUND_DOWN_ULL(max_link_rate_kbps * 8, 10);
> > + * max_link_rate = max_link_rate_kbps / 8;
> > + */
>
> Not sure why we are repeating the nuts and bolts detils in the
> comments so much? Doesn't drm_dp_bw_channel_coding_efficiency()
> explain all this already?
I simply copied the function, but yes in this context there is
duplication, thanks for reading through all that. Will consolidate both
the above and the bigger comment before the function with the existing
docs here.
>
> > + return DIV_ROUND_DOWN_ULL(mul_u32_u32(max_link_rate_kbps * max_lanes,
> > + ch_coding_efficiency),
> > + 1000000 * 8);
> > +}
> > +EXPORT_SYMBOL(drm_dp_max_dprx_data_rate);
> > diff --git a/include/drm/display/drm_dp_helper.h b/include/drm/display/drm_dp_helper.h
> > index 863b2e7add29e..454ae7517419a 100644
> > --- a/include/drm/display/drm_dp_helper.h
> > +++ b/include/drm/display/drm_dp_helper.h
> > @@ -813,4 +813,6 @@ int drm_dp_bw_overhead(int lane_count, int hactive,
> > int bpp_x16, unsigned long flags);
> > int drm_dp_bw_channel_coding_efficiency(bool is_uhbr);
> >
> > +int drm_dp_max_dprx_data_rate(int max_link_rate, int max_lanes);
> > +
> > #endif /* _DRM_DP_HELPER_H_ */
> > --
> > 2.39.2
>
> --
> Ville Syrjälä
> Intel
