Re: [Intel-gfx] [PATCH] drm/i915: Add a DP1.2 compatible way to read LTTPR capabilities

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, Mar 01, 2022 at 08:14:25PM +0200, Imre Deak wrote:
> On Tue, Mar 01, 2022 at 04:14:24PM +0200, Ville Syrjälä wrote:
> > On Mon, Feb 28, 2022 at 10:12:34PM +0200, Imre Deak wrote:
> > > At least some DELL monitors (P2715Q) with DPCD_REV 1.2 return corrupted
> > > DPCD register values when reading from the 0xF0000- LTTPR range with an
> > > AUX transaction block size bigger than 1. The DP standard requires 0 to
> > > be returned - as for any other reserved/invalid addresses - but these
> > > monitors return the DPCD_REV register value repeated in each byte of the
> > > read buffer. This will in turn corrupt the values returned by the LTTPRs
> > > between the source and the monitor: LTTPRs must adjust the values they
> > > read from the downstream DPRX, for instance left-shift/init the
> > > downstream DP_PHY_REPEATER_CNT value. Since the value returned by the
> > > monitor's DPRX is non-zero the adjusted values will be corrupt.
> > > 
> > > Reading the LTTPR registers one-by-one instead of reading all of them
> > > with a single AUX transfer works around the issue.
> > > 
> > > According to the DP standard's 0xF0000 register description:
> > > "LTTPR-related registers at DPCD Addresses F0000h through F02FFh are
> > > valid only for DPCD r1.4 (or higher)." While it's unclear if DPCD r1.4
> > > refers to the DPCD_REV or to the
> > > LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV register (tickets filed
> > > at the VESA site to clarify this haven't been addressed), one
> > > possibility is that it's a restriction due to non-compliant monitors
> > > described above. Disabling the non-transparent LTTPR mode for all such
> > > monitors is not a viable solution: the transparent LTTPR mode has its
> > > own issue causing link training failures and this would affect a lot of
> > > monitors in use with DPCD_REV < 1.4. Instead this patch works around
> > > the problem by reading the LTTPR common and PHY cap registers one-by-one
> > > for any monitor with a DPCD_REV < 1.4.
> > > 
> > > The standard requires the DPCD capabilites to be read after the LTTPR
> > > common capabilities are read, so re-read the DPCD capabilities after
> > > the LTTPR common and PHY caps were read out.
> > > 
> > > Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/4531
> > > Signed-off-by: Imre Deak <imre.deak@intel.com>
> > > ---
> > >  drivers/gpu/drm/dp/drm_dp.c                   | 58 ++++++++++++-------
> > >  .../drm/i915/display/intel_dp_link_training.c | 30 +++++++---
> > >  include/drm/dp/drm_dp_helper.h                |  2 +
> > >  3 files changed, 59 insertions(+), 31 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/dp/drm_dp.c b/drivers/gpu/drm/dp/drm_dp.c
> > > index 703972ae14c64..f3950d42980f9 100644
> > > --- a/drivers/gpu/drm/dp/drm_dp.c
> > > +++ b/drivers/gpu/drm/dp/drm_dp.c
> > > @@ -2390,9 +2390,36 @@ int drm_dp_dsc_sink_supported_input_bpcs(const u8 dsc_dpcd[DP_DSC_RECEIVER_CAP_S
> > >  }
> > >  EXPORT_SYMBOL(drm_dp_dsc_sink_supported_input_bpcs);
> > >  
> > > +static int drm_dp_read_lttpr_regs(struct drm_dp_aux *aux, const u8 dpcd[DP_RECEIVER_CAP_SIZE], int address,
> > > +				  u8 *buf, int buf_size)
> > > +{
> > > +	/*
> > > +	 * Some monitors with a DPCD_REV < 0x14 return corrupted values when
> > > +	 * reading from the 0xF0000- range with a block size bigger than 1.
> > > +	 */
> > 
> > This sounds really scary. Have we checked what other registers might
> > end up corrupted? Eg. couple of rounds of comparing full dd bs=1 vs. 
> > dd bs=16.
> 
> Yes, checked now on a DELL P2715Q/ADLP/native-DP (w/o any LTTPR):
> 
> dd bs=1 count=1M failes at offset 81 and 83, bs=16 doesn't have this
> problem.

Huh. Oh well, it's a Dell monitor after all. Not sure what else I was
expecting tbh.

-- 
Ville Syrjälä
Intel