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

[Imre Deak <imre.deak@intel.com>]

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.

Replacing the above two bytes with 0s in the bs=1 output, the only
difference is at 0xf0000:

bs=1:  0x12 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
bs=16: 0x12 0x12 0x12 0x12 0x12 0x12 0x12 0x12 0x12 0x12 0x12 0x12 0x12 0x12 0x12 0x12

I attached the edid and bin files to the bugzilla ticket.

> > +	int block_size = dpcd[DP_DPCD_REV] < 0x14 ? 1 : buf_size;
> > +	int offset = 0;
> > +	int ret;
> > +
> > +	while (offset < buf_size) {
> 
> Can we use a for loop?

Yes, will change this.

> > +		ret = drm_dp_dpcd_read(aux,
> > +				       address + offset,
> > +				       &buf[offset], block_size);
> > +		if (ret < 0)
> > +			return ret;
> > +
> > +		WARN_ON(ret != block_size);
> > +
> > +		offset += block_size;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> 
> -- 
> Ville Syrjälä
> Intel