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@xxxxxxxxx> > > > --- > > > 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
