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.
- 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?
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;
+}