[PATCH] drm/dp: Add DSC virtual DPCD quirk for Realtek MST branch device

[PATCH] drm/dp: Add DSC virtual DPCD quirk for Realtek MST branch device

[Imre Deak]

The ASUS DC301 USB-C dock containing a Realtek MST branch device
supports the DSC decompression functionality on each of the dock's
downstream connectors, even though there is no discoverable peer-to-peer
virtual device in the MST topology (which the DP Standard
requires/suggests to control the DSC functionality on a per-DFP basis).
Add the DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD quirk for this branch
device as well to enable the DSC decompression functionality on all DFP
connectors of the dock, similarly to how this is done for dock's
containing older Synaptics branch devices.

Cc: Lyude Paul <lyude@redhat.com>
Reported-and-tested-by: Shawn C Lee <shawn.c.lee@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
---
 drivers/gpu/drm/display/drm_dp_helper.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
index a6067471dd563..9c31e14cc413b 100644
--- a/drivers/gpu/drm/display/drm_dp_helper.c
+++ b/drivers/gpu/drm/display/drm_dp_helper.c
@@ -2546,6 +2546,8 @@ static const struct dpcd_quirk dpcd_quirk_list[] = {
 	{ OUI(0x00, 0x00, 0x00), DEVICE_ID('C', 'H', '7', '5', '1', '1'), false, BIT(DP_DPCD_QUIRK_NO_SINK_COUNT) },
 	/* Synaptics DP1.4 MST hubs can support DSC without virtual DPCD */
 	{ OUI(0x90, 0xCC, 0x24), DEVICE_ID_ANY, true, BIT(DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD) },
+	/* Realtek DP1.4 MST hubs can support DSC without virtual DPCD */
+	{ OUI(0x00, 0xe0, 0x4c), DEVICE_ID('D', 'p', '1', '.', '4', 0), true, BIT(DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD) },
 	/* Synaptics DP1.4 MST hubs require DSC for some modes on which it applies HBLANK expansion. */
 	{ OUI(0x90, 0xCC, 0x24), DEVICE_ID_ANY, true, BIT(DP_DPCD_QUIRK_HBLANK_EXPANSION_REQUIRES_DSC) },
 	/* MediaTek panels (at least in U3224KBA) require DSC for modes with a short HBLANK on UHBR links. */
-- 
2.49.1

Claude review: drm/dp: Add DSC virtual DPCD quirk for Realtek MST branch device

[Claude Code Review Bot]

Patch Review

**Code change:**
```c
/* Realtek DP1.4 MST hubs can support DSC without virtual DPCD */
{ OUI(0x00, 0xe0, 0x4c), DEVICE_ID('D', 'p', '1', '.', '4', 0), true, BIT(DP_DPCD_QUIRK_DSC_WITHOUT_VIRTUAL_DPCD) },
```

**Observations:**

1. **Correct placement**: The new entry is inserted directly after the existing Synaptics `DSC_WITHOUT_VIRTUAL_DPCD` entry (line 2548), grouping related quirks together. Good.

2. **OUI `00:e0:4c`**: This is Realtek's registered IEEE OUI. Correct.

3. **Specific device ID vs. DEVICE_ID_ANY**: Unlike the Synaptics entry which uses `DEVICE_ID_ANY` (matching all Synaptics branch devices), this patch specifies a particular device ID string `"Dp1.4\0"`. This is a more conservative approach — it limits the quirk to only the specific Realtek device that has been tested, rather than all Realtek MST hubs. This is appropriate since Realtek may ship other MST devices where this quirk might not apply.

4. **Null byte in device ID**: The sixth byte is `0` (null), not a printable character. Looking at the quirk matching logic in `drm_dp_get_quirks()` (line 2590-2591), the device_id comparison uses `memcmp` over the full 6-byte array, so a null byte is compared correctly — it's treated as data, not a string terminator. No issue here.

5. **`is_branch = true`**: Correct — this is an MST branch device (hub), not a sink.

6. **Commit message quality**: The commit message properly explains the problem (no discoverable peer-to-peer virtual device), the hardware context (ASUS DC301 dock with Realtek MST), and the solution (reuse the existing quirk). It includes `Reported-and-tested-by` and `Cc` tags appropriately.

**No issues found.** This is a clean, minimal, well-tested quirk addition following established patterns.

Reviewed-by criteria: **Acceptable as-is.**

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Generated by Claude Code Patch Reviewer

Claude review: drm/dp: Add DSC virtual DPCD quirk for Realtek MST branch device

[Claude Code Review Bot]

Overall Series Review

Subject: drm/dp: Add DSC virtual DPCD quirk for Realtek MST branch device
Author: Imre Deak <imre.deak@intel.com>
Patches: 1
Reviewed: 2026-05-26T07:08:44.389106

---

This is a single-patch series adding a DPCD quirk entry for a Realtek MST branch device (OUI `00:e0:4c`) found in the ASUS DC301 USB-C dock. The quirk enables DSC decompression support on downstream connectors when no virtual DPCD peer-to-peer device is discoverable in the MST topology. This follows the exact same pattern already established for Synaptics hubs.

The patch is straightforward, well-motivated, and low-risk. The commit message clearly explains the hardware situation and references the existing Synaptics precedent.

**Verdict: Looks good to merge.**

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Generated by Claude Code Patch Reviewer