Hi all,
This is a design-only RFC to solicit feedback on a proposed relaxation of the
cloned-modeset selection logic in the DRM fbdev client. No patch is attached;
pseudocode only.
Background and motivation
On the HiBMC SoC (drivers/gpu/drm/hisilicon/hibmc, a dual-output VGA +
DP PCI display device, single CRTC, clone output) we observe the following
call chain during fbdev client hotplug:
+-----------------------------+
| drm_fbdev_client_hotplug |
+-----------------------------+
|
v
+-----------------------------+
| drm_fb_helper_hotplug_event |
+-----------------------------+
|
| fb_helper->fb(640x480) <derived from vga GRUB>
v
+-----------------------------+
| drm_client_modeset_probe |
| <width=640, height=480> |
+-----------------------------+ +-------------------+
| | DP probed modes: |
v | "640x480" 25200 |
+-----------------------------------+ | "640x480" 25175 |
| .fill_modes | | |
| <Rejected other modes: VIRTUAL_X> |-------------| VGA probed modes: |
+-----------------------------------+ | "640x480" 25175 |
| +-------------------+
v
+-----------------------------+ +---------------------------------+
| drm_client_target_cloned | --------> | drm_connector_pick_cmdline_mode |
+-----------------------------+ | + drm_mode_match |
| <No dp GRUB> |
+---------------------------------+
|
| fail
v
+-------------------------------------+
| DMT(1024x768) |
| <Already rejected in .fill_modes> |
+-------------------------------------+
fail |
---------------------------------------------
| can_clone = false; <can't enable cloning when we probably
wanted to.>
v
+-----------------------------+
| drm_client_target_preferred |
| <dp: 640x480 25200> |
| <vga: 640x480 25175> |
+-----------------------------+
|
v
+-----------------------------+
| drm_client_pick_crtcs |
| <drm_mode_equal fail> |
+-----------------------------+
* GRUB brings the console up at 640x480 on the VGA connector, so the fbdev
client is probed with width=640, height=480.
* The VGA connector gets a command-line mode of 640x480@25175 (derived
from the GRUB framebuffer).
* The DP connector has *no* command-line mode, but its probed mode list
contains "640x480" 25200, "640x480" 25175, "640x480" 25175.
* VGA's probed mode list contains "640x480" 25175.
The current drm_client_target_cloned() requires *every* enabled connector to
have a command-line mode, and all of them to be equal. Because DP has none,
this path fails immediately. The fallback then tries to find DMT 1024x768 on
every connector, but on this hardware 1024x768 has already been rejected in
drm_mode_prune_invalid() (VIRTUAL_X), so the fallback fails too. The code
prints "kms: can't enable cloning when we probably wanted to." and falls
through to drm_client_target_preferred(), which independently picks
"640x480" 25200 for DP and "640x480" 25175 for VGA. Those two modes are not
drm_mode_equal(), so drm_client_pick_crtcs() cannot place both connectors on
the single CRTC and one output ends up disabled.
The net effect: even though both connectors advertise a perfectly good
640x480@25175 mode, clone setup fails because (a) the command-line check
insists on every connector having a cmdline mode and (b) the only fallback
is a hardcoded 1024x768 that this hardware cannot scan out.
Proposed design (pseudocode)
The idea is to relax the mode-selection part of drm_client_target_cloned()
to handle three cases, in order, before the existing 1024x768 fallback:
Case 1 — All enabled connectors have a cmdline mode and they are all
equal. Clone succeeds. (Existing behaviour; unchanged.)
Case 2 — Some enabled connectors have a cmdline mode and all such cmdline
modes are equal. For every enabled connector WITHOUT a cmdline
mode, look the shared cmdline mode up in that connector's probed
mode list. If every connector without a cmdline mode has a
matching probed mode, clone succeeds.
Case 3 — No enabled connector has a cmdline mode. Walk the first enabled
connector's probed mode list and, for each candidate, check that
every other enabled connector has a drm_mode_match() equivalent
in its probed list. The first candidate present on all
connectors wins.
Pseudocode:
drm_client_target_cloned(dev, connectors, connector_count,
modes, offsets, enabled, width, height):
// --- existing guards, unchanged ---
if dev->mode_config.num_crtc > 1:
return false
count = number of enabled connectors
if count <= 1:
return false
// --- Phase 1: classify cmdline modes ---
common_cmdline_mode = NULL
have_cmdline_mode = false
have_no_cmdline_mode = false
can_clone = true
for each enabled connector i:
modes[i] = drm_connector_pick_cmdline_mode(connectors[i])
if modes[i] != NULL:
have_cmdline_mode = true
if common_cmdline_mode == NULL:
common_cmdline_mode = modes[i]
else if not drm_mode_match(modes[i], common_cmdline_mode,
TIMINGS | CLOCK | FLAGS | 3D_FLAGS):
can_clone = false
break
else:
have_no_cmdline_mode = true
// --- Case 1: all have same cmdline mode ---
if can_clone and have_cmdline_mode and not have_no_cmdline_mode:
return true
// --- Case 2: partial cmdline, resolve the rest from probed lists ---
if can_clone and have_cmdline_mode and have_no_cmdline_mode:
for each enabled connector i without a cmdline mode:
modes[i] = NULL
for each mode M in connectors[i]->modes:
if drm_mode_match(M, common_cmdline_mode,
TIMINGS | CLOCK | FLAGS | 3D_FLAGS):
modes[i] = M
break
if modes[i] == NULL:
can_clone = false
break
if can_clone:
return true
// --- Case 3: no cmdline mode anywhere, find a common probed mode ---
if not have_cmdline_mode:
first = index of first enabled connector
for each candidate mode M in connectors[first]->modes:
can_clone = true
modes[first] = M
for each other enabled connector j:
modes[j] = NULL
for each mode P in connectors[j]->modes:
if drm_mode_match(P, M,
TIMINGS | CLOCK | FLAGS | 3D_FLAGS):
modes[j] = P
break
if modes[j] == NULL:
can_clone = false
break
if can_clone:
return true
// else: reset all modes[] and try next candidate
// --- clean up before fallback ---
for each enabled connector i:
modes[i] = NULL
// --- existing 1024x768 DMT fallback, unchanged ---
...
// --- existing failure message, unchanged ---
drm_info(dev, "kms: can't enable cloning when we probably wanted to.")
return false
Only the mode-selection logic is touched. The single-CRTC guard, the
1-enabled-connector guard, the 1024x768 fallback, and the failure message
are all kept. Matching uses DRM_MODE_MATCH_TIMINGS | CLOCK | FLAGS |
3D_FLAGS — the same set the original code already uses — so two modes that
the rest of the DRM client considers "the same mode" are treated as
clone-compatible here too.
No UAPI change. No new helper. No change to drm_connector_pick_cmdline_mode,
drm_client_target_preferred, or drm_client_pick_crtcs.
RFC-specific questions
I am specifically looking for feedback on:
* Whether case 2 (partial cmdline, look up shared cmdline mode in
probed lists) is the right boundary, or whether the community would
prefer the fbdev client to stay strict and require cmdline modes on
all connectors.
* Whether case 3 (no cmdline mode at all, search for a common probed
mode) is acceptable in principle, or whether the community would
prefer to keep that case relying solely on the 1024x768 fallback.
* Whether the candidate-walk in case 3 (first enabled connector's
modes, first match wins) is the right selection policy, or whether
a "preferred mode first" / "largest mode" policy would be better.
The simple "first found" style mirrors the existing 1024x768
fallback.
* Naming suggestions for the shared cmdline mode and presence-tracking
variables, if/when this becomes a formal patch.
For now the prototype is against v7.2.0 of the local tree.
Comments very welcome.
Thanks,
Yongbang.
