[PATCH v6 0/6] drm: simplify support for transparent DRM bridges

[Bryan O'Donoghue]

On 03/11/2023 23:03, Dmitry Baryshkov wrote:
> Supporting DP/USB-C can result in a chain of several transparent
> bridges (PHY, redrivers, mux, etc). All attempts to implement DP support
> in a different way resulted either in series of hacks or in device tree
> not reflecting the actual hardware design. This results in drivers
> having similar boilerplate code for such bridges.
> 
> Next, these drivers are susceptible to -EPROBE_DEFER loops: the next
> bridge can either be probed from the bridge->attach callback, when it is
> too late to return -EPROBE_DEFER, or from the probe() callback, when the
> next bridge might not yet be available, because it depends on the
> resources provided by the probing device. Device links can not fully
> solve this problem since there are mutual dependencies between adjancent
> devices.
> 
> Last, but not least, this results in the the internal knowledge of DRM
> subsystem slowly diffusing into other subsystems, like PHY or USB/TYPEC.
> 
> To solve all these issues, define a separate DRM helper, which creates
> separate aux device just for the bridge. During probe such aux device
> doesn't result in the EPROBE_DEFER loops. Instead it allows the device
> drivers to probe properly, according to the actual resource
> dependencies. The bridge auxdevs are then probed when the next bridge
> becomes available, sparing drivers from drm_bridge_attach() returning
> -EPROBE_DEFER.

Dmitry,

Looking to give you a Tested-by: here but got the below splat.

https://git.codelinaro.org/bryan.odonoghue/kernel/-/tree/next-20231123-tcpm-fix?ref_type=heads

- Boot via fastboot
- Remove USB cable
- Attach DisplayPort cable
- Get some activity on the DP
- Then this

root at linaro-gnome:~# [  376.861822] xhci-hcd xhci-hcd.4.auto: xHCI Host 
Controller
[  376.867584] xhci-hcd xhci-hcd.4.auto: new USB bus registered, 
assigned bus number 3
[  376.875775] xhci-hcd xhci-hcd.4.auto: hcc params 0x0230ffe5 hci 
version 0x110 quirks 0x0000000000000010
[  376.885666] xhci-hcd xhci-hcd.4.auto: irq 229, io mem 0x0a600000
[  376.892140] xhci-hcd xhci-hcd.4.auto: xHCI Host Controller
[  376.897951] xhci-hcd xhci-hcd.4.auto: new USB bus registered, 
assigned bus number 4
[  376.905869] xhci-hcd xhci-hcd.4.auto: Host supports USB 3.1 Enhanced 
SuperSpeed
[  376.914130] hub 3-0:1.0: USB hub found
[  376.918030] hub 3-0:1.0: 1 port detected
[  376.922417] usb usb4: We don't know the algorithms for LPM for this 
host, disabling LPM.
[  376.931540] hub 4-0:1.0: USB hub found
[  376.935439] hub 4-0:1.0: 1 port detected
[  377.885638] Unable to handle kernel NULL pointer dereference at 
virtual address 0000000000000060
[  377.892927] msm_dpu ae01000.display-controller: [drm] Cannot find any 
crtc or sizes
[  377.894724] Mem abort info:
[  377.905504]   ESR = 0x0000000096000004
[  377.909375]   EC = 0x25: DABT (current EL), IL = 32 bits
[  377.914852]   SET = 0, FnV = 0
[  377.918005]   EA = 0, S1PTW = 0
[  377.921250]   FSC = 0x04: level 0 translation fault
[  377.926269] Data abort info:
[  377.929239]   ISV = 0, ISS = 0x00000004, ISS2 = 0x00000000
[  377.934881]   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
[  377.940095]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
[  377.945563] user pgtable: 4k pages, 48-bit VAs, pgdp=0000000101992000
[  377.952441] [0000000000000060] pgd=0000000000000000, p4d=0000000000000000
[  377.959448] Internal error: Oops: 0000000096000004 [#1] PREEMPT SMP
[  377.965882] Modules linked in: typec_displayport nf_tables libcrc32c 
nfnetlink q6asm_dai q6routing q6afe_clocks q6afe_dai q6asm q6adm 
snd_q6dsp_common q6afe q6core apr pdr_interfacer
[  377.965984]  aux_bridge crct10dif_ce snd_soc_lpass_macro_common 
drm_kms_helper qnoc_sm8250 qcom_wdt icc_osm_l3 fuse drm backlight dm_mod 
ip_tables x_tables
[  378.072201] CPU: 5 PID: 379 Comm: dp_hpd_handler Not tainted 
6.7.0-rc2-next-20231123-00008-g812004aeedc0-dirty #30
[  378.082817] Hardware name: Qualcomm Technologies, Inc. Robotics RB5 (DT)
[  378.088884] msm_dpu ae01000.display-controller: [drm] Cannot find any 
crtc or sizes
[  378.089697] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS 
BTYPE=--)
[  378.089700] pc : drm_object_property_set_value+0x0/0x88 [drm]
[  378.110607] lr : drm_dp_set_subconnector_property+0x58/0x64 
[drm_display_helper]
[  378.118205] sp : ffff800081fbbda0
[  378.121616] x29: ffff800081fbbda0 x28: 0000000000000000 x27: 
0000000000000000
[  378.128940] x26: 0000000000000000 x25: 0000000000000000 x24: 
ffff38d1ccef2880
[  378.136264] x23: ffff38d1ccef2a10 x22: ffff38d1ccef2880 x21: 
ffff38d1ccef29f0
[  378.143587] x20: 0000000000000000 x19: ffff38d1ccef2880 x18: 
0000000000000000
[  378.150911] x17: 000000040044ffff x16: ffffa79c03e1fe34 x15: 
0000000000000000
[  378.158235] x14: ffff38d1c5861000 x13: 00000000000003ec x12: 
0000000000000001
[  378.165560] x11: 071c71c71c71c71c x10: 0000000000000b00 x9 : 
ffff800081fbb9d0
[  378.172884] x8 : ffffa79b9b4d9000 x7 : 0000000000000001 x6 : 
ffffa79b9b6d74b0
[  378.180207] x5 : 0000000000000000 x4 : ffff38d1cb2d3800 x3 : 
ffff38d1c28e169f
[  378.187530] x2 : 000000000000000f x1 : 0000000000000000 x0 : 
ffff38d1cb2d3840
[  378.194853] Call trace:
[  378.197376]  drm_object_property_set_value+0x0/0x88 [drm]
[  378.202947]  dp_display_process_hpd_high+0xa0/0x14c [msm]
[  378.208526]  dp_hpd_plug_handle.isra.0+0x8c/0x10c [msm]
[  378.213918]  hpd_event_thread+0xc4/0x56c [msm]
[  378.218508]  kthread+0x110/0x114
[  378.221828]  ret_from_fork+0x10/0x20
[  378.225506] Code: 128002a0 d65f03c0 d4210000 17ffffea (f9403024)
[  378.231763] ---[ end trace 0000000000000000 ]---
[  384.505974] msm_dpu ae01000.display-controller: [drm] Cannot find any 
crtc or sizes
[  385.538016] msm_dpu ae01000.display-controller: [drm] Cannot find any 
crtc or sizes
[  385.666018] msm_dpu ae01000.display-controller: [drm] Cannot find any 
crtc or sizes