Re: [PATCH] cxl/acpi: Defer probe when ACPI0016 PCI root bridge is not ready

[Alison Schofield <alison.schofield@intel.com>]

On Thu, May 14, 2026 at 10:32:38AM +0800, Chen Pei wrote:
> On some platforms (e.g., RISC-V and ARM64) that use the generic
> pci_acpi_scan_root() implementation, cxl_acpi_probe may run before
> acpi_pci_root driver has bound to ACPI0016 (CXL host bridge) devices.
> In this case, acpi_pci_find_root() returns NULL, causing
> to_cxl_host_bridge() to skip the device silently. This results in
> incomplete CXL port enumeration on first boot.
> 
> Fix this by detecting the case where an ACPI0016 device exists but its
> PCI root bridge is not yet ready, and returning -EPROBE_DEFER to trigger
> a deferred probe retry.
> 
> Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>

Hi Chen Pei,

As Richard suggested, this fails for the mock platform in cxl-test.
(stack trace appended at end)

With this diff applied on top of your patch, it works for cxl-test
AND I think it works for your case too. With real hardware,
ACPI_COMPANION returns the device, and with the mock platform,
ACPI_COMPANION returns NULL and the defer check is skipped.

Try it out, and note that I didn't consider if any of the comments
need updating.


diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
index 9952d0cff903..ec037668afba 100644
--- a/drivers/cxl/acpi.c
+++ b/drivers/cxl/acpi.c
@@ -631,7 +631,7 @@ static int add_host_bridge_dport(struct device *match, void *arg)
 	struct acpi_pci_root *pci_root;
 	struct cxl_port *root_port = arg;
 	struct device *host = root_port->dev.parent;
-	struct acpi_device *adev = to_acpi_device(match);
+	struct acpi_device *adev = ACPI_COMPANION(match);
 	struct acpi_device *hb;
 
 	/*
@@ -639,7 +639,7 @@ static int add_host_bridge_dport(struct device *match, void *arg)
 	 * found the PCI root yet (driver not probed), defer the probe
 	 * to allow acpi_pci_root to bind first.
 	 */
-	if (strcmp(acpi_device_hid(adev), "ACPI0016") == 0 &&
+	if (adev && strcmp(acpi_device_hid(adev), "ACPI0016") == 0 &&
 	    !acpi_pci_find_root(adev->handle)) {
 		dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
 		return -EPROBE_DEFER;
@@ -701,7 +701,7 @@ static int add_host_bridge_uport(struct device *match, void *arg)
 {
 	struct cxl_port *root_port = arg;
 	struct device *host = root_port->dev.parent;
-	struct acpi_device *adev = to_acpi_device(match);
+	struct acpi_device *adev = ACPI_COMPANION(match);
 	struct acpi_device *hb;
 	struct acpi_pci_root *pci_root;
 	struct cxl_dport *dport;
@@ -711,8 +711,7 @@ static int add_host_bridge_uport(struct device *match, void *arg)
 	resource_size_t component_reg_phys;
 	int rc;
 
-	/* Same deferral check as in add_host_bridge_dport() */
-	if (strcmp(acpi_device_hid(adev), "ACPI0016") == 0 &&
+	if (adev && strcmp(acpi_device_hid(adev), "ACPI0016") == 0 &&
 	    !acpi_pci_find_root(adev->handle)) {
 		dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
 		return -EPROBE_DEFER;

==========

Failure loading cxl-test module:

[    6.523556] calling  cxl_test_init+0x0/0xff0 [cxl_test] @ 622
[    6.524952] BUG: kernel NULL pointer dereference, address: 0000000000000091
[    6.526022] #PF: supervisor read access in kernel mode
[    6.526988] #PF: error_code(0x0000) - not-present page
[    6.527855] PGD 0 P4D 0 
[    6.528331] Oops: Oops: 0000 [#1] SMP NOPTI
[    6.529268] CPU: 3 UID: 0 PID: 622 Comm: systemd-modules Tainted: G           O        7.1.0-rc1+ #212 PREEMPT(lazy) 
[    6.530655] Tainted: [O]=OOT_MODULE
[    6.531238] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
[    6.532321] RIP: 0010:acpi_device_hid+0x18/0x30
[    6.533008] Code: cc cc 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 48 8b 87 98 00 00 00 48 81 c7 98 00 00 00 48 39 f8 74 0e 48 85 c0 74 09 <48> 8b 40 10 c3 cc cc cc cc 48 c7 c0 ad c9 a7 82 c3 cc cc cc cc 0f
[    6.535011] RSP: 0018:ffffc90002077870 EFLAGS: 00010206
[    6.535729] RAX: 0000000000000081 RBX: ffff8882000eb010 RCX: ffffffffa00504f0
[    6.536514] RDX: ffff88800199acc8 RSI: ffff888006ca6000 RDI: ffff8882000eae48
[    6.537284] RBP: ffffc900020778c0 R08: ffffffffa0e67491 R09: 0000000000000040
[    6.538122] R10: ffff888203667c00 R11: ffffffff835ccf70 R12: ffff888006ba3010
[    6.538961] R13: ffff888006ca6000 R14: 0000000000000000 R15: ffff888006ca6000
[    6.539777] FS:  00007f9001205480(0000) GS:ffff8880fa501000(0000) knlGS:0000000000000000
[    6.540639] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    6.541313] CR2: 0000000000000091 CR3: 0000000007037003 CR4: 0000000000370ef0
[    6.542125] Call Trace:
[    6.542540]  <TASK>
[    6.542903]  ? add_host_bridge_dport+0x23/0x200 [cxl_acpi]
[    6.543538]  ? klist_next+0xb0/0x170
[    6.544019]  ? __pfx_add_host_bridge_dport+0x10/0x10 [cxl_acpi]
[    6.544716]  bus_for_each_dev+0x65/0xa0
[    6.545204]  cxl_acpi_probe+0xe5/0x2d0 [cxl_acpi]
[    6.545758]  ? acpi_dev_pm_attach+0x20/0xf0
[    6.546300]  platform_probe+0x3a/0x70
[    6.546834]  really_probe+0xda/0x3e0
[    6.547302]  ? __pfx___device_attach_driver+0x10/0x10
[    6.547913]  __driver_probe_device+0x10b/0x1a0
[    6.548422]  driver_probe_device+0x1f/0x90
[    6.548949]  __device_attach_driver+0x8f/0x130
[    6.549448]  bus_for_each_drv+0x73/0xb0
[    6.549947]  __device_attach+0xb1/0x1c0
[    6.550371]  device_initial_probe+0x43/0x50
[    6.550882]  bus_probe_device+0x29/0x90
[    6.551340]  device_add+0x682/0x860
[    6.551836]  ? dev_set_name+0x3e/0x50
[    6.552282]  platform_device_add+0x176/0x260
[    6.552820]  cxl_test_init+0x80c/0xff0 [cxl_test]
[    6.553348]  ? __pfx_cxl_test_init+0x10/0x10 [cxl_test]
[    6.553954]  do_one_initcall+0x46/0x220
[    6.554411]  do_init_module+0x63/0x240
[    6.554926]  load_module+0x2826/0x2b40
[    6.555410]  ? kernel_read+0x3f/0x50
[    6.555931]  ? kernel_read_file+0x27b/0x2f0
[    6.556414]  init_module_from_file+0xbc/0xf0
[    6.556964]  __x64_sys_finit_module+0x267/0x380
[    6.557473]  x64_sys_call+0x1d68/0x2010
[    6.557958]  do_syscall_64+0x5a/0x470
[    6.558399]  entry_SYSCALL_64_after_hwframe+0x71/0x79
[    6.558981] RIP: 0033:0x7f900110b27d
[    6.559405] Code: 5d c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 73 cb 0e 00 f7 d8 64 89 01 48
[    6.561245] RSP: 002b:00007fff4b386268 EFLAGS: 00000246 ORIG_RAX: 0000000000000139
[    6.562035] RAX: ffffffffffffffda RBX: 000055b7253f34b0 RCX: 00007f900110b27d
[    6.562779] RDX: 0000000000000000 RSI: 00007f900178f43c RDI: 000000000000000c
[    6.563473] RBP: 00007f900178f43c R08: 0000000000000000 R09: 000055b7253f8f80
[    6.564187] R10: 000000000000000c R11: 0000000000000246 R12: 0000000000020000
[    6.564918] R13: 000055b7253f4cc0 R14: 0000000000000000 R15: 000055b7253f97c0
[    6.565601]  </TASK>
[    6.565895] Modules linked in: cxl_test(O+) cxl_acpi(O) cxl_pmem(O) cxl_mem(O) cxl_port(O) cxl_mock(O) cxl_core(O) fwctl libnvdimm
[    6.566999] CR2: 0000000000000091
[    6.567417] ---[ end trace 0000000000000000 ]---
[    6.567990] RIP: 0010:acpi_device_hid+0x18/0x30
[    6.568661] Code: cc cc 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 48 8b 87 98 00 00 00 48 81 c7 98 00 00 00 48 39 f8 74 0e 48 85 c0 74 09 <48> 8b 40 10 c3 cc cc cc cc 48 c7 c0 ad c9 a7 82 c3 cc cc cc cc 0f
[    6.573628] RSP: 0018:ffffc90002077870 EFLAGS: 00010206
[    6.575230] RAX: 0000000000000081 RBX: ffff8882000eb010 RCX: ffffffffa00504f0
[    6.577279] RDX: ffff88800199acc8 RSI: ffff888006ca6000 RDI: ffff8882000eae48
[    6.579343] RBP: ffffc900020778c0 R08: ffffffffa0e67491 R09: 0000000000000040
[    6.581407] R10: ffff888203667c00 R11: ffffffff835ccf70 R12: ffff888006ba3010
[    6.583237] R13: ffff888006ca6000 R14: 0000000000000000 R15: ffff888006ca6000
[    6.584595] FS:  00007f9001205480(0000) GS:ffff8880fa501000(0000) knlGS:0000000000000000
[    6.586087] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[    6.587256] CR2: 0000000000000091 CR3: 0000000007037003 CR4: 0000000000370ef0
[    6.588634] note: systemd-modules[622] exited with irqs disabled