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

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

[Chen Pei]

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>
---
 drivers/cxl/acpi.c | 26 ++++++++++++++++++++++++--
 1 file changed, 24 insertions(+), 2 deletions(-)

diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
index 127537628817..9952d0cff903 100644
--- a/drivers/cxl/acpi.c
+++ b/drivers/cxl/acpi.c
@@ -631,8 +631,21 @@ 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 *hb = to_cxl_host_bridge(host, match);
+	struct acpi_device *adev = to_acpi_device(match);
+	struct acpi_device *hb;
 
+	/*
+	 * If this is an ACPI0016 device but acpi_pci_find_root() hasn't
+	 * 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 &&
+	    !acpi_pci_find_root(adev->handle)) {
+		dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
+		return -EPROBE_DEFER;
+	}
+
+	hb = to_cxl_host_bridge(host, match);
 	if (!hb)
 		return 0;
 
@@ -688,7 +701,8 @@ 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 *hb = to_cxl_host_bridge(host, match);
+	struct acpi_device *adev = to_acpi_device(match);
+	struct acpi_device *hb;
 	struct acpi_pci_root *pci_root;
 	struct cxl_dport *dport;
 	struct cxl_port *port;
@@ -697,6 +711,14 @@ 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 &&
+	    !acpi_pci_find_root(adev->handle)) {
+		dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
+		return -EPROBE_DEFER;
+	}
+
+	hb = to_cxl_host_bridge(host, match);
 	if (!hb)
 		return 0;
 
-- 
2.50.1

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

[Richard Cheng]

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>
> ---
>  drivers/cxl/acpi.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
>

Hi Chen Pei,

Thanks for the patch.
I have a few questions and suggestions regarding to your changes.

First of all I would like in which scenario did you encounter the bug?
Any specific CONFIG options and the devices ? what's the error log ?

It would be nice if you can attach it for us. 

> diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> index 127537628817..9952d0cff903 100644
> --- a/drivers/cxl/acpi.c
> +++ b/drivers/cxl/acpi.c
> @@ -631,8 +631,21 @@ 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 *hb = to_cxl_host_bridge(host, match);
> +	struct acpi_device *adev = to_acpi_device(match);
> +	struct acpi_device *hb;
>  
> +	/*
> +	 * If this is an ACPI0016 device but acpi_pci_find_root() hasn't
> +	 * 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 &&
> +	    !acpi_pci_find_root(adev->handle)) {
> +		dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
> +		return -EPROBE_DEFER;
> +	}

What about strncpy() here since we already know we're comparing against "ACPI0016" ?
At the same time, why not just use "acpi_dev_hid_match()" ? it's widely used across
numerous files.

> +
> +	hb = to_cxl_host_bridge(host, match);
>  	if (!hb)
>  		return 0;
>  
> @@ -688,7 +701,8 @@ 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 *hb = to_cxl_host_bridge(host, match);
> +	struct acpi_device *adev = to_acpi_device(match);
> +	struct acpi_device *hb;
>  	struct acpi_pci_root *pci_root;
>  	struct cxl_dport *dport;
>  	struct cxl_port *port;
> @@ -697,6 +711,14 @@ 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 &&
> +	    !acpi_pci_find_root(adev->handle)) {
> +		dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
> +		return -EPROBE_DEFER;
> +	}
> +
> +	hb = to_cxl_host_bridge(host, match);
>  	if (!hb)
>  		return 0;
>  
> -- 
> 2.50.1
> 
> 

These 2 checks are basically the same, can we put it in a static inline helper or
a macro if possible? something like the following might be better

```
static int cxl_acpi_defer_host_bridge(struct device *host,
				      struct acpi_device *adev)
{
	if (acpi_dev_hid_match(adev, "ACPI0016") &&
	    !acpi_pci_find_root(adev->handle)) {
		dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
		return -EPROBE_DEFER;
	}
	return 0;
}
```
and use it in your code like

```
int rc = cxl_acpi_defer_host_bridge(host, adev);
if (rc)
	return rc;
```


Last but not least, have you run the kselftests of CXL ? some mock bridges
are platform devices, not ACPI devices, you are using "to_acpi_device(match)", this
is not a safe runtime check when "match" is a platform_device, the code will read the memory
layout wrongly.


Best regards,
Richard Cheng.

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

[Dave Jiang]

On 5/13/26 7:32 PM, 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>
> ---
>  drivers/cxl/acpi.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> index 127537628817..9952d0cff903 100644
> --- a/drivers/cxl/acpi.c
> +++ b/drivers/cxl/acpi.c
> @@ -631,8 +631,21 @@ 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 *hb = to_cxl_host_bridge(host, match);
> +	struct acpi_device *adev = to_acpi_device(match);
> +	struct acpi_device *hb;
>  
> +	/*
> +	 * If this is an ACPI0016 device but acpi_pci_find_root() hasn't
> +	 * 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 &&
> +	    !acpi_pci_find_root(adev->handle)) {
> +		dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
> +		return -EPROBE_DEFER;
> +	}
> +
> +	hb = to_cxl_host_bridge(host, match);
>  	if (!hb)
>  		return 0;
>  
> @@ -688,7 +701,8 @@ 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 *hb = to_cxl_host_bridge(host, match);
> +	struct acpi_device *adev = to_acpi_device(match);
> +	struct acpi_device *hb;
>  	struct acpi_pci_root *pci_root;
>  	struct cxl_dport *dport;
>  	struct cxl_port *port;
> @@ -697,6 +711,14 @@ 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 &&
> +	    !acpi_pci_find_root(adev->handle)) {
> +		dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
> +		return -EPROBE_DEFER;
> +	}

I don't believe this check in add_host_bridge_uport() is necessary. add_host_bridge_dport() happens first and the check should've been done there. There is no reason to do it again and you'll never reach here if it's not ready. 

DJ

> +
> +	hb = to_cxl_host_bridge(host, match);
>  	if (!hb)
>  		return 0;
>  

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

[Alison Schofield]

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

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

[Chen Pei]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain; charset=y, Size: 6567 bytes --]

On Thu, 14 May 2026 15:31:11 +0800, Richard Cheng 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>
> > ---
> >  drivers/cxl/acpi.c | 26 ++++++++++++++++++++++++--
> >  1 file changed, 24 insertions(+), 2 deletions(-)
> >
> 
> Hi Chen Pei,
> 
> Thanks for the patch.
> I have a few questions and suggestions regarding to your changes.
> 
> First of all I would like in which scenario did you encounter the bug?
> Any specific CONFIG options and the devices ? what's the error log ?
> 
> It would be nice if you can attach it for us.

Hi Richard,

Thanks for the review.

I'm currently working on bringing up CXL support on the RISC-V QEMU
virt platform with ACPI (EDK2 UEFI firmware). This is still in the
early debugging/enabling stage.

During testing, I found that cxl_acpi (ACPI0017) probes before
acpi_pci_root has bound to the ACPI0016 (CXL host bridge) device.
RISC-V uses the generic pci_acpi_scan_root() implementation, where
the probe ordering of acpi_pci_root relative to cxl_acpi is not
guaranteed.

On x86, acpi_pci_root uses subsys_initcall and binds very early,
so this race does not manifest there.

This is a silent failure (no explicit error log), which makes it
particularly hard to diagnose. When cxl_acpi probes before
acpi_pci_root has bound the ACPI0016 device:
1. acpi_pci_find_root() returns NULL in to_cxl_host_bridge()
2. to_cxl_host_bridge() returns NULL
2. Both add_host_bridge_dport() and add_host_bridge_uport() return 0
   (not an error), silently skipping the host bridge
3. cxl_acpi_probe() returns success, but the CXL port topology is
   incomplete — no dports or uports are registered

The observable result after boot:
    # memdev is visible but decoder hierarchy is missing
    $ cxl list -M
    [
      {
        "memdev":"mem0",
        ...
      }
    ]

    # No decoders or ports registered for the host bridge
    $ cxl list -BDP
    []

    # Workaround: manually unbind/bind triggers re-probe after
    # acpi_pci_root is ready, and CXL topology enumerates correctly
    $ echo ACPI0017:00 > /sys/bus/platform/drivers/cxl_acpi/unbind
    $ echo ACPI0017:00 > /sys/bus/platform/drivers/cxl_acpi/bind

    # After re-probe, full topology is available and CXL memory
    # can be enabled/onlined successfully
    $ cxl enable-memdev mem0
    $ cxl create-region -m -t ram -d decoder0.0 -w 1 mem0 -s 4G
    $ daxctl online-memory dax0.0

> > diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> > index 127537628817..9952d0cff903 100644
> > --- a/drivers/cxl/acpi.c
> > +++ b/drivers/cxl/acpi.c
> > @@ -631,8 +631,21 @@ 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 *hb = to_cxl_host_bridge(host, match);
> > + struct acpi_device *adev = to_acpi_device(match);
> > + struct acpi_device *hb;
> >  
> > + /*
> > +  * If this is an ACPI0016 device but acpi_pci_find_root() hasn't
> > +  * 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 &&
> > +     !acpi_pci_find_root(adev->handle)) {
> > +  dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
> > +  return -EPROBE_DEFER;
> > + }
> 
> What about strncpy() here since we already know we're comparing against "ACPI0016" ?
> At the same time, why not just use "acpi_dev_hid_match()" ? it's widely used across
> numerous files.

Good point, thanks. I will switch to the acpi_dev_hid_match() in v2.

> > +
> > + hb = to_cxl_host_bridge(host, match);
> >   if (!hb)
> >    return 0;
> >  
> > @@ -688,7 +701,8 @@ 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 *hb = to_cxl_host_bridge(host, match);
> > + struct acpi_device *adev = to_acpi_device(match);
> > + struct acpi_device *hb;
> >   struct acpi_pci_root *pci_root;
> >   struct cxl_dport *dport;
> >   struct cxl_port *port;
> > @@ -697,6 +711,14 @@ 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 &&
> > +     !acpi_pci_find_root(adev->handle)) {
> > +  dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
> > +  return -EPROBE_DEFER;
> > + }
> > +
> > + hb = to_cxl_host_bridge(host, match);
> >   if (!hb)
> >    return 0;
> >  
> > -- 
> > 2.50.1
> > 
> > 
> 
> These 2 checks are basically the same, can we put it in a static inline helper or
> a macro if possible? something like the following might be better
> 
> ```
> static int cxl_acpi_defer_host_bridge(struct device *host,
          > struct acpi_device *adev)
> {
 > if (acpi_dev_hid_match(adev, "ACPI0016") &&
     > !acpi_pci_find_root(adev->handle)) {
  > dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
  > return -EPROBE_DEFER;
 > }
 > return 0;
> }
> ```
> and use it in your code like
> 
> ```
> int rc = cxl_acpi_defer_host_bridge(host, adev);
> if (rc)
 > return rc;
> ```

Agreed, will extract it into a helper in v2.

> Last but not least, have you run the kselftests of CXL ? some mock bridges
> are platform devices, not ACPI devices, you are using "to_acpi_device(match)", this
> is not a safe runtime check when "match" is a platform_device, the code will read the memory
> layout wrongly.

Good catch, I haven't run the CXL kselftests. You're right that the
mock bridges are platform devices and unconditionally calling
to_acpi_device(match) would be unsafe there.

I'll fix this in v2 by adding a dev_is_platform() guard in the
helper, so it only applies to real ACPI devices. I'll also run
the CXL kselftests to validate before sending v2.

Thanks,
Pei

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

[Chen Pei]

On Thu, 14 May 2026 10:10:50 -0700, Dave Jiang 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>
> > ---
> >  drivers/cxl/acpi.c | 26 ++++++++++++++++++++++++--
> >  1 file changed, 24 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> > index 127537628817..9952d0cff903 100644
> > --- a/drivers/cxl/acpi.c
> > +++ b/drivers/cxl/acpi.c
> > @@ -631,8 +631,21 @@ 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 *hb = to_cxl_host_bridge(host, match);
> > + struct acpi_device *adev = to_acpi_device(match);
> > + struct acpi_device *hb;
> >  
> > + /*
> > +  * If this is an ACPI0016 device but acpi_pci_find_root() hasn't
> > +  * 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 &&
> > +     !acpi_pci_find_root(adev->handle)) {
> > +  dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
> > +  return -EPROBE_DEFER;
> > + }
> > +
> > + hb = to_cxl_host_bridge(host, match);
> >   if (!hb)
> >    return 0;
> >  
> > @@ -688,7 +701,8 @@ 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 *hb = to_cxl_host_bridge(host, match);
> > + struct acpi_device *adev = to_acpi_device(match);
> > + struct acpi_device *hb;
> >   struct acpi_pci_root *pci_root;
> >   struct cxl_dport *dport;
> >   struct cxl_port *port;
> > @@ -697,6 +711,14 @@ 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 &&
> > +     !acpi_pci_find_root(adev->handle)) {
> > +  dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
> > +  return -EPROBE_DEFER;
> > + }
> 
> I don't believe this check in add_host_bridge_uport() is necessary. add_host_bridge_dport() happens first and the check should've been done there. There is no reason to do it again and you'll never reach here if it's not ready. 
> 
> DJ

Hi Dave,

You're right. Since add_host_bridge_dport() is called first and
cxl_acpi_probe() returns early on error, add_host_bridge_uport()
will never be reached if the PCI root is not ready. I'll drop the
duplicate check in v2.

Thanks,
Pei

> > +
> > + hb = to_cxl_host_bridge(host, match);
> >   if (!hb)
> >    return 0;

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

[Chen Pei]

On Thu, 14 May 2026 10:19:43 -0700, Alison Schofield 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.

Hi Alison,

Thanks for the fix! 

I'll test it on my RISC-V QEMU setup and fold it into v2 along with
the other review feedback (switching to acpi_dev_hid_match(),
dropping the redundant check in add_host_bridge_uport(), etc.).

Will also run the CXL kselftests to make sure the mock platform path
works correctly.

Thanks,
Pei

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

[Dave Jiang]

On 5/13/26 7:32 PM, 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>
> ---
>  drivers/cxl/acpi.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/cxl/acpi.c b/drivers/cxl/acpi.c
> index 127537628817..9952d0cff903 100644
> --- a/drivers/cxl/acpi.c
> +++ b/drivers/cxl/acpi.c
> @@ -631,8 +631,21 @@ 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 *hb = to_cxl_host_bridge(host, match);
> +	struct acpi_device *adev = to_acpi_device(match);
> +	struct acpi_device *hb;
>  
> +	/*
> +	 * If this is an ACPI0016 device but acpi_pci_find_root() hasn't
> +	 * 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 &&
> +	    !acpi_pci_find_root(adev->handle)) {

I would leave the assignment of 'hb' as is above and just what's below after looking at to_cxl_host_bridge() function.

if (!hb) {
	if (!acpi_pci_find_root(adev->handle)) {
		...
		return -EPROBE_DEFER;
	}
	return 0;
}

I am concerned of other failure cases that causes acpi_pci_find_root() to fail beyond the driver not being attached. Will cxl_acpi probe() be stuck permanently in the defer probe queue when that happens? This is also something that Sashiko raised. Is there no way to adjust the RISCV ACPI setup to go around subsys_initcall()? That would be preferred solution.

DJ

> +		dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
> +		return -EPROBE_DEFER;
> +	}
> +
> +	hb = to_cxl_host_bridge(host, match);
>  	if (!hb)
>  		return 0;
>  
> @@ -688,7 +701,8 @@ 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 *hb = to_cxl_host_bridge(host, match);
> +	struct acpi_device *adev = to_acpi_device(match);
> +	struct acpi_device *hb;
>  	struct acpi_pci_root *pci_root;
>  	struct cxl_dport *dport;
>  	struct cxl_port *port;
> @@ -697,6 +711,14 @@ 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 &&
> +	    !acpi_pci_find_root(adev->handle)) {
> +		dev_dbg(host, "deferring probe, ACPI0016 PCI root not ready\n");
> +		return -EPROBE_DEFER;
> +	}
> +
> +	hb = to_cxl_host_bridge(host, match);
>  	if (!hb)
>  		return 0;
>  

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

[Dan Williams (nvidia)]

Chen Pei wrote:
> On Thu, 14 May 2026 15:31:11 +0800, Richard Cheng 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>
> > > ---
> > >  drivers/cxl/acpi.c | 26 ++++++++++++++++++++++++--
> > >  1 file changed, 24 insertions(+), 2 deletions(-)
> > >
> > 
> > Hi Chen Pei,
> > 
> > Thanks for the patch.
> > I have a few questions and suggestions regarding to your changes.
> > 
> > First of all I would like in which scenario did you encounter the bug?
> > Any specific CONFIG options and the devices ? what's the error log ?
> > 
> > It would be nice if you can attach it for us.
> 
> Hi Richard,
> 
> Thanks for the review.
> 
> I'm currently working on bringing up CXL support on the RISC-V QEMU
> virt platform with ACPI (EDK2 UEFI firmware). This is still in the
> early debugging/enabling stage.
>
> During testing, I found that cxl_acpi (ACPI0017) probes before
> acpi_pci_root has bound to the ACPI0016 (CXL host bridge) device.
> RISC-V uses the generic pci_acpi_scan_root() implementation, where
> the probe ordering of acpi_pci_root relative to cxl_acpi is not
> guaranteed.
> 
> On x86, acpi_pci_root uses subsys_initcall and binds very early,
> so this race does not manifest there.

If the platform is defined to defer PCI root scans then this dependency
must be declared. Specifically firmware needs to tell Linux about the
dependency given it does not order PCI enumeration before ACPI0017
enumeration by default. Something like: 

  Device (CXLM) { // ACPI0017
      Name (_HID, "ACPI0017")
      Name (_DEP, Package () {
          \_SB.CXL0, // ACPI0016 host bridge
          \_SB.CXL1,
      })
  }

...in the firmware, and then:

acpi_dev_clear_dependencies()

...for each acpi_pci_root_add().

Then ACPI0017 will naturally await all of the ACPI0016 devices that the
firmware knows about.

Otherwise EPROBE_DEFER and scanning for ACPI0016 attachment is
introducing a mess that _DEP was meant to solve.