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@xxxxxxxxxxxxxxxxx>
> > ---
> >  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;