Re: [Resend with Ack][PATCH v1] PCI: allow acpiphp to handle PCIeports without native PCIe hotplug capability

From: Don Dutile
Date: Tue Jul 03 2012 - 15:50:15 EST

On 07/03/2012 11:59 AM, Bjorn Helgaas wrote:
On Mon, Jul 2, 2012 at 10:16 PM, Bjorn Helgaas<bhelgaas@xxxxxxxxxx> wrote:
On Mon, Jun 4, 2012 at 1:44 AM, Jiang Liu<jiang.liu@xxxxxxxxxx> wrote:
Commit 0d52f54e2ef64c189dedc332e680b2eb4a34590a (PCI / ACPI: Make acpiphp
ignore root bridges using PCIe native hotplug) added code that made the
acpiphp driver completely ignore PCIe root complexes for which the kernel
had been granted control of the native PCIe hotplug feature by the BIOS
through _OSC. Later commit 619a5182d1f38a3d629ee48e04fa182ef9170052
"PCI hotplug: Always allow acpiphp to handle non-PCIe bridges" relaxed
the constraints to allow acpiphp driver handle non-PCIe bridges under
such a complex. The constraint needs to be relaxed further to allow
acpiphp driver to hanlde PCIe ports without native PCIe hotplug capability.

Some MR-IOV switch chipsets, such PLX8696, support multiple virtual PCIe
switches and may migrate downstream ports among virtual switches.
To migrate a downstream port from the source virtual switch to the target,
the port needs to be hot-removed from the source and hot-added into the
target. pciehp driver can't be used here because there's no slots within
the virtual PCIe switch. So acpiphp driver is used to support downstream
port migration. A typical configuration is as below:
[Root w/o native PCIe HP]
[Upstream port of vswitch w/o native PCIe HP]
[Downstream port of vswitch w/ native PCIe HP]
[PCIe enpoint]

Here acpiphp driver will be used to handle root ports and upstream port
in the virtual switch, and pciehp driver will be used to handle downstream
ports in the virtual switch.

Acked-by: Rafael J. Wysocki<rjw@xxxxxxx>
Signed-off-by: Jiang Liu<liuj97@xxxxxxxxx>

drivers/pci/hotplug/acpiphp_glue.c | 49 ++++++++++++++++++++++++++++-------
1 files changed, 39 insertions(+), 10 deletions(-)

diff --git a/drivers/pci/hotplug/acpiphp_glue.c b/drivers/pci/hotplug/acpiphp_glue.c
index 806c44f..4889448 100644
--- a/drivers/pci/hotplug/acpiphp_glue.c
+++ b/drivers/pci/hotplug/acpiphp_glue.c
@@ -115,6 +115,43 @@ static const struct acpi_dock_ops acpiphp_dock_ops = {
.handler = handle_hotplug_event_func,

+/* Check whether device is managed by native PCIe hotplug driver */
+static bool device_is_managed_by_native_pciehp(struct pci_dev *pdev)
+ int pos;
+ u16 reg16;
+ u32 reg32;
+ acpi_handle tmp;
+ struct acpi_pci_root *root;
+ if (!pci_is_pcie(pdev))
+ return false;
+ /* Check whether PCIe port supports native PCIe hotplug */
+ pos = pci_pcie_cap(pdev);

Add "if (!pos) return false;" here and you can drop the "if
(!pci_is_pcie())" test above.

+ pci_read_config_word(pdev, pos + PCI_EXP_FLAGS,&reg16);
+ if (!(reg16& PCI_EXP_FLAGS_SLOT))

I think this is unsafe. Per the PCIe v3.0 spec, sec 7.8.2 on p648,
the "Slot Implemented" bit is undefined except for Downstream Ports,
so we're using an undefined bit to decide whether to read

If the device has a v1 PCIe Capability, it is not required to even
implement PCI_EXP_SLTCAP, so we could be reading garbage out of an
unrelated capability. This is in sec 7.8, p363, of the v1.1 PCIe
spec. I think v3.0 of the spec is dangerously incomplete because it
doesn't include enough information to handle the v1 PCIe Capability

There's a fair amount of work to fix this. I started doing it, but
decided I didn't have time to complete it. Here's what I think we
(and by "we," I'm afraid I mean "you" :)) should do:

- Add a "u16 pcie_flags" field in struct pci_dev and save the "PCI
Express Capabilities Register" there in set_pcie_port_type(). All
fields in that register are read-only, so it should be safe to cache
- Remove pcie_type from struct pci_dev and replace it with a
pcie_type() inline that extracts it from pcie_flags.
- Rework the pcie_cap_has_*() macros in drivers/pci/pci.c to take a
struct pci_dev * and use pcie_flags instead of type and flags. This
will remove the need for callers to read the flags themselves.
- Move the pcie_cap_has_*() macros to include/linux/pci_reg.h so
they can be shared.
- Audit all uses of the Link registers (PCI_EXP_LNKCAP,
PCI_EXP_RTCTL, PCI_EXP_RTSTA) to make sure the register exists, either
by using pcie_cap_has_*() or some other knowledge of the device.

Thinking about this some more, this still leaves the callers
responsible for using pcie_cap_has_*(), which feels pretty

I wonder if it'd be worth adding interfaces like:

pcie_cap_read_word(const struct pci_dev *, int where, u16 *val);
pcie_cap_read_dword(const struct pci_dev *, int where, u32 *val);
pcie_cap_write_word(const struct pci_dev *, int where, u16 val);
pcie_cap_write_dword(const struct pci_dev *, int where, u32 val);

I like your thinking!

We might be able to encapsulate the v1/v2 differences inside these, e.g.,

int pcie_cap_read_word(const struct pci_dev *dev, int where, u16 *val)
int pos;

pos = pci_pcie_cap(dev);
if (!pos)
return -EINVAL;

may want to change read value to 0 just in case callers are doing rtn value
check and just value-read mask & go. I believe for all the optional/version'd
registers below, non-existent regs are required to be rtn-zero if not implemented.

switch (where) {
return pci_read_config_word(dev, pos + where, val);
if (pcie_cap_has_lnkctl(dev))
return pci_read_config_word(dev, pos + where, val);
else {
*val = 0;
return 0;
if (pcie_cap_has_sltctl(dev))
return pci_read_config_word(dev, pos + where, val);
else {
*val = 0;
if (where == PCI_EXP_SLTSTA&& dev->pcie_type ==
return 0;
return -EINVAL;

Any thoughts?

only one is that 'cap' is overused in PCI space, just like 'domain' in
various kernel subsystems. cap could be 'cap list structure'
or a specific 'capability'. I wish we had a better TLA for 'cap' and what
it refers to. ... but that's my pet peeve...

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