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SubjectRe: [PATCH v4] PCI: introduce two interfaces to walk PCI buses
On Fri, Sep 21, 2012 at 10:07 AM, Jiang Liu <liuj97@gmail.com> wrote:
> The pci_find_next_bus() is not hotplug safe, so introduce PCI hotplug
> safe interfaces to walk PCI buses. To avoid some deadlock scenarios,
> two interfaces are introduced.
>
> The first one is pci_for_each_bus(), which walks all PCI buses holding
> read lock on the pci_bus_sem.
>
> The second one is pci_for_each_started_bus(), which walks all started
> PCI buses without holding any global locks. Started PCI buses are those
> which have been added to the device tree by calling device_add().
> ---
> Hi Bjorn,
> How about this PCI bus iterator design? It's a little ugly that
> we need to two interfaces to work around some deadlock scenarios.
> And I plan to split the task into two parts:
> 1) a hotplug safe PCI bus iteraror to replace pci_find_next_bus
> 2) handle hotplug notifications to update bus related states.
>
> My patchset at http://www.spinics.net/lists/linux-pci/msg17515.html has
> patially solved issue 2 above for x86/ACPI, and will add more supports
> for other platforms.
> Thanks!
> Gerry

I like this interface:

int pci_for_each_bus(int (* cb)(struct pci_bus *, void *), void *data)

quite a bit because it has the potential for removing all the list
knowledge from the callers, but there are two things I don't like:

1) The interface would allow the PCI core to call the callback for
future hot-added buses, but your implementation doesn't have that yet.

2) I'd rather have something like "pci_for_each_dev()" so this is
device-based instead of bus-based. The struct pci_bus is of limited
usefulness outside the core, except as a container for a set of
devices. So the users of pci_for_each_bus() would often iterate
through that set of devices, and given the complications of SR-IOV
virtual buses, I don't think it's really clear how to do that
iteration correctly.

> ---
> drivers/pci/bus.c | 42 ++++++++++++++++++++++++++++++++++++++++++
> include/linux/pci.h | 1 +
> 2 files changed, 43 insertions(+)
>
> diff --git a/drivers/pci/bus.c b/drivers/pci/bus.c
> index e16a8f0f..21b0ade 100644
> --- a/drivers/pci/bus.c
> +++ b/drivers/pci/bus.c
> @@ -327,6 +327,48 @@ void pci_walk_bus(struct pci_bus *top, int (*cb)(struct pci_dev *, void *),
> }
> EXPORT_SYMBOL_GPL(pci_walk_bus);
>
> +static int pci_bus_iter(struct pci_bus *bus,
> + int (* cb)(struct pci_bus *, void *), void *data)
> +{
> + int rc;
> +
> + rc = cb(bus, data);
> + if (rc == 0)
> + list_for_each_entry(bus, &bus->children, node) {
> + rc = pci_bus_iter(bus, cb, data);
> + if (rc)
> + break;
> + }
> +
> + return rc;
> +}
> +
> +/** pci_for_each_bus - walk all PCI buses and call the provided callback.
> + * @cb callback to be called for each bus found
> + * @data arbitrary pointer to be passed to callback.
> + *
> + * Walk all PCI buses and call the provided callback with pci_bus_sem held.
> + *
> + * We check the return of @cb each time. If it returns anything
> + * other than 0, we break out.
> + */
> +int pci_for_each_bus(int (* cb)(struct pci_bus *, void *), void *data)
> +{
> + int rc = 0;
> + struct pci_bus *bus;
> +
> + down_read(&pci_bus_sem);
> + list_for_each_entry(bus, &pci_root_buses, node) {
> + rc = pci_bus_iter(bus, cb, data);
> + if (rc)
> + break;
> + }
> + up_read(&pci_bus_sem);
> +
> + return rc;
> +}
> +EXPORT_SYMBOL(pci_for_each_bus);
> +
> struct pci_bus *pci_bus_get(struct pci_bus *bus)
> {
> if (bus)
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index 3c5017d..1423a24 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -1060,6 +1060,7 @@ int pci_scan_bridge(struct pci_bus *bus, struct pci_dev *dev, int max,
>
> void pci_walk_bus(struct pci_bus *top, int (*cb)(struct pci_dev *, void *),
> void *userdata);
> +int pci_for_each_bus(int (* cb)(struct pci_bus *, void *), void *data);
> int pci_cfg_space_size_ext(struct pci_dev *dev);
> int pci_cfg_space_size(struct pci_dev *dev);
> unsigned char pci_bus_max_busnr(struct pci_bus *bus);
> --
> 1.7.9.5
>


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