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SubjectRe: [PATCH v6 1/4] PCI: Consider alignment of hot-added bridges when distributing available resources
Mika, this patch changes code you added in 1a5767725cec ("PCI:
Distribute available resources to hotplug-capable bridges"). Is there
any chance you could help review this?

On Wed, May 22, 2019 at 02:30:44PM +0000, Nicholas Johnson wrote:
> Rewrite pci_bus_distribute_available_resources to better handle bridges
> with different resource alignment requirements. Pass more details
> arguments recursively to track the resource start and end addresses
> relative to the initial hotplug bridge. This is especially useful for
> Thunderbolt with native PCI enumeration, enabling external graphics
> cards and other devices with bridge alignment higher than 0x100000
> bytes.
>
> Change extend_bridge_window to resize the actual resource, rather than
> using add_list and dev_res->add_size. If an additional resource entry
> exists for the given resource, zero out the add_size field to avoid it
> interfering. Because add_size is considered optional when allocating,
> using add_size could cause issues in some cases, because successful
> resource distribution requires sizes to be guaranteed. Such cases
> include hot-adding nested hotplug bridges in one enumeration, and
> potentially others which are yet to be encountered.
>
> Signed-off-by: Nicholas Johnson <nicholas.johnson-opensource@outlook.com.au>
> ---
> drivers/pci/setup-bus.c | 169 ++++++++++++++++++++--------------------
> 1 file changed, 84 insertions(+), 85 deletions(-)
>
> diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
> index 0cdd5ff38..1b5b851ca 100644
> --- a/drivers/pci/setup-bus.c
> +++ b/drivers/pci/setup-bus.c
> @@ -1835,12 +1835,10 @@ static void extend_bridge_window(struct pci_dev *bridge, struct resource *res,
> }
>
> static void pci_bus_distribute_available_resources(struct pci_bus *bus,
> - struct list_head *add_list,
> - resource_size_t available_io,
> - resource_size_t available_mmio,
> - resource_size_t available_mmio_pref)
> + struct list_head *add_list, struct resource io,
> + struct resource mmio, struct resource mmio_pref)

Follow the parameter indentation style of the rest of the file.

> {
> - resource_size_t remaining_io, remaining_mmio, remaining_mmio_pref;
> + resource_size_t io_per_hp, mmio_per_hp, mmio_pref_per_hp, align;
> unsigned int normal_bridges = 0, hotplug_bridges = 0;
> struct resource *io_res, *mmio_res, *mmio_pref_res;
> struct pci_dev *dev, *bridge = bus->self;
> @@ -1850,29 +1848,36 @@ static void pci_bus_distribute_available_resources(struct pci_bus *bus,
> mmio_pref_res = &bridge->resource[PCI_BRIDGE_RESOURCES + 2];
>
> /*
> - * Update additional resource list (add_list) to fill all the
> - * extra resource space available for this port except the space
> - * calculated in __pci_bus_size_bridges() which covers all the
> - * devices currently connected to the port and below.
> + * The alignment of this bridge is yet to be considered, hence it must
> + * be done now before extending its bridge window. A single bridge
> + * might not be able to occupy the whole parent region if the alignment
> + * differs - for example, an external GPU at the end of a Thunderbolt
> + * daisy chain.

The example seems needlessly specific. There isn't anything GPU- or
Thunderbolt-specific about this, is there?

Bridge windows can be aligned to any multiple of 1MB. But a device
BAR must be aligned on its size, so any BAR larger than 1MB should be
able to cause this, e.g.,

[mem 0x100000-0x3fffff] (bridge A 3MB window)
[mem 0x200000-0x3fffff] (bridge B 2MB window)
[mem 0x200000-0x3fffff] (device 2MB BAR)

Bridge B *could* occupy the the entire 3MB parent region, but it
doesn't need to. But you say it "might not be *able* to", so maybe
you're thinking of something different?

> - extend_bridge_window(bridge, io_res, add_list, available_io);
> - extend_bridge_window(bridge, mmio_res, add_list, available_mmio);
> - extend_bridge_window(bridge, mmio_pref_res, add_list,
> - available_mmio_pref);
> + align = pci_resource_alignment(bridge, io_res);
> + if (!io_res->parent && align)
> + io.start = ALIGN(io.start, align);
> +
> + align = pci_resource_alignment(bridge, mmio_res);
> + if (!mmio_res->parent && align)
> + mmio.start = ALIGN(mmio.start, align);
> +
> + align = pci_resource_alignment(bridge, mmio_pref_res);
> + if (!mmio_pref_res->parent && align)
> + mmio_pref.start = ALIGN(mmio_pref.start, align);
>
> /*
> - * Calculate the total amount of extra resource space we can
> - * pass to bridges below this one. This is basically the
> - * extra space reduced by the minimal required space for the
> - * non-hotplug bridges.
> + * Update the resources to fill as much remaining resource space in the
> + * parent bridge as possible, while considering alignment.
> */
> - remaining_io = available_io;
> - remaining_mmio = available_mmio;
> - remaining_mmio_pref = available_mmio_pref;
> + extend_bridge_window(bridge, io_res, add_list, resource_size(&io));
> + extend_bridge_window(bridge, mmio_res, add_list, resource_size(&mmio));
> + extend_bridge_window(bridge, mmio_pref_res, add_list,
> + resource_size(&mmio_pref));
>
> /*
> * Calculate how many hotplug bridges and normal bridges there
> - * are on this bus. We will distribute the additional available
> + * are on this bus. We will distribute the additional available

This whitespace change is pointless and distracting.

> * resources between hotplug bridges.
> */
> for_each_pci_bridge(dev, bus) {
> @@ -1882,104 +1887,98 @@ static void pci_bus_distribute_available_resources(struct pci_bus *bus,
> normal_bridges++;
> }
>
> + /*
> + * There is only one bridge on the bus so it gets all possible
> + * resources which it can then distribute to the possible
> + * hotplug bridges below.
> + */
> + if (hotplug_bridges + normal_bridges == 1) {
> + dev = list_first_entry(&bus->devices, struct pci_dev, bus_list);
> + if (dev->subordinate)
> + pci_bus_distribute_available_resources(dev->subordinate,
> + add_list, io, mmio, mmio_pref);
> + return;
> + }

Moving this "single bridge" case up makes sense, and I think it could
be done by a separate patch preceding this one. Mika, I remember some
discussion about this case, but I can't remember if there's some
reason you didn't do this initially.

The current code is:

for_each_pci_bridge(dev, bus)
# compute hotplug_bridges, normal_bridges

for_each_pci_bridge(dev, bus)
# compute remaining_io, etc

if (hotplug_bridges + normal_bridges == 1)
# handle single bridge case

for_each_pci_bridge(dev, bus)
# use remaining_io, etc here

AFAICT the single bridge case has no dependency on the remaining_io
computation.

> + /*
> + * Reduce the available resource space by what the
> + * bridge and devices below it occupy.
> + */
> for_each_pci_bridge(dev, bus) {
> - const struct resource *res;
> + struct resource *res;
> + resource_size_t used_size;
>
> if (dev->is_hotplug_bridge)
> continue;
>
> - /*
> - * Reduce the available resource space by what the
> - * bridge and devices below it occupy.
> - */
> res = &dev->resource[PCI_BRIDGE_RESOURCES + 0];
> - if (!res->parent && available_io > resource_size(res))
> - remaining_io -= resource_size(res);
> + align = pci_resource_alignment(dev, res);
> + align = align ? ALIGN(io.start, align) - io.start : 0;
> + used_size = align + resource_size(res);
> + if (!res->parent && used_size <= resource_size(&io))
> + io.start += used_size;
>
> res = &dev->resource[PCI_BRIDGE_RESOURCES + 1];
> - if (!res->parent && available_mmio > resource_size(res))
> - remaining_mmio -= resource_size(res);
> + align = pci_resource_alignment(dev, res);
> + align = align ? ALIGN(mmio.start, align) - mmio.start : 0;
> + used_size = align + resource_size(res);
> + if (!res->parent && used_size <= resource_size(&mmio))
> + mmio.start += used_size;
>
> res = &dev->resource[PCI_BRIDGE_RESOURCES + 2];
> - if (!res->parent && available_mmio_pref > resource_size(res))
> - remaining_mmio_pref -= resource_size(res);
> + align = pci_resource_alignment(dev, res);
> + align = align ? ALIGN(mmio_pref.start, align) -
> + mmio_pref.start : 0;
> + used_size = align + resource_size(res);
> + if (!res->parent && used_size <= resource_size(&mmio_pref))
> + mmio_pref.start += used_size;
> }
>
> - /*
> - * There is only one bridge on the bus so it gets all available
> - * resources which it can then distribute to the possible hotplug
> - * bridges below.
> - */
> - if (hotplug_bridges + normal_bridges == 1) {
> - dev = list_first_entry(&bus->devices, struct pci_dev, bus_list);
> - if (dev->subordinate) {
> - pci_bus_distribute_available_resources(dev->subordinate,
> - add_list, available_io, available_mmio,
> - available_mmio_pref);
> - }
> + if (!hotplug_bridges)
> return;

I like the addition of this early return when there are no hotplug
bridges. The following loop is a no-op if there are no hotplug
bridges, so it doesn't *fix* anything, but it does make it more
obvious that we don't even have to bother with the loop at all, and
it makes the "Here hotplug_bridges is always != 0" comment
unnecessary.

I think this could be done in a separate patch before this one, too.
Anything we can do to simplify these patches is a win because the code
is so complicated.

> /*
> - * Go over devices on this bus and distribute the remaining
> - * resource space between hotplug bridges.
> + * Distribute any remaining resources equally between
> + * the hotplug-capable downstream ports.
> */
> - for_each_pci_bridge(dev, bus) {
> - resource_size_t align, io, mmio, mmio_pref;
> - struct pci_bus *b;
> + io_per_hp = div64_ul(resource_size(&io), hotplug_bridges);
> + mmio_per_hp = div64_ul(resource_size(&mmio), hotplug_bridges);
> + mmio_pref_per_hp = div64_ul(resource_size(&mmio_pref),
> + hotplug_bridges);
>
> - b = dev->subordinate;
> - if (!b || !dev->is_hotplug_bridge)
> + for_each_pci_bridge(dev, bus) {
> + if (!dev->subordinate || !dev->is_hotplug_bridge)
> continue;
>
> - /*
> - * Distribute available extra resources equally between
> - * hotplug-capable downstream ports taking alignment into
> - * account.
> - *
> - * Here hotplug_bridges is always != 0.
> - */
> - align = pci_resource_alignment(bridge, io_res);
> - io = div64_ul(available_io, hotplug_bridges);
> - io = min(ALIGN(io, align), remaining_io);
> - remaining_io -= io;
> -
> - align = pci_resource_alignment(bridge, mmio_res);
> - mmio = div64_ul(available_mmio, hotplug_bridges);
> - mmio = min(ALIGN(mmio, align), remaining_mmio);
> - remaining_mmio -= mmio;
> + io.end = io.start + io_per_hp - 1;
> + mmio.end = mmio.start + mmio_per_hp - 1;
> + mmio_pref.end = mmio_pref.start + mmio_pref_per_hp - 1;
>
> - align = pci_resource_alignment(bridge, mmio_pref_res);
> - mmio_pref = div64_ul(available_mmio_pref, hotplug_bridges);
> - mmio_pref = min(ALIGN(mmio_pref, align), remaining_mmio_pref);
> - remaining_mmio_pref -= mmio_pref;
> + pci_bus_distribute_available_resources(dev->subordinate,
> + add_list, io, mmio, mmio_pref);
>
> - pci_bus_distribute_available_resources(b, add_list, io, mmio,
> - mmio_pref);
> + io.start = io.end + 1;
> + mmio.start = mmio.end + 1;
> + mmio_pref.start = mmio_pref.end + 1;
> }

I like the simplification of this loop.

> }
>
> static void pci_bridge_distribute_available_resources(struct pci_dev *bridge,
> struct list_head *add_list)
> {
> - resource_size_t available_io, available_mmio, available_mmio_pref;
> - const struct resource *res;
> + struct resource io_res, mmio_res, mmio_pref_res;
>
> if (!bridge->is_hotplug_bridge)
> return;
>
> + io_res = bridge->resource[PCI_BRIDGE_RESOURCES + 0];
> + mmio_res = bridge->resource[PCI_BRIDGE_RESOURCES + 1];
> + mmio_pref_res = bridge->resource[PCI_BRIDGE_RESOURCES + 2];
> +
> /* Take the initial extra resources from the hotplug port */
> - res = &bridge->resource[PCI_BRIDGE_RESOURCES + 0];
> - available_io = resource_size(res);
> - res = &bridge->resource[PCI_BRIDGE_RESOURCES + 1];
> - available_mmio = resource_size(res);
> - res = &bridge->resource[PCI_BRIDGE_RESOURCES + 2];
> - available_mmio_pref = resource_size(res);
>
> pci_bus_distribute_available_resources(bridge->subordinate,
> - add_list, available_io,
> - available_mmio,
> - available_mmio_pref);
> + add_list, io_res, mmio_res, mmio_pref_res);
> }
>
> void pci_assign_unassigned_bridge_resources(struct pci_dev *bridge)
> --
> 2.20.1
>

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