Hi Bjorn,
I figured it out. The difference between the functions is whether they
use struct pci_bus * or struct pci_dev * as cursor. I found that the
functions are in fact equivalent. The last loop iteration in
pci_enable_atomic_ops_to_root is equivalent to the code after the loop
in qedr_pci_set_atomic. Both handle the root port.
I think my confusion was based on an incorrect assumption that
bridge->bus->self is the same as bridge. But brigde->bus is in fact the
parent bus.
I sent out an updated patch that addresses your comments to the previous
version. This should be general enough that it can replace
qedr_pci_set_atomic.
Regards,
 Felix
On 2018-01-04 06:40 PM, Bjorn Helgaas wrote:
> On Tue, Jan 02, 2018 at 06:41:17PM -0500, Felix Kuehling wrote:
>> On 2017-12-12 06:27 PM, Bjorn Helgaas wrote:
>>> [+cc Ram, Michal, Ariel, Doug, Jason]
>>>
>>> The [29/37] in the subject makes it look like this is part of a larger
>>> series, but I can't find the rest of it on linux-pci or linux-kernel.
>>>
>>> I don't want to merge a new interface unless there's an in-tree user
>>> of it. I assume the rest of the series includes a user.
>>>
>>> On Fri, Dec 08, 2017 at 11:09:07PM -0500, Felix Kuehling wrote:
>> [snip]
>>>> + * all upstream bridges support AtomicOp routing, egress blocking is disabled
>>>> + * on all upstream ports, and the root port supports 32-bit, 64-bit and/or
>>>> + * 128-bit AtomicOp completion, or negative otherwise.
>>>> + */
>>>> +int pci_enable_atomic_ops_to_root(struct pci_dev *dev)
>>>> +{
>>>> + struct pci_bus *bus = dev->bus;
>>>> +
>>>> + if (!pci_is_pcie(dev))
>>>> + return -EINVAL;
>>>> +
>>>> + switch (pci_pcie_type(dev)) {
>>>> + /*
>>>> + * PCIe 3.0, 6.15 specifies that endpoints and root ports are permitted
>>>> + * to implement AtomicOp requester capabilities.
>>>> + */
>>>> + case PCI_EXP_TYPE_ENDPOINT:
>>>> + case PCI_EXP_TYPE_LEG_END:
>>>> + case PCI_EXP_TYPE_RC_END:
>>>> + break;
>>>> + default:
>>>> + return -EINVAL;
>>>> + }
>>>> +
>>>> + while (bus->parent) {
>>>> + struct pci_dev *bridge = bus->self;
>>>> + u32 cap;
>>>> +
>>>> + pcie_capability_read_dword(bridge, PCI_EXP_DEVCAP2, &cap);
>>>> +
>>>> + switch (pci_pcie_type(bridge)) {
>>>> + /*
>>>> + * Upstream, downstream and root ports may implement AtomicOp
>>>> + * routing capabilities. AtomicOp routing via a root port is
>>>> + * not considered.
>>>> + */
>>>> + case PCI_EXP_TYPE_UPSTREAM:
>>>> + case PCI_EXP_TYPE_DOWNSTREAM:
>>>> + if (!(cap & PCI_EXP_DEVCAP2_ATOMIC_ROUTE))
>>>> + return -EINVAL;
>>>> + break;
>>>> +
>>>> + /*
>>>> + * Root ports are permitted to implement AtomicOp completion
>>>> + * capabilities.
>>>> + */
>>>> + case PCI_EXP_TYPE_ROOT_PORT:
>>>> + if (!(cap & (PCI_EXP_DEVCAP2_ATOMIC_COMP32 |
>>>> + PCI_EXP_DEVCAP2_ATOMIC_COMP64 |
>>>> + PCI_EXP_DEVCAP2_ATOMIC_COMP128)))
>>>> + return -EINVAL;
>>>> + break;
>>>> + }
>>> IIUC, you want to enable an endpoint, e.g., an AMD Fiji-class GPU, to
>>> initiate AtomicOps that target system memory. This interface
>>> (pci_enable_atomic_ops_to_root()) doesn't specify what size operations
>>> the driver wants to do. If the GPU requests a 128-bit op and the Root
>>> Port doesn't support it, I think we'll see an Unsupported Request
>>> error.
>>>
>>> Do you need to extend this interface so the driver can specify what
>>> sizes it wants?
>>>
>>> The existing code in qedr_pci_set_atomic() is very similar. We should
>>> make this new interface work for both places, then actually use it in
>>> qedr_pci_set_atomic().
>> Hi Bjorn, Doug, Ram,
>>
>> I just discussed this with Jay, and he noticed that qedr_pci_set_atomic
>> seems to use a different criteria to find the completer for atomic
>> requests. Jay's function expects the root port to have a parent, which
>> was the case on the systems he tested. But Ram's function looks for a
>> bridge without a parent and checks completion capabilities on that. Jay
>> believes that to be a root complex, not a root port.
> By "Ram's function", I guess you mean qedr_pci_set_atomic()?
>
> That starts with a PCIe device ("pdev"; it assumes but does not check
> that this is a PCIe device), and traverses through all the bridges
> leading to it. Usually this will be:
>
> endpoint -> root port
> endpoint -> switch downstream port -> switch upstream port -> root port
>
> Or there may be additional switches in the middle. The code is
> actually not quite correct because it is legal to have this:
>
> endpoint -> PCI-to-PCIe bridge -> conventional PCI bridge -> ...
>
> and qedr_pci_set_atomic() will traverse up through the conventional
> part of the hierarchy, where there is no PCI_EXP_DEVCAP2.
>
> In general, a Root Port is the root of a PCIe hierarchy and there is
> no parent device. E.g., on my laptop:
>
> 00:1c.0 Intel Root Port (bridge to [bus 02])
> 00:1c.2 Intel Root Port (bridge to [bus 04])
>
> What sort of parent do you expect? As I mentioned, it's legal to have
> a PCI/PCI-X to PCIe bridge inside a conventional PCI hierarchy, but
> that's a little unusual.
>
>> According to the spec, "Root ports are permitted to implement AtomicOp
>> completion capabilities." It talks about a root port, not a root complex.
>>
>> Can you help us understand, which interpretation is correct? And how to
>> correctly identify the root port for checking completion capabilities?
> If you start with a PCIe device and traverse upstream, you should
> eventually reach a Root Port or a PCI/PCI-X to PCIe bridge.
>
>> Are there valid topologies where a root port does not have a parent?
> I don't understand this because Root Ports normally do not have
> parents.
>
> PCIe devices other than Root Ports normally have a Root Port (or
> PCI/PCI-X to PCIe bridge) at the root of the PCIe hierarchy, but there
> are definitely exceptions.
>
> For example, there some systems where the Root Port is not visible to
> Linux, e.g.,
> http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=ee8bdfb6568d
> On systems like that, I don't think you can safely use AtomicOps.
>
> Bjorn
