Re: [PATCH v3 2/3] drivers/perf: add DesignWare PCIe PMU driver

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On 2023/5/18 00:27, Bjorn Helgaas wrote:
> On Wed, May 17, 2023 at 10:54:21AM +0100, Jonathan Cameron wrote:
>> On Tue, 16 May 2023 14:17:52 -0500
>> Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
>>> On Tue, May 16, 2023 at 04:03:04PM +0100, Jonathan Cameron wrote:
>> ...
>>>> The approach used here is to separately walk the PCI topology and
>>>> register the devices.  It can 'maybe' get away with that because no
>>>> interrupts and I assume resets have no nasty impacts on it because
>>>> the device is fairly simple.  In general that's not going to work.
>>>> CXL does a similar trick (which I don't much like, but too late
>>>> now), but we've also run into the problem of how to get interrupts
>>>> if not the main driver.  
>>> Yes, this is a real problem.  I think the "walk all PCI devices
>>> looking for one we like" approach is terrible because it breaks a lot
>>> of driver model assumptions (no device ID to autoload module via udev,
>>> hotplug doesn't work, etc), but we don't have a good alternative right
>>> now.
>>> I think portdrv is slightly better because at least it claims the
>>> device in the usual way and gives a way for service drivers to
>>> register with it.  But I don't really like that either because it
>>> created a new weird /sys/bus/pci_express hierarchy full of these
>>> sub-devices that aren't really devices, and it doesn't solve the
>>> module load and hotplug issues.
>>> I would like to have portdrv be completely built into the PCI core and
>>> not claim Root Ports or Switch Ports.  Then those devices would be
>>> available via the usual driver model for driver loading and binding
>>> and for hotplug.
>> Let me see if I understand this correctly as I can think of a few options
>> that perhaps are inline with what you are thinking.
>> 1) All the portdrv stuff converted to normal PCI core helper functions
>>    that a driver bound to the struct pci_dev can use.
>> 2) Driver core itself provides a bunch of extra devices alongside the
>>    struct pci_dev one to which additional drivers can bind? - so kind
>>    of portdrv handling, but squashed into the PCI device topology?
>> 3) Have portdrv operated under the hood, so all the services etc that
>>    it provides don't require a driver to be bound at all.  Then
>>    allow usual VID/DID based driver binding.
>> If 1 - we are going to run into class device restrictions and that will
>> just move where we have to handle the potential vendor specific parts.
>> We probably don't want that to be a hydra with all the functionality
>> and lookups etc driven from there, so do we end up with sub devices
>> of that new PCI port driver with a discover method based on either
>> vsec + VID or DVSEC with devices created under the main pci_dev.
>> That would have to include nastiness around interrupt discovery for
>> those sub devices. So ends up roughly like port_drv.
>> I don't think 2 solves anything.
>> For 3 - interrupts and ownership of facilities is going to be tricky
>> as initially those need to be owned by the PCI core (no device driver bound)
>> and then I guess handed off to the driver once it shows up?  Maybe that
>> driver should call a pci_claim_port() that gives it control of everything
>> and pci_release_port() that hands it all back to the core.  That seems
>> racey.
> Yes, 3 is the option I want to explore.  That's what we already do for
> things like ASPM.  Agreed, interrupts is a potential issue.  I think
> the architected parts of config space should be implicitly owned by
> the PCI core, with interfaces à la pci_disable_link_state() if drivers
> need them.

I agree "walk all PCI devices looking for one we like" approach is terrible
in general. And I am glad to modify my code to adapt to a more suitable solution
when it comes.

For now, I will collect comments from v3 and v4 and send a new version after
addressed them. Any alternative option is welcomed, always :)

> Bjorn

Thank you.

Best Regards,

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