Re: [PATCH 1/7] PCI/portdrv: Create a platform device for the perf uncore driver

[Bjorn Helgaas]

[+cc Stephane in case he has thoughts on the perf driver claim issue]

On Thu, Jul 02, 2020 at 10:05:11AM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> 
> On Snow Ridge server, several performance monitoring counters are added
> in the Root Port Configuration Space of CPU Complex PCIe Root Ports A,
> which can be used to collect the performance data between the PCIe
> devices and the components (in M2IOSF) which are responsible for
> translating and managing the requests to/from the device. The
> performance data is very useful for analyzing the performance of the
> PCIe devices.
> 
> However, the perf uncore driver cannot be loaded to register a
> performance monitoring unit (PMU) for the counters, because the PCIe
> Root Ports device already has a bonded driver portdrv_pci.
> 
> To enable the uncore PMU support for these counters on the uncore
> driver, a new solution should be introduced, which has to meet the
> requirements as below:
> - must have a reliable way to find the PCIe Root Port device from the
>   uncore driver;
> - must be able to access the uncore counters of the PCIe Root Port
>   device from the uncore driver;
> - must support hotplug. When the PCIe Root Port device is removed, the
>   uncore driver has to be notified and unregisters the uncore PMU.
> 
> A new platform device 'perf_uncore_pcieport' is introduced as part of
> the new solution, which can facilitate the enabling of the uncore PMU in
> the uncore driver. The new platform device
> - is a child device of the PCIe Root Port device. It's allocated when
>   the PCIe Root Ports A device is probed. (For SNR, the PMU counters are
>   only located in the configuration space of the PCIe Root Ports A.)
> - stores its pdev as the private driver data pointer of the PCIe Root
>   Ports A. The pdev can be easily retrieved to check the existence of
>   the platform device when removing the PCIe Root Ports A.
> - is unregistered when the PCIe Root Port A is removed. The remove()
>   method which is provided in the uncore driver will be invoked. The
>   uncore PMU will be unregistered as well.
> - doesn't share any memory and IRQ resources. The uncore driver will
>   only touch the PMU counters in the configuration space of the PCIe
>   Root Port A.

I have to admit this is clever.  I don't really *like* it, but we
don't have any very good alternatives at the moment.

I don't like the idea of a list of PCI IDs
(perf_uncore_pcieport_ids[]) below that must be updated for every
device that needs something like this.  That PCI ID information is
normally in the drivers themselves, not in bus-level code like this.

And I don't like the way this subverts the device ownership model.
Now we have several drivers (pciehp, aer, dpc, etc, plus this new perf
driver) that share the same PCI device.  And we rely on the assumption
that none of these drivers interferes with the others.

I think the best way to deal with this would be to incorporate the
existing portdrv users (pciehp, aer, dpc, etc) directly into the PCI
core so portdrv would not use pci_register_driver(), leaving the Root
Port device available for the perf driver to claim it the normal way.
But realistically I don't know when or even whether this will be done.

I think Stephane has worked around this problem in a different way,
IIRC by using pci_get_device() in a perf driver to find Ports of
interest.  That also subverts the device ownership model, and it
doesn't work naturally with hotplug, but at least it gets the device
IDs out of the PCI core and into the driver where they belong.  And
there's value in solving the same problem in the same way.

Wait a minute!  You've already used the pci_get_device() strategy
several times:

  2b3b76b5ec67 ("perf/x86/intel/uncore: Add Ice Lake server uncore support")
  fdb64822443e ("perf/x86: Add Intel Tiger Lake uncore support")
  ee49532b38dd ("perf/x86/intel/uncore: Add IMC uncore support for Snow Ridge")

So what's really different about *this* situation?  Why would you not
just continue using the same strategy?

> Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> ---
>  drivers/pci/pcie/portdrv_pci.c | 38 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 38 insertions(+)
> 
> diff --git a/drivers/pci/pcie/portdrv_pci.c b/drivers/pci/pcie/portdrv_pci.c
> index 3acf151..47e33b2 100644
> --- a/drivers/pci/pcie/portdrv_pci.c
> +++ b/drivers/pci/pcie/portdrv_pci.c
> @@ -15,6 +15,7 @@
>  #include <linux/init.h>
>  #include <linux/aer.h>
>  #include <linux/dmi.h>
> +#include <linux/platform_device.h>
>  
>  #include "../pci.h"
>  #include "portdrv.h"
> @@ -90,6 +91,40 @@ static const struct dev_pm_ops pcie_portdrv_pm_ops = {
>  #define PCIE_PORTDRV_PM_OPS	NULL
>  #endif /* !PM */
>  
> +static const struct pci_device_id perf_uncore_pcieport_ids[] = {
> +	{ PCI_DEVICE(PCI_VENDOR_ID_INTEL, 0x334a) },
> +	{ },
> +};
> +
> +static void perf_platform_device_register(struct pci_dev *dev)
> +{
> +	struct platform_device *pdev;
> +
> +	if (!pci_match_id(perf_uncore_pcieport_ids, dev))
> +		return;
> +
> +	pdev = platform_device_alloc("perf_uncore_pcieport", PLATFORM_DEVID_AUTO);
> +	if (!pdev)
> +		return;
> +
> +	pdev->dev.parent = &dev->dev;
> +
> +	if (platform_device_add(pdev)) {
> +		platform_device_put(pdev);
> +		return;
> +	}
> +
> +	pci_set_drvdata(dev, pdev);
> +}
> +
> +static void perf_platform_device_unregister(struct pci_dev *dev)
> +{
> +	struct platform_device *pdev = pci_get_drvdata(dev);
> +
> +	if (pdev)
> +		platform_device_unregister(pdev);
> +}
> +
>  /*
>   * pcie_portdrv_probe - Probe PCI-Express port devices
>   * @dev: PCI-Express port device being probed
> @@ -113,6 +148,8 @@ static int pcie_portdrv_probe(struct pci_dev *dev,
>  	if (status)
>  		return status;
>  
> +	perf_platform_device_register(dev);
> +
>  	pci_save_state(dev);
>  
>  	dev_pm_set_driver_flags(&dev->dev, DPM_FLAG_NO_DIRECT_COMPLETE |
> @@ -142,6 +179,7 @@ static void pcie_portdrv_remove(struct pci_dev *dev)
>  		pm_runtime_dont_use_autosuspend(&dev->dev);
>  	}
>  
> +	perf_platform_device_unregister(dev);
>  	pcie_port_device_remove(dev);
>  }
>  
> -- 
> 2.7.4
>