Re: [PATCH v9 3/4] drivers/perf: add DesignWare PCIe PMU driver

[Robin Murphy <robin.murphy@arm.com>]

On 2023-10-23 13:32, Will Deacon wrote:
[...]
>> +
>> +	/*
>> +	 * The Group#1 event measures the amount of data processed in 16-byte
>> +	 * units. Simplify the end-user interface by multiplying the counter
>> +	 * at the point of read.
>> +	 */
>> +	if (event_id >= 0x20 && event_id <= 0x23)
>> +		return (((u64)hi << 32) | lo) << 4;
>> +	else
>> +		return (((u64)hi << 32) | lo);
> 
> nit, but I think it would be clearer to do:
> 
> 	ret = ((u64)hi << 32) | lo;
> 
> 	/* ... */
> 	if (event_id >= 0x20 && event_id <= 0x23)
> 		ret <<= 4;

Nit: "ret *= 16;" since the comment says it's multiplying a value, not 
moving a bitfield. The compiler already knows the most efficient way to 
implement constant multiplication.

> 
> 	return ret;
> 
[...]
>> +static int __init dwc_pcie_pmu_init(void)
>> +{
>> +	int ret;
>> +
>> +	ret = cpuhp_setup_state_multi(CPUHP_AP_ONLINE_DYN,
>> +				      "perf/dwc_pcie_pmu:online",
>> +				      dwc_pcie_pmu_online_cpu,
>> +				      dwc_pcie_pmu_offline_cpu);
>> +	if (ret < 0)
>> +		return ret;
>> +
>> +	dwc_pcie_pmu_hp_state = ret;
>> +
>> +	ret = platform_driver_register(&dwc_pcie_pmu_driver);
>> +	if (ret)
>> +		goto platform_driver_register_err;
>> +
>> +	dwc_pcie_pmu_dev = platform_device_register_simple(
>> +				"dwc_pcie_pmu", PLATFORM_DEVID_NONE, NULL, 0);
>> +	if (IS_ERR(dwc_pcie_pmu_dev)) {
>> +		ret = PTR_ERR(dwc_pcie_pmu_dev);
>> +		goto platform_device_register_error;
>> +	}
> 
> I'm a bit confused as to why you're having to create a platform device
> for a PCI device -- is this because the main designware driver has already
> bound to it? A comment here explaining why you need to do this would be
> very helpful. In particular, is there any dependency on another driver
> to make sure that e.g. config space accesses work properly? If so, we
> probably need to enforce module load ordering or something like that.

AFAICS the platform device/driver serve no purpose other than being a 
hilariously roundabout way to run the for_each_pci_dev() loop in 
dwc_pcie_pmu_probe() upon module init, and to save explicitly freeing 
the PMU name/data. Furthermore the devres action for 
dwc_pcie_pmu_remove_cpuhp_instance() is apparently going for even more 
style points at module exit by not even relying on the corresponding 
.remove callback of the tenuous platform driver to undo what its .probe 
did, but (ab)using the device's devres list to avoid having to keep 
track of an explicit list of PMU instances at all.

Frankly I think it would be a lot more straightforward to just maintain 
that explicit list of PMU instances, do the PMU creation directly in 
dwc_pcie_pmu_init(), then unregister and free them in 
dwc_pcie_pmu_exit(). Not every driver has to contain a literal struct 
device_driver.

It also smells a bit odd that it handles PCI hot-remove but not hot-add 
- if the underlying device really is hotpluggable, wouldn't we also want 
to handle new ones turning up after module load? Conversely if it isn't, 
why pretend to handle it being removed? Even if it's not to do with 
physical hotplug of the PMU but with the user unloading the PCI 
controller driver itself (since there's no module/driver-level 
dependency enforced) and thus tearing down the whole PCI bus, then the 
same point still applies - if that *can* happen, then what if the user 
then re-loads it again, or indeed if this module loads first to begin 
with; wouldn't we want to be able to (re-)discover the PMUs rather than 
leave the whole PMU driver degraded to a useless state?

Thanks,
Robin.