PCI host bridge hotplug test question (was Re: [PATCH v2] bus: hisi_lpc: Don't use devm_kzalloc() to allocate logical PIO range)

[John Garry <john.garry@huawei.com>]

On 18/06/2019 18:50, Bjorn Helgaas wrote:
> [+cc Rafael, Mika, Jiang, linux-pci for ACPI host bridge hotplug test question]
>
> On Tue, Jun 18, 2019 at 3:44 AM John Garry <john.garry@huawei.com> wrote:
>
>>>>> Could you just move the logic_pio_register_range() call farther down
>>>>> in hisi_lpc_probe()?  IIUC, once logic_pio_register_range() returns,
>>>>> an inb() with the right port number will try to access that port, so
>>>>> we should be prepared for that, i.e., maybe this in the wrong order to
>>>>> begin with?
>>>>
>>>> No, unfortunately we can't. The reason is that we need the logical PIO
>>>> base for that range before we enumerate the children of that host. We
>>>> need that base address for "translating" the child bus addresses to
>>>> logical PIO addresses.
>
>>> Ah, yeah, that makes sense.  I think.  We do assume that we know all
>>> the MMIO and I/O port translations before enumerating devices.  It's
>>> *conceivable* that could be changed someday since we don't actually
>>> need the translations until a driver claims the device,
>>
>> We actually need them before a driver claims the device.
>>
>> The reason is that when we create that child platform device we set the
>> device's IORESOURCE_IO resources according to the translated logic PIO
>> addresses, and not the host bus address. This is what makes the host
>> transparent to the child device driver.
>
> I think you need it to set pdev->resource[], which is currently done
> long before the driver claims the device (though one could imagine
> delaying it even as far as pci_enable_device()-time).  I don't think
> the translation is actually *used* until the driver claims the device
> because only the driver knows how to do any inb/outb to the device.
>
> But of course, that's all speculative and doesn't change what you need
> to do now.  The current code assumes we know the translations during
> enumeration, so you need to do the logic_pio registration before
> enumerating.
>
>>> and it would
>>> gain some flexibility if we didn't have to program the host bridge
>>> windows until we know how much space is required.  But I don't see
>>> that happening anytime soon.
>
>> My problem is that I need to ensure that the new logical PIO unregister
>> function works ok for hot-pluggable host bridges. I need to get some way
>> to test this. Advice?
>

Hi Bjorn,

> Good question.  The ACPI host bridge driver (drivers/acpi/pci_root.c)
> should support hotplug, but I'm not sure if there's a manual way to
> trigger it via sysfs or something similar.  If there is, and you have
> a machine with more than one host bridge, you might be able to remove
> one that leads to non-essential devices.

For one of our earlier boards I don't think that it had any essential 
devices on the host bridge. But I need to find out about possibility of 
removal. Hmmm.

>
> Bjorn
>

Further to the topic of supporting hotplug and unregistering IO port 
regions, we don't even release IO port regions in the error path of PCI 
host enumeration. We have pci_register_io_range(), but no unregister 
equivalent.

Looking at the history here, pci_register_io_range() was originally in 
OF code. And in the OF code, calling pci_register_io_range() is a 
side-effect of parsing the device tree. As such, I can see why there was 
no unregister function.

It would be worth noting this discussion, where the same was mentioned:
https://lore.kernel.org/linux-pci/20180403140410.GE27789@ulmo/

The tegra PCI host probe can defer, but, since there is no tidy-up of 
pci_register_io_range() when deferring, we need to ensure that the port 
IO management code can handle re-attempts to register the same range.

It looks like this can be cleaned up also.

Thanks,
John

> .
>