Re: [PATCH v8 10/13] PCI/acpiphp: do not use ACPI PCI subdriver mechanism

[Bjorn Helgaas]

On Wed, Apr 10, 2013 at 7:50 PM, Yijing Wang <wangyijing@xxxxxxxxxx> wrote:
>>> Hi Bjorn,
>>>         Thanks for review.
>>>
>>>> My goal is that a user should never have to specify a kernel boot
>>>> parameter or edit a modules.conf file, but the user did previously
>>>> have some way to influence whether we use pciehp or acpiphp.  I know
>>>> we still have some issues, particularly with acpiphp, so I'm a little
>>>> concerned that by removing the CONFIG_HOTPLUG_PCI_ACPI=m, we might be
>>>> removing a way to work around those issues.
>>>>
>>>> A distro that previously used CONFIG_HOTPLUG_PCI_ACPI=m will now have
>>>> to use =y, so modules.conf is no longer applicable.  Can you convince
>>>> me that the user still has a way to work around issues?  I spent quite
>>>> a while trying to understand the pciehp/acpiphp dependencies, but it's
>>>> pretty tangled web.
>>> I will try my best to explain the relationships between pciehp and acpiphp
>>> as of v3.9-rc6.
>>>
>>> The pciehp driver always have priority over the acpiphp driver.
>>> That is, the acpiphp driver rejects binding to an ACPI PCI hotplug slot if
>>> a) The slot's parent is a PCIe port with native hotplug capability
>>> b) OSPM has taken over PCIe native hotplug control from BIOS.
>>>         !(root->osc_control_set & OSC_PCI_EXPRESS_NATIVE_HP_CONTROL)
>>> The above check has no dependency on the loading order of pciehp and acpiphp
>>> drivers. So converting acpiphp driver to builit-in should be ok.
>>>
>>> On the other hand, I remember Yinghai has mentioned that some PCIe ports
>>> with native hotplug capability doesn't work as expected with the pciehp driver
>>> and should be managed by the acpiphp driver. Currently we could achieve that
>>> by using boot param "pcie_ports=compat", but this will disable PCIe port
>>> drivers altogether. And I also remember that Rafael has mentioned that
>>> some BIOSes exhibit strange dependency among PCIe OSC controls, so it's
>>> not feasible to only disable PCIe native hotplug.
>>>
>>> For "pciehp_force", it does only affect the way pciehp to detect a hotplug
>>> slot, it doesn't affect acpiphp at all.
>>>
>>> To sum up, converting acpiphp as built-in should not affect the relationship
>>> between pciehp and acpiphp driver.
>>
>> My concern is that a user used to be able to remove acpiphp from
>> modules.conf.  Now removing acpiphp will require a kernel rebuild.
>> But maybe that won't turn out to be a problem.
>
> Hi Bjorn,
>    If user don't want to occupy the slot by acpiphp. Conservative approach, what about add a kernel parameter
> to control acpiphp to enumerate slot ?

Yes, that's what I'm thinking.  A "Please report a bug if you have to
use this parameter" chicken switch.  Hopefully nobody will need to use
it, but if somebody *does* need it, it's a lot better than having to
tell him to rebuild the kernel.

Bjorn
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