linux-kernel - Re: [PATCH] PCI: Set device power state to PCI

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Date:	Tue, 18 Oct 2011 19:22:21 +0530
From:	Ajaykumar Hotchandani <ajaykumar.hotchandani@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Yinghai Lu <yhlu.kernel@...il.com>,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PCI: Set device power state to PCI_D0 for device without
 native PM support



On 10/13/2011 08:00 PM, Stefano Stabellini wrote:
> On Mon, 10 Oct 2011, Ajaykumar Hotchandani wrote:
>> On 10/06/2011 09:47 PM, Stefano Stabellini wrote:
>>> On Thu, 6 Oct 2011, Yinghai Lu wrote:
>>>> On Thu, Oct 6, 2011 at 3:39 AM, Stefano Stabellini
>>>> <stefano.stabellini@...citrix.com>   wrote:
>>>>> I had the same issue and sent a patch a while ago to fix it, adding
>>>>>
>>>>> current_state = PCI_D0 in acpiphp_glue.c:register_slot
>>>>>
>>>>> it is strange that this does not work for you:
>>>>>
>>>>> http://marc.info/?l=linux-kernel&m=129891002722845&w=2
>>>> So guest os has to load acpiphp instead of pciehp?
>>> maybe pciehp needs to make sure that current_state = D0 in
>>> pciehp_enable_slot, like acpiphp does
>> Here, acpi hotplugging is involved.
>> With your change in register_slot(), device will have proper power state when module is being loaded for the first time after booting.
>> However, while unload of pci module; following is in pci_device_remove():
>>           if (pci_dev->current_state == PCI_D0)
>>                   pci_dev->current_state = PCI_UNKNOWN;
>>
>> So, device power state state will remain PCI_UNKNOWN while module is loaded again. Subsequently, MSI write will do nothing.
> Does this mean that this bug would actually trigger even with devices that
> do support _EJ0 and power management?
Nope. I don't have system to verify. But, for the scenario you mentioned 
(device is hot pluggable, acpi bus power manageable and device with 
pm_cap supported; if I understand it correctly), following is what I 
think should happen:
- Inside pci_platform_power_transition(), 
platform_pci_power_manageable() will be successful. However, 
platform_pci_set_power_state() will fail (as device supports EJ0) and 
subsequently pci_update_current_state() will not get called.
- So, current power state of device will not be read and power state of 
device will remain PCI_UNKNOWN.
- Now, pci_raw_set_power_state() will get called. Here, as current power 
state is PCI_UNKNOWN, pci_write_config_word() will be called with pmcsr 
value as 0. As, last two bits of pmcsr is 0, written power state of 
device will be PCI_D0 now. And subsequently, dev->current_state will be 
assigned as PCI_D0 (by using pci_read_config_word() ).
> Because acpi_pci_set_power_state won't set current_state to PCI_D0
> because the "hotplug driver will take care of _PSx" (see
> 10b3dcae0f275e2546e55303d64ddbb58cec7599) but the hotplug driver is not
> actually invoked when loading again a driver module of an existing pci
> device (the example you mention above)?
Can you please elaborate more on this?
acpi_bus_set_power() will update state of acpi_dev (ACPI_STATE_XX) and 
not pci_dev (PCI_XX).
And here, issue lies with current_state of pci_dev.

Thanks,
Ajay

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