Re: [PATCH] PCI/ASPM: Enable ASPM for links under VMD domain

[Kai-Heng Feng]

> On Sep 18, 2020, at 01:20, Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> 
> On Thu, Sep 10, 2020 at 07:51:05PM +0000, Derrick, Jonathan wrote:
>> On Thu, 2020-09-10 at 14:17 -0500, Bjorn Helgaas wrote:
>>> On Thu, Sep 10, 2020 at 06:52:48PM +0000, Derrick, Jonathan wrote:
>>>> On Thu, 2020-09-10 at 12:38 -0500, Bjorn Helgaas wrote:
>>>>> On Thu, Sep 10, 2020 at 04:33:39PM +0000, Derrick, Jonathan wrote:
>>>>>> On Wed, 2020-09-09 at 20:55 -0500, Bjorn Helgaas wrote:
>>>>>>> On Fri, Aug 21, 2020 at 08:32:20PM +0800, Kai-Heng Feng wrote:
>>>>>>>> New Intel laptops with VMD cannot reach deeper power saving state,
>>>>>>>> renders very short battery time.
>>>>>>>> 
>>>>>>>> As BIOS may not be able to program the config space for devices under
>>>>>>>> VMD domain, ASPM needs to be programmed manually by software. This is
>>>>>>>> also the case under Windows.
>>>>>>>> 
>>>>>>>> The VMD controller itself is a root complex integrated endpoint that
>>>>>>>> doesn't have ASPM capability, so we can't propagate the ASPM settings to
>>>>>>>> devices under it. Hence, simply apply ASPM_STATE_ALL to the links under
>>>>>>>> VMD domain, unsupported states will be cleared out anyway.
>>>>>>>> 
>>>>>>>> Signed-off-by: Kai-Heng Feng <kai.heng.feng@xxxxxxxxxxxxx>
>>>>>>>> ---
>>>>>>>> drivers/pci/pcie/aspm.c |  3 ++-
>>>>>>>> drivers/pci/quirks.c    | 11 +++++++++++
>>>>>>>> include/linux/pci.h     |  2 ++
>>>>>>>> 3 files changed, 15 insertions(+), 1 deletion(-)
>>>>>>>> 
>>>>>>>> diff --git a/drivers/pci/pcie/aspm.c b/drivers/pci/pcie/aspm.c
>>>>>>>> index 253c30cc1967..dcc002dbca19 100644
>>>>>>>> --- a/drivers/pci/pcie/aspm.c
>>>>>>>> +++ b/drivers/pci/pcie/aspm.c
>>>>>>>> @@ -624,7 +624,8 @@ static void pcie_aspm_cap_init(struct pcie_link_state *link, int blacklist)
>>>>>>>> 		aspm_calc_l1ss_info(link, &upreg, &dwreg);
>>>>>>>> 
>>>>>>>> 	/* Save default state */
>>>>>>>> -	link->aspm_default = link->aspm_enabled;
>>>>>>>> +	link->aspm_default = parent->dev_flags & PCI_DEV_FLAGS_ENABLE_ASPM ?
>>>>>>>> +			     ASPM_STATE_ALL : link->aspm_enabled;
>>>>>>> 
>>>>>>> This function is ridiculously complicated already, and I really don't
>>>>>>> want to make it worse.
>>>>>>> 
>>>>>>> What exactly is the PCIe topology here?  Apparently the VMD controller
>>>>>>> is a Root Complex Integrated Endpoint, so it's a Type 0 (non-bridge)
>>>>>>> device.  And it has no Link, hence no Link Capabilities or Control and
>>>>>>> hence no ASPM-related bits.  Right?
>>>>>> 
>>>>>> That's correct. VMD is the Type 0 device providing config/mmio
>>>>>> apertures to another segment and MSI/X remapping. No link and no ASPM
>>>>>> related bits.
>>>>>> 
>>>>>> Hierarchy is usually something like:
>>>>>> 
>>>>>> Segment 0           | VMD segment
>>>>>> Root Complex -> VMD | Type 0 (RP/Bridge; physical slot) - Type 1
>>>>>>                    | Type 0 (RP/Bridge; physical slot) - Type 1
>>>>>> 
>>>>>>> And the devices under the VMD controller?  I guess they are regular
>>>>>>> PCIe Endpoints, Switch Ports, etc?  Obviously there's a Link involved
>>>>>>> somewhere.  Does the VMD controller have some magic, non-architected
>>>>>>> Port on the downstream side?
>>>>>> 
>>>>>> Correct: Type 0 and Type 1 devices, and any number of Switch ports as
>>>>>> it's usually pinned out to physical slot.
>>>>>> 
>>>>>>> Does this patch enable ASPM on this magic Link between VMD and the
>>>>>>> next device?  Configuring ASPM correctly requires knowledge and knobs
>>>>>>> from both ends of the Link, and apparently we don't have those for the
>>>>>>> VMD end.
>>>>>> 
>>>>>> VMD itself doesn't have the link to it's domain. It's really just the
>>>>>> config/mmio aperture and MSI/X remapper. The PCIe link is between the
>>>>>> Type 0 and Type 1 devices on the VMD domain. So fortunately the VMD
>>>>>> itself is not the upstream part of the link.
>>>>>> 
>>>>>>> Or is it for Links deeper in the hierarchy?  I assume those should
>>>>>>> just work already, although there might be issues with latency
>>>>>>> computation, etc., because we may not be able to account for the part
>>>>>>> of the path above VMD.
>>>>>> 
>>>>>> That's correct. This is for the links within the domain itself, such as
>>>>>> between a type 0 and NVMe device.
>>>>> 
>>>>> OK, great.  So IIUC, below the VMD, there is a Root Port, and the Root
>>>>> Port has a link to some Endpoint or Switch, e.g., an NVMe device.  And
>>>>> we just want to enable ASPM on that link.
>>>>> 
>>>>> That should not be a special case; we should be able to make this so
>>>>> it Just Works.  Based on this patch, I guess the reason it doesn't
>>>>> work is because link->aspm_enabled for that link isn't set correctly.
>>>>> 
>>>>> So is this just a consequence of us depending on the initial Link
>>>>> Control value from BIOS?  That seems like something we shouldn't
>>>>> really depend on.
>> Seems like a good idea, that it should instead be quirked if ASPM is
>> found unusable on a link. Though I'm not aware of how many platforms
>> would require a quirk..
>> 
>>>>> 
>>>> That's the crux. There's always pcie_aspm=force.
>>>> Something I've wondered is if there is a way we could 'discover' if the
>>>> link is ASPM safe?
>>> 
>>> Sure.  Link Capabilities is supposed to tell us that.  If aspm.c
>>> depends on the BIOS settings, I think that's a design mistake.
>>> 
>>> But what CONFIG_PCIEASPM_* setting are you using?  The default
>>> is CONFIG_PCIEASPM_DEFAULT, which literally means "Use the BIOS
>>> defaults".  If you're using that, and BIOS doesn't enable ASPM below
>>> VMD, I guess aspm.c will leave it disabled, and that seems like it
>>> would be the expected behavior.
>>> 
>>> Does "pcie_aspm=force" really help you?  I don't see any uses of it
>>> that should apply to your situation.
>>> 
>>> Bjorn
>> 
>> No you're right. I don't think we need pcie_aspm=force, just the policy
>> configuration.
> 
> I'm not sure where we're at here.
> 
> If the kernel is built with CONFIG_PCIEASPM_DEFAULT=y (which means
> "use the BIOS defaults"), and the BIOS doesn't enable ASPM on these
> links below VMD, then Linux will leave things alone.  I think that's
> working as intended.

Yes and that's the problem here. BIOS doesn't enable ASPM for links behind VMD.
The ASPM is enabled by VMD driver under Windows.

> 
> If desired, we should be able to enable ASPM using sysfs in that case.

I hope to keep everything inside kernel. Of course we can use udev rules to change sysfs value, if anyone prefers that approach.

> 
> We have a pci_disable_link_state() kernel interface that drivers can
> use to *disable* ASPM for their device.  But I don't think there's any
> corresponding interface for drivers to *enable* ASPM.  Maybe that's an
> avenue to explore?

Okay, I will work on pci_enable_link_state() helper and let VMD driver as its first user.

Kai-Heng

> 
> Bjorn