linux-kernel - Re: [PATCH] PCI/ASPM: fix reverse ASPM L0s assignment of upstream and downstream

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Date:	Wed, 25 May 2016 12:50:39 -0500
From:	Bjorn Helgaas <helgaas@...nel.org>
To:	Sinan Kaya <okaya@...eaurora.org>
Cc:	Ocean HY1 He <hehy1@...ovo.com>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>,
	"wangyijing@...wei.com" <wangyijing@...wei.com>,
	"luto@...nel.org" <luto@...nel.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"prarit@...hat.com" <prarit@...hat.com>,
	"jcm@...hat.com" <jcm@...hat.com>,
	Nagananda Chumbalkar <nchumbalkar@...ovo.com>
Subject: Re: [PATCH] PCI/ASPM: fix reverse ASPM L0s assignment of upstream
 and downstream

On Wed, May 25, 2016 at 01:21:12PM -0400, Sinan Kaya wrote:
> Hi Bjorn,
> 
> OK. I see that we are dealing with two different questions.
> 
> > I thought you were talking about booting with
> > "pcie_aspm.policy=powersave", where pcie_aspm_set_policy() sets
> > aspm_policy = POLICY_POWERSAVE, then configures each link with
> > ASPM_STATE_ALL.  But pcie_config_aspm_link() does AND the desired
> > state (ASPM_STATE_ALL) with link->aspm_capable, which only has
> > ASPM_STATE_L0S set if both the upstream and downstream components
> > advertised PCIE_LINK_STATE_L0S.
> > This path is very complicated, but I don't *think* it will enable L0s
> > if the other end of the link doesn't support it.
> 
> Thanks for clarifying this. You are saying that if one side of the
> link doesn't support L0s, Linux doesn't enable L0s on the other side.
> This makes sense.
> 
> I think there is a confusion between what supported means vs. when
> L0s can be enabled on which side of the link.
> 
> You clarified the supported case above that code will not enable
> L0s if the other side doesn't support L0s.
> 
> 
> > Now we enable the downstream component's transmitter to enter L0s.
> > Per PCIe spec r3.0, sec 7.8.7, the receiver, i.e., the upstream
> > component, must be capable of entering L0s even when its transmitter
> > is disabled from entering L0s.
> 
> Let's assume that both sides actually support L0s but the link parameters
> on one side is not compatible.
> 
> You are saying that it is OK to enable L0s on just one side of the
> link as long as both sides support L0s. 

I'm not sure what you mean by the link parameters not being
compatible, but I think it is legal to enable L0s on only one
direction.

> This part is a little bit misleading. I had HW people telling me
> that both sides need to enable L0s at about the same time.

I don't remember seeing anything like that in the spec.  Do they have
a pointer?  "At about the same time" is too hand-wavey to be useful to
software.

> I'm actually seeing Linux enabling L0s on one side only as you
> described and both supports L0s.