linux-kernel - Re: [PATCH v2 1/6] PCI/ASPM: Add locked helper for enabling link state

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Message-ID: <20231214172848.GA1095194@bhelgaas>
Date:   Thu, 14 Dec 2023 11:28:48 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     "David E. Box" <david.e.box@...ux.intel.com>
Cc:     Kai-Heng Feng <kai.heng.feng@...onical.com>,
        Johan Hovold <johan+linaro@...nel.org>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Manivannan Sadhasivam <mani@...nel.org>,
        Rob Herring <robh@...nel.org>,
        Nirmal Patel <nirmal.patel@...ux.intel.com>,
        Jonathan Derrick <jonathan.derrick@...ux.dev>,
        linux-arm-msm@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Subject: Re: [PATCH v2 1/6] PCI/ASPM: Add locked helper for enabling link
 state

On Wed, Dec 13, 2023 at 03:39:24PM -0800, David E. Box wrote:
> On Wed, 2023-12-13 at 14:45 -0600, Bjorn Helgaas wrote:
> ...

> > I'd be shocked if Windows treated the BIOS config as a "do not exceed
> > this" situation, so my secret hope is that some of these "broken"
> > devices are really caused by defects in the Linux ASPM support or the
> > driver, and that we can fix them if we find out about them.
> > 
> > But I have no details about any of these alleged broken devices, so
> > it's hard to make progress on them.  
> 
> I don't have a sense of the scope either. But I could see BIOS not
> enabling features that would provide no added power savings benefit.
> We use ASPM to manage package power. There are Intel devices that
> certainly don't require L1SS for the SoC to achieve the deepest
> power savings. L1 alone is fine for them. I don't know what the test
> coverage is for unenabled features. I've sent these questions to
> our BIOS folks.

Once upon a time there was a push to make it so firmware only had to
enumerate boot and console devices and it could skip enumeration and
configuration of other devices.  But I don't think we've made much
progress on that, at least for x86, possibly because Linux depends so
much on BIOS resource assignment.  IMO that's a Linux deficiency.

Bjorn