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

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Date:   Wed, 26 Aug 2020 13:53:56 +0800
From:   Kai-Heng Feng <kai.heng.feng@...onical.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        "Derrick, Jonathan" <jonathan.derrick@...el.com>,
        Mario.Limonciello@...l.com, Heiner Kallweit <hkallweit1@...il.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Xiongfeng Wang <wangxiongfeng2@...wei.com>,
        Krzysztof Wilczynski <kw@...ux.com>,
        "open list:PCI SUBSYSTEM" <linux-pci@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "Huffman, Amber" <amber.huffman@...el.com>
Subject: Re: [PATCH] PCI/ASPM: Enable ASPM for links under VMD domain



> On Aug 25, 2020, at 14:56, Christoph Hellwig <hch@...radead.org> wrote:
> 
> On Tue, Aug 25, 2020 at 02:39:55PM +0800, Kai Heng Feng wrote:
>> Hi Christoph,
>> 
>>> On Aug 25, 2020, at 2:23 PM, Christoph Hellwig <hch@...radead.org> 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.
>>> 
>>> So what about just disabling VMD given how bloody pointless it is?
>>> Hasn't anyone learned from the AHCI remapping debacle?
>>> 
>>> I'm really pissed at all this pointless crap intel comes up with just
>>> to make life hard for absolutely no gain.  Is it so hard to just leave
>>> a NVMe device as a standard NVMe device instead of f*^&ing everything
>>> up in the chipset to make OS support a pain and I/O slower than by
>>> doing nothing?
>> 
>> From what I can see from the hardwares at my hand, VMD only enables a PCI domain and PCI bridges behind it.
>> 
>> NVMe works as a regular NVMe under those bridges. No magic remapping happens here.
> 
> It definitively is less bad than the AHCI remapping, that is for sure.
> 
> But it still requires:
> 
> - a new OS driver just to mak the PCIe device show up
> - indirections in the irq handling
> - indirections in the DMA handling
> - hacks for ASPSM
> - hacks for X (there were a few more)
> 
> while adding absolutely no value.  Basically we have to add a large
> chunk of kernel code just to undo silicone/firmware Intel added to their
> platform to make things complicated.  I mean it is their platform and if
> they want a "make things complicated" option that is fine, but it should
> not be on by default.

Yes, I do want it to be a regular PCIe bridge... but it's not the reality here.
Almost all next-gen Intel laptops will have VMD enabled, so users are forced to have it.
I would really like to have this patch in upstream instead of carrying it as a downstream distro-only patch.

Kai-Heng