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

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Message-ID: <20200825065634.GA2691@infradead.org>
Date:   Tue, 25 Aug 2020 07:56:34 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Kai Heng Feng <kai.heng.feng@...onical.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        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 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.