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Date:   Tue, 25 Aug 2020 07:56:34 +0100
From:   Christoph Hellwig <hch@infradead.org>
To:     Kai Heng Feng <kai.heng.feng@canonical.com>
Cc:     Christoph Hellwig <hch@infradead.org>,
        Bjorn Helgaas <bhelgaas@google.com>,
        jonathan.derrick@intel.com, Mario.Limonciello@dell.com,
        Heiner Kallweit <hkallweit1@gmail.com>,
        Mika Westerberg <mika.westerberg@linux.intel.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Xiongfeng Wang <wangxiongfeng2@huawei.com>,
        Krzysztof Wilczynski <kw@linux.com>,
        "open list:PCI SUBSYSTEM" <linux-pci@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        "Huffman, Amber" <amber.huffman@intel.com>
Subject: Re: [PATCH] PCI/ASPM: Enable ASPM for links under VMD domain
Message-ID: <20200825065634.GA2691@infradead.org>
References: <20200821123222.32093-1-kai.heng.feng@canonical.com>
 <20200825062320.GA27116@infradead.org>
 <08080FC7-861B-472A-BD7D-02D33926677F@canonical.com>
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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@infradead.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.