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Subject: Re: [PATCH] PCI / PM: Don't runtime suspend when device only supports
 wakeup from D0
From:   Kai Heng Feng <kai.heng.feng@canonical.com>
In-Reply-To: <20190522185339.pfo5xeopyz2i5iem@wunner.de>
Date:   Thu, 23 May 2019 03:05:08 +0800
Cc:     Alan Stern <stern@rowland.harvard.edu>,
        Bjorn Helgaas <helgaas@kernel.org>,
        Rafael Wysocki <rafael.j.wysocki@intel.com>,
        linux-pci@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
        Mathias Nyman <mathias.nyman@intel.com>,
        linux-usb@vger.kernel.org
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Message-Id: <850CC1CD-2043-4C32-8BB1-5F5BAC1DDF55@canonical.com>
References: <20190522181157.GC79339@google.com>
 <Pine.LNX.4.44L0.1905221433310.1410-100000@iolanthe.rowland.org>
 <20190522185339.pfo5xeopyz2i5iem@wunner.de>
To:     Lukas Wunner <lukas@wunner.de>
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> On May 23, 2019, at 2:53 AM, Lukas Wunner <lukas@wunner.de> wrote:
> 
> On Wed, May 22, 2019 at 02:39:56PM -0400, Alan Stern wrote:
>> According to Kai, PME signalling doesn't work in D0 -- or at least, it
>> is _documented_ not to work in D0 -- even though it is enabled and the
>> device claims to support it.
>> 
>> In any case, I don't really see any point in "runtime suspending" a 
>> device while leaving it in D0.  We might as well just leave it alone.
> 
> There may be devices whose drivers are able to reduce power consumption
> through some device-specific means when runtime suspending, even though
> the device remains in PCI_D0.  The patch would cause a power regression
> for those.
> 
> In particular, pci_target_state() returns PCI_D0 if the device lacks the
> PM capability.

So an explicit device_can_wakeup() check before calling pci_target_state()
is needed to avoid the case you described.

I’ll add this in patch v2.

Kai-Heng

> 
> Thanks,
> 
> Lukas