Re: [PATCH] PCI / PM: Don't runtime suspend when device only supports wakeup from D0

From: Lukas Wunner <lukas@wunner.de>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Bjorn Helgaas <helgaas@kernel.org>,
	Kai Heng Feng <kai.heng.feng@canonical.com>,
	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
Subject: Re: [PATCH] PCI / PM: Don't runtime suspend when device only supports wakeup from D0
Date: Wed, 22 May 2019 20:53:39 +0200	[thread overview]
Message-ID: <20190522185339.pfo5xeopyz2i5iem@wunner.de> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1905221433310.1410-100000@iolanthe.rowland.org>

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.

Thanks,

Lukas