linux-kernel - Re: [PATCH 2/2] PCI PM: Read device power state from register after updating it

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Message-Id: <200906112153.16853.rjw@sisk.pl>
Date:	Thu, 11 Jun 2009 21:53:16 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Jesse Barnes <jesse.barnes@...el.com>
Cc:	pm list <linux-pm@...ts.linux-foundation.org>,
	Linux PCI <linux-pci@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jirislaby@...il.com>
Subject: Re: [PATCH 2/2] PCI PM: Read device power state from register after updating it

On Thursday 11 June 2009, Jesse Barnes wrote:
> On Tue, 26 May 2009 21:52:29 +0200
> "Rafael J. Wysocki" <rjw@...k.pl> wrote:
> 
> > On Monday 18 May 2009, Rafael J. Wysocki wrote:
> > > From: Rafael J. Wysocki <rjw@...k.pl>
> > > 
> > > After attempting to change the power state of a PCI device
> > > pci_raw_set_power_state() doesn't check if the value it wrote into
> > > the device's PCI_PM_CTRL register has been stored in there.  Still,
> > > it modifies the device's current_state field as though that's the
> > > case.  This may cause the driver of the device to think that its
> > > power state has been changed while in fact it hasn't.
> > > 
> > > To prevent such situations from happening modify
> > > pci_raw_set_power_state() so that it reads the device's PCI_PM_CTRL
> > > register after writing into it and uses the value read from the
> > > register to update the device's current_state field.  Also make it
> > > return -EIO if the new state of the device is not equal to the state
> > > requested by the called.
> > > 
> > > To distinguish this error condition from the other ones make
> > > pci_raw_set_power_state() return -ENOSYS instead of -EIO when it is
> > > impossible to change the power state of the device, because it
> > > doesn't support the native PCI PM at all or the requested target
> > > state is not supported by it.
> > 
> > Having reconsidered it I think that -ENODEV is probably better than
> > -ENOSYS for this purpose.  Updated patch follows.
> 
> Applied to linux-next, thanks Rafael.

Please drop.  It has been reported by Jiri to cause a regression to happen.

Best,
Rafael
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