Re: [PACTH,v6,1/2] usb: xhci: plat: Enable runtime PM

[Brian Norris <briannorris@chromium.org>]

Corrected a bouncing email address (sorry for the noise)

On Fri, Aug 26, 2016 at 03:11:36PM -0700, Brian Norris wrote:
> Hi,
> 
> On Wed, Aug 24, 2016 at 04:48:01PM -0400, Robert Foss wrote:
> > On 2016-08-22 11:23 PM, Brian Norris wrote:
> > >+ others
> > >
> > >Hi Robert and Felipe,
> > >
> > >I have a few questions for one or both of you. I'm not really an expert
> > >on runtime PM, so please take my questions with a grain of salt.
> > >
> > >On Wed, Aug 10, 2016 at 04:32:15PM -0400, robert.foss@collabora.com wrote:
> > >>From: Robert Foss <robert.foss@collabora.com>
> > >>
> > >>Enable runtime PM for the xhci-plat device so that the parent device
> > >>may implement runtime PM.
> > >>
> > >>Signed-off-by: Robert Foss <robert.foss@collabora.com>
> > >>
> > >>Tested-by: Robert Foss <robert.foss@collabora.com>
> > >>---
> > >> drivers/usb/host/xhci-plat.c | 29 +++++++++++++++++++++++++++--
> > >> 1 file changed, 27 insertions(+), 2 deletions(-)
> > >>
> > >>diff --git a/drivers/usb/host/xhci-plat.c b/drivers/usb/host/xhci-plat.c
> > >>index ed56bf9..ba4efe7 100644
> > >>--- a/drivers/usb/host/xhci-plat.c
> > >>+++ b/drivers/usb/host/xhci-plat.c
> > >>@@ -246,6 +246,9 @@ static int xhci_plat_probe(struct platform_device *pdev)
> > >> 	if (ret)
> > >> 		goto dealloc_usb2_hcd;
> > >>
> > >>+	pm_runtime_set_active(&pdev->dev);
> > >>+	pm_runtime_enable(&pdev->dev);
> > >>+
> > >
> > >How does it help to enable PM runtime like this, if you don't have any
> > >kind of runtime_{suspend,resume}() callbacks?
> > 
> > Andrew, I think you understand the inner workings of this code
> > better than me, maybe you could give a short summary?
> 
> I believe Andrew is fairly busy, and I already talked with him a bit
> about this. This is needed (as per your (or his?) commit message) only
> if you have a parent device that wants to implement runtime PM. Now
> depending on what you want to do with "runtime PM", this might mean
> the parent device has to suspend xhci_{suspend,resume}() on behalf of
> xhci-plat.c. Not very nice layering IMO, but it has been done before...
> 
> So I guess this comes down to "what does a parent device/driver want to
> do"? If that's (e.g.) just putting some PHY into a slightly lower power
> mode, then maybe it's fine for xhci-plat not to do anything else. But if
> you actually want to completely power down the parent bus, reset an
> accompanying dual-role/OTG controller, etc., then this really isn't
> sufficient, AFAICT. But maybe that's just an indictment of the poor
> structure of dwc3's host-mode support, more than it is an indictment of
> this patch.
> 
> Brian