linux-kernel - Re: [PATCH v10 6/7] usb: pci-quirks: add Intel USB drcfg mux device

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Date:	Wed, 8 Jun 2016 08:45:27 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Lu Baolu <baolu.lu@...ux.intel.com>
Cc:	felipe.balbi@...ux.intel.com,
	Mathias Nyman <mathias.nyman@...el.com>,
	Lee Jones <lee.jones@...aro.org>,
	Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
	Liam Girdwood <lgirdwood@...il.com>,
	Mark Brown <broonie@...nel.org>, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 6/7] usb: pci-quirks: add Intel USB drcfg mux device

On Wed, Jun 08, 2016 at 03:56:04PM +0800, Lu Baolu wrote:
> Hi Greg,
> 
> On 06/08/2016 12:45 PM, Greg Kroah-Hartman wrote:
> > On Thu, Jun 02, 2016 at 09:37:28AM +0800, Lu Baolu wrote:
> >> In some Intel platforms, a single usb port is shared between USB host
> >> and device controllers. The shared port is under control of a switch
> >> which is defined in the Intel vendor defined extended capability for
> >> xHCI.
> >>
> >> This patch adds the support to detect and create the platform device
> >> for the port mux switch.
> > Why do you need a platform device for this?  You do nothing with this
> > device, why create it at all?
> 
> In this patch series, I have a generic framework for port mux devices
> and two port mux drivers sitting on top the generic code.
> 
> In this patch, I create a platform device for the real mux device in
> Intel Cherry Trail or Broxton SOCs. In it's driver, I registered a mux
> into the generic framework and handle the power management
> things in driver's pm entries (otherwise, the system can't be waken
> up from system suspend).
> 
> > And why is it a platform device, isn't is really a PCI device?  Why
> > would you ever find a "platform" device below a PCI device?  Don't abuse
> > platform devices for things that aren't.  It makes me want to delete
> > that whole interface more and more...
> 
> Port mux devices are physical devices in Intel Cherry Trail and Broxton
> SOCs. It doesn't sit on any PCIe bus. But it maps its registers in xHCI
> space. OS kernel can enumerate it by looking up the xhci extended
> capability list with a vendor specific capability ID.

A physical device that maps registers into PCI space seems like a PCI
device of some type to me :)

Again, I hate platform devices for obvious reasons like this...

greg k-h