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 15:56:04 +0800
From:	Lu Baolu <baolu.lu@...ux.intel.com>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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

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.

Best regards,
Lu Baolu