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

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Date:	Thu, 9 Jun 2016 10:39:30 +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 11:45 PM, Greg Kroah-Hartman wrote:
> 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...
>

It's not PCI configure space, but xhci's io memory. XHCI spec reserves
a range in its extended capability list for vendor specific things. Intel's
platform leverages this for the port mux device register mapping.
It looks odd though. :)

Best regards,
Lu Baolu