Date:	Tue, 14 Apr 2015 12:21:25 +0300
From:	Roger Quadros <rogerq@...com>
To:	Mathias Nyman <mathias.nyman@...ux.intel.com>,
	<mathias.nyman@...el.com>, Alan Stern <stern@...land.harvard.edu>
CC:	<gregkh@...uxfoundation.org>, <linux-usb@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/5] usb: xhci: cleanup xhci_hcd allocation

On 13/04/15 15:48, Mathias Nyman wrote:
> Hi
> 
> On 09.04.2015 12:22, Roger Quadros wrote:
>> Hi,
>>
>> On 07/04/15 17:23, Mathias Nyman wrote:
>>> Hi
>>>
>>> On 02.04.2015 15:23, Roger Quadros wrote:
>>>> HCD core allocates memory for HCD private data in
>>>> usb_create_[shared_]hcd() so make use of that
>>>> mechanism to allocate the struct xhci_hcd.
>>>>
>>>> Introduce struct xhci_driver_overrides to provide
>>>> the size of HCD private data and hc_driver operation
>>>> overrides. As of now we only need to override the
>>>> reset and start methods.
>>>>
>>>> Signed-off-by: Roger Quadros <rogerq@...com>
>>>
>>> I'm not sure I fully understand the what's going on, or what the
>>> intention of this patch is.
>>
>> The main intention is to have both primary and shared HCDs allocated
>> before calling usb_add_hcd() for the primary hcd.
>> This is so that at the first usb_add_hcd() the OTG core knows the HCD topology
>> (i.e. whether it uses a shared HCD or not).
>>
>> From the OTG perspective we have to prevent the actual usb_add_hcd() till the
>> OTG state machine says so.
>> This means that xhci_gen_setup() won't be necessarily called immediately and
>> so we need to allocate for xhci somewhere else.
> 
> Ok, thanks for explaining. I now understand the reason behind this.
> 
>>
>>>
>>> So currently xhci driver manages the allocation and freeing of
>>> the xhci_hcd structure. We store a pointer to the xhci_hcd structure in
>>> the content of both the primary and shared usb_hcds structures hcd_priv
>>> field.
>>>
>>> With this patch xhci would be part of the usb_hcd structure,
>>> starting at hcd_priv[0]. (Like EHCI I think) It allocates enough space to include
>>> the xhci_hcd in both the primary and shared usb_hcd, but always only use the one
>>> in the primary hcd.
>>
>> precisely.
>>
>>> I'm not sure what to do with the space allocated for the shared hcd's
>>> hcd_priv field.
>>
>> we just ignore the space allocated for the shared hcd.
> 
> Ok, not a big loss.
> 
>>
>>>
>>> This also means that xhci goes away together with the primary hcd. It's possible
>>> this has some impact as the xhci driver expects xhci to always exists.
>>
>> Can you please point out where this impact is.
>>
>> I've been testing add/remove HCD extensively and didn't observe any issues after applying
>> these 5 patches. Well there is one issue that comes up but it has nothing to do with xhci
>> not being allocated. It has more to do with command being queued after the HCD has gone away
>> and so getting stuck forever without timing out.
> 
> I went through the codepaths and you're right, should work fine. My concern wasn't valid. 
> This patchset doesn't even touch the order how primary and shared HCDs are created and added
> in the PCI case, only for the platform device case.   

I was not very sure how to do it for the PCI case.
usb_hcd_pci_probe() wants to create and add the hcd. We need something else to split
the create_hcd() and add_hcd() operations.

> 
> I'll try it out and send forward once rc1 is out.

Thanks.

cheers,
-roger
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