Date:	Tue, 12 May 2015 17:22:28 +0300
From:	Mathias Nyman <mathias.nyman@...ux.intel.com>
To:	Roger Quadros <rogerq@...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 11.05.2015 17:18, Roger Quadros wrote:
> Hi Mathias,
> 
> 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'll try it out and send forward once rc1 is out.
> 
> did you get a chance to try this series?
> 

Sorry, not yet, got delayed by other internal tasks.
I'll try it out as soon as possible.

-Mathias



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