Re: [PATCH] usb: host: ohci-at91: suspend/resume ports after/before OHCI accesses

[<Claudiu.Beznea@microchip.com>]

On 23.06.2021 16:59, Alan Stern wrote:
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> 
> On Wed, Jun 23, 2021 at 12:47:56PM +0000, Claudiu.Beznea@microchip.com wrote:
>> On 10.06.2021 02:07, Alan Stern wrote:
>>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>>>
>>> On Wed, Jun 09, 2021 at 03:10:27PM +0300, Claudiu Beznea wrote:
>>>> On SAMA7G5 suspending ports will cut the access to OHCI registers and
>>>> any subsequent access to them will lead to CPU being blocked trying to
>>>> access that memory. Same thing happens on resume: if OHCI memory is
>>>> accessed before resuming ports the CPU will block on that access. The
>>>> OCHI memory is accessed on suspend/resume though
>>>> ohci_suspend()/ohci_resume().
>>>
>>> That sounds very strange.
>>
>> The clock scheme for OHCI and EHCI IPs on SAMA7G5 is as follows
>> (I hope it is readable):
>>
>>                                             Main Xtal
>>                                                |
>>                                                +-------------+
>>                                                |             |
>> +---------------------------+                 \ /            |
>> |                 +------+  | 60MHz  +--------------------+  |
>> |                 |      |  |        |                    |------+
>> |                 | Port |<----------| UTMI Transceiver A |  |   |
>> |                 |      |  |        |                    |----+ |
>> |  USB 2.0 EHCI   |Router|  |        +--------------------+  | | |
>> | Host Controller |      |  | 60MHz  +--------------------+  | | |
>> |                 |      |<----------| UTMI Transceiver B |<-+ | |
>> |                 |      |  |        +--------------------+  | | |
>> |                 |      |  | 60MHz  +--------------------+  | | |
>> |                 |      |<----------| UTMI Transceiver C |<-+ | |
>> |                 |      |  |        +--------------------+    | |
>> |                 +------+  |                                  | |
>> |                           |                                  | |
>> +---------------------------+                                  | |
>>                                                                | |
>> +---------------------------+                                  | |
>> |                 +------+  |         UHP48M                   | |
>> |                 | Root |  |<---------------------------------+ |
>> |  USB 1.1 OHCI   | hub  |  |                                    |
>> | Host Controller | and  |  |         UHP12M                     |
>> |                 | host |  |<-----------------------------------+
>> |                 | SIE  |  |
>> |                 +------+  |
>> |                           |
>> +---------------------------+
>>
>> Where UTMI transceiver A generates the 48MHz and 12MHz clocks for OHCI
>> full-speed operations.
>>
>> The ports control is done through AT91_SFR_OHCIICR via
>> ohci_at91_port_suspend() function where. Setting 0 in AT91_SFR_OHCIICR
>> means suspend is controlled by EHCI-OHCI and 1 forces the port suspend.
>> Suspending the port A will cut the clocks for OHCI. I want to mention that
>> AT91_SFR_OHCIICR register is not in the same memory space of OHCI, EHCI IPs
>> and is clocked by other clocks.
>>
>>> Suppose one of the ports is suspended, so access to the
>>> OHCI registers is blocked.  Then how can you resume the port?
>>
>> For run-time control (via ohci_at91_hub_control()), I agree with you that
>> the current implemented approach is not healthy (taking into account the
>> clock scheme above) and the fact that we do force the ports suspend on
>> ohci_at91_hub_control(). For suspend/resume it should be OK as the ports
>> are suspended at the end of any OHCI accesses (I don't know how the Linux
>> USB subsystem behaves so please do correct me if I'm wrong).
> 
> (I haven't checked the details recently, so I'm not certain about
> this.)  In some -- perhaps all -- cases, we don't suspend the ports at
> all during system suspend.  We just rely on the USB devices
> automatically going into suspend when the root hub stops sending
> packets.
> 
>>> Don't you have to
>>> access the OHCI registers in order to tell the controller to do the port resume?
>>
>> On our implementation we control the port suspend/resume via
>> AT91_SFR_OHCIICR, a special kind of register, memory mapped at different
>> address (compared w/ OHCI, EHCI IPs), so clocked by other clocks.
>>
>>>
>>> What happens when there's more than one port, and one of them is suspended while
>>> the other one is still running?  How can you communicate with the active port if
>>> access to the OHCI registers is blocked?
>>
>> For this kind of scenario (the run-time suspend of a port, not system
>> suspend/resume) a different mechanism should be implemented taking into
>> account the clock schema presented above.
> 
> Okay, I see.  It seems like the driver will need some significant
> changes to handle runtime power management properly.
> 
> One thing you might consider changing: The name of the
> ohci_at91_port_suspend routine is misleading.  It doesn't really
> handle suspending the port; instead it handles the clocks that drive
> the entire OHCI controller.  Right?

It does both as far as I can tell at the moment.

> 
> Alan Stern
> 

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