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

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Message-ID: <0621eaba-db4d-a174-1b15-535e804b52ac@microchip.com>
Date:   Wed, 23 Jun 2021 12:47:56 +0000
From:   <Claudiu.Beznea@...rochip.com>
To:     <stern@...land.harvard.edu>
CC:     <gregkh@...uxfoundation.org>, <Nicolas.Ferre@...rochip.com>,
        <alexandre.belloni@...tlin.com>, <Ludovic.Desroches@...rochip.com>,
        <Cristian.Birsan@...rochip.com>, <linux-usb@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] usb: host: ohci-at91: suspend/resume ports after/before
 OHCI accesses

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).

> 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.

Thank you,
Claudiu Beznea

> 
> Alan Stern
>