[PATCH 7/8] mailbox: f_mhu: add driver for Fujitsu MHU controller

Sudeep Holla sudeep.holla at arm.com
Thu Jul 17 11:51:42 PDT 2014

On 17/07/14 18:07, Jassi Brar wrote:
> On 17 July 2014 20:39, Sudeep Holla <sudeep.holla at arm.com> wrote:
>> On 17/07/14 13:56, Jassi Brar wrote:
>>> On 17 July 2014 16:01, Sudeep Holla <sudeep.holla at arm.com> wrote:


>>>>>> This note could be added as how this mailbox works in general and
>>>>>> it's not just Rx right ? Even Tx done is based on this logic.
>>>>>> Basically the logic is valid on both directions.
>>>>> Yes that is a protocol level agreement. Different f/w may behave
>>>>> differently.
>>>> Ok, I am not sure if that's entirely true because the MHU spec says it
>>>> drives
>>>> the signal using a 32-bit register, with all 32 bits logically ORed
>>>> together.
>>>> Usually only Rx signals are wired to interrupts and Tx needs to be polled
>>>> but that's entirely implementation choice I believe.
>>> On my platform, _STAT register only carries the command code but
>>> actual data is exchanged via SharedMemory/SHM. Now we need to know
>>> when the sent data packet (in SHM) has been consumed by the remote -
>>> for which our protocol mandates the remote clear the TX_STAT register.
>>> And vice-versa for RX.
>>>    Some other f/w may choose some other mechanism for TX-Done - say some
>>> ACK bit set or even some time bound guarantee.
>>>> More over if it's protocol level agreement it should not belong here :)
>>> My f/w expects the RX_STAT cleared after reading data from SHM. Where
>>> do you think is the right place to clear RX_STAT?
>> I understand that and what I am saying is the MHU is designed like that
>> and protocol is just using it. There's nothing specific to protocol. Ideally
>> if an implementation has both Rx and Tx interrupts, the RX_CLEAR from here
>> raises an interrupt to the firmware. In absence of it we need polling that's
>> what both Linux and firmware does for Tx case.
>> Even on Juno, it's same. But we should be able to extend it to support Tx
>> if an implementation supports. Similarly the firmware can make use of the
>> same when Linux clears Rx(it would be Tx complete/ack for the firmware)
>> When we need to make this driver work across different firmware, you just
>> can't rely on the firmware protocol, hence I am asking to drop those
>> comments.
> OK, I will remove the comment.
>>> I have said many times in many threads... its the mailbox controller
>>> _and_ the remote f/w that should be seen as one entity. It may not be
>>> possible to write a controller driver that works with any remote
>>> firmware.
>> It should be possible if the remote protocol just use the same hardware
>> feature without any extra software policy at the lowest level(raw Tx and
>> Rx).
> I wouldn't count on f/w always done the right way. And I am speaking
> from my whatever first hand experience :D
> And sometimes there may just be bugs that need some quirks at controller level.

Agreed, and I too have similar experience. This is exact reason why I am
urging for threaded irq, unless we have real requirement for hard irq.

>> I believe that's what we need here if we want this driver to work
>> on both Juno and your platform. Agree ?
> Does this driver not work for Juno?

I have not yet tried yet. For sure secure access will explode.

> If no, may I see your driver and the MHU manual (mine is 90% Japanese)?

It's quite similar to your one expect few comments which I have made.
Here is the public version of Juno spec[1]. Not sure if it covers MHU in

>>>>>>> +static int mhu_startup(struct mbox_chan *chan)
>>>>>>> +{
>>>>>>> +       struct mhu_link *mlink = (struct mhu_link *)chan->con_priv;
>>>>>>> +       unsigned long flags;
>>>>>>> +       u32 val;
>>>>>>> +       int ret;
>>>>>>> +
>>>>>>> +       pr_debug("%s:%d\n", __func__, __LINE__);
>>>>>>> +       spin_lock_irqsave(&mlink->lock, flags);
>>>>>>> +       val = readl_relaxed(mlink->tx_reg + INTR_STAT_OFS);
>>>>>>> +       writel_relaxed(val, mlink->tx_reg + INTR_CLR_OFS);
>>>>>>> +       spin_unlock_irqrestore(&mlink->lock, flags);
>>>>>>> +
>>>>>>> +       ret = request_irq(mlink->irq, mhu_rx_interrupt,
>>>>>>> +                         IRQF_SHARED, "mhu_link", chan);
>>>>>> Just a thought: Can this be threaded_irq instead ?
>>>>>> Can move request_irq to probe instead esp. if threaded_irq ?
>>>>>> That provides some flexibility to client's rx_callback.
>>>>> This is controller driver, and can not know which client want
>>>>> rx_callback in hard-irq context and which in thread_fn context.
>>>>> Otherwise, request_irq() does evaluate to request_threaded_irq(), if
>>>>> thats what you mean.
>>>> Agreed but on contrary since MHU involves external firmware(running
>>>> on different processor) which might have it's own latency, I prefer
>>>> threaded over hard irq. And yes request_irq does evaluate to
>>>> request_threaded_irq but with thread_fn = NULL which is not what I want.
>>> If remote has its own latency, that does not mean we add some more :)
>> No what I meant is unless there is a real need to use hard irq, we
>> should prefer threaded one otherwise.
> And how does controller discern a "real need" from a "soft need" to
> use hard irq?
> Even if the controller driver pushes data up from a threaded function,
> the client can't know the context and can't sleep because the
> intermediate API says the rx_callback should be assumed to be atomic.

Yes I am not arguing on that, it should assume atomic and not sleep.
I am saying we can avoid rx_callback in interrupt context if possible.
I will try to look at the protocol implementation tomorrow.

> Again, it maybe more efficient if I see your implementation of the
> driver and understand your concerns about mine.

As I said its almost same as this, except I call mbox_chan_received_data
in irq thread context. I prefer enabling other interrupts while copying
payload data.

>> Also by latency I meant what if
>> the remote firmware misbehaves. In threaded version you have little
>> more flexibility whereas hard irq is executed with interrupts disabled.
>> At least the system is responsive and only MHU interrupts are disabled.
> If the remote firmware misbehaves, that is a bug of the platform. No
> mailbox API could/should account for such malfunctions.

No I didn't intend for any mailbox API to account it.


[1] http://infocenter.arm.com/help/topic/com.arm.doc.dto0038a/index.html

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