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

Sudeep Holla sudeep.holla at arm.com
Thu Jul 17 08:09:16 PDT 2014

On 17/07/14 13:56, Jassi Brar wrote:
> On 17 July 2014 16:01, Sudeep Holla <sudeep.holla at arm.com> wrote:
>> On 17/07/14 07:25, Jassi Brar wrote:
>>>>> +       u32 val;
>>>>> +
>>>>> +       pr_debug("%s:%d\n", __func__, __LINE__);
>>>> Please remove all these debug prints.
>>> These are as good as absent unless DEBUG is defined.
>> Yes, but with mailbox framework, the controller driver(esp. this one)is
>> almost like a shim layer. Instead of each driver adding these debugs,
>> IMO it would be good to have these in the framework.
> OK
>>>>> +       /* See NOTE_RX_DONE */
>>>>> +       val = readl_relaxed(mlink->rx_reg + INTR_STAT_OFS);
>>>>> +       mbox_chan_received_data(chan, (void *)val);
>>>>> +
>>>>> +       /*
>>>>> +        * It is agreed with the remote firmware that the receiver
>>>>> +        * will clear the STAT register indicating it is ready to
>>>>> +        * receive next data - NOTE_RX_DONE
>>>>> +        */
>>>> 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

> 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 believe that's what we need here if we want this driver to work
on both Juno and your platform. Agree ?

>>>>> +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. 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.


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