linux-kernel - Re: [PATCH] mailbox: add support for doorbell/signal mode controllers

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Date:   Thu, 2 Nov 2017 16:56:40 +0530
From:   Jassi Brar <jassisinghbrar@...il.com>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Bjorn Andersson <bjorn.andersson@...aro.org>
Subject: Re: [PATCH] mailbox: add support for doorbell/signal mode controllers

On Thu, Nov 2, 2017 at 4:17 PM, Sudeep Holla <sudeep.holla@....com> wrote:
> On 02/11/17 02:39, Jassi Brar wrote:

>>>>>
>>>>> Such controllers don't need to transmit any data, they just transmit
>>>>> the signal. In such controllers the data pointer passed to
>>>>> mbox_send_message is passed to client via it's tx_prepare callback.
>>>>> Controller doesn't need any data to be passed from the client.
>>>>>
>>>> Some controllers need a non-zero value written to a register in order
>>>> to trigger the signal.
>>>
>>> You are right, just right non-zero or whatever controller value to
>>> trigger the interrupt to remote.
>>>
>>>> That register is visible to the remote. While the data/packet is setup
>>>> during tx_prepare() callback.
>>>
>>> Agreed.
>>>
>>>> You are overlooking this class of doorbell controllers.
>>>>
>>>
>>> Not sure what do you mean by that ?
>>>
>> Such doorbell controllers can't use send_signal(chan) because they
>> need that non-zero value from client to send over the shared register.
>> You are assuming every protocol implements just one command.
>>
>
> No that non-zero value is not client specific, it's entirely controller
> specific.
>
??
For example BCM2835 has such a controller. Have a look at
bcm2835_send_data() and let me know what is that controller specific
value.


>>> Again agreed. But see below for reason to create this API.
>>>
>>>>> The new API send_signal will eliminate the
>>>>> issue Jassi has explained in earlier discussion with respect to generic
>>>>> message format using Rockchip example.
>>>>>
>>>> Sorry I don't see how.
>>>> Please explain how can send_signal() api be used by, say, rockchip to
>>>> support SCMI?
>>>>
>>>
>>>  80         writel_relaxed(msg->cmd, mb->mbox_base +
>>> MAILBOX_A2B_CMD(chans->idx));
>>>  81         writel_relaxed(msg->rx_size, mb->mbox_base +
>>>
>>>  82                        MAILBOX_A2B_DAT(chans->idx));
>>>
>>>  83
>>>
>>>  will be replaced with
>>>
>>> writel(whatever_value_to trigger_signal, MAILBOX_A2B_CMD(chans->idx));
>>>
>>> in its send_signal function.
>>>
>> 1) Where does the  "whatever_value_to_trigger_signal"  come from?
>
> Controller specific.
>
>> That has to come from client.
>
> No.
>
Again, let me know what does the controller expect 'val' to be

  writel(val, MAILBOX_A2B_CMD(chans->idx))


Your entire post is based on your assertion that the controller
expects a particular non-zero value to trigger a signal, which is
wrong. So lets first get that straight and not stray from the point.