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

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Message-ID: <4268c9d9-dc1e-0bd7-3fac-790c2d42b54b@arm.com>
Date:   Thu, 2 Nov 2017 10:47:46 +0000
From:   Sudeep Holla <sudeep.holla@....com>
To:     Jassi Brar <jassisinghbrar@...il.com>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        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 02/11/17 02:39, Jassi Brar wrote:
> On Wed, Nov 1, 2017 at 11:45 PM, Sudeep Holla <sudeep.holla@....com> wrote:
>>
>>
>> On 01/11/17 18:03, Jassi Brar wrote:
>>> On Wed, Nov 1, 2017 at 10:02 PM, Sudeep Holla <sudeep.holla@....com> 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. Not sure why do you think I am assuming every protocol
implements just one command.

>>>>
>>>> This is rough idea I have on extending mailbox interface to support
>>>> the doorbell requirements.
>>>>
>>> What doorbell requirements does the api not support?
>>> QComm's APCS IPC is what you call a "doorbell" controller and is
>>> already supported by the API. 

After looking at this, you will see that doorbell has not data specific
to client in the above case.

	unsigned long idx = (unsigned long)chan->con_priv;

	writel(BIT(idx), apcs->reg);

So it's channel specific, same in mailbox-sti

>> 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.

> You can not dictate the channel transfers a fixed u32 value over its
>lifetime. SCMI may use one command code but other protocols use more.

Yes if it's just a doorbell, see the above 2 cases I have pointed out.

> 
> 2) Using 'rx_size' is not a software choice made in the driver. The
> _hardware_ has two registers shared with remote side - a CMD and a
> DATA register. So the driver (written agnostic to any particular
> client) would naturally expect the command+data from the client to be
> programmed in to CMD and DAT registers.
> 

OK, if this controller needs to be used in doorbell mode for SCMI, we
can send one fixed cmd and fixed rx_size() or 1 based on inclusive or
exclusive).

> 
>>> I am not convinced we should clone an api just so that a client driver
>>> becomes simpler. Esp when it shifts, and not avoid, the additional
>>> code (to support the client) onto the provider side.
>>>
>>
>> It doesn't tie the data format with particular mailbox controller.
>> send_data has void *data and the interpretation is controller specific.
>> send_signal on the other handle can implemented by the controllers which
>> knows how and can trigger the specific signal to the remote.
>>
> Yeah that's what I said - you want to make a client simpler by pushing
> the code requirement onto the provider side.
> 

No, I want to support generic case of mailbox doorbell instead of
creating another unnecessary abstraction layer
> For example, you mean we modify the provider rockchip-mailbox.c by implementing
> 
> rockchip_send_signal(chan)
> {
>   struct rockchip_mbox_msg msg;
> 
>   msg.cmd = chan->idx;  //only one command supported by the channel !!!

Yes, it's just a doorbell. That actual data is transmitted or shared
elsewhere. This doorbell is a signal to the remote to examine that,

>   msg.rx_size = 0;
> 
>   rockchip_send_data(chan, (void*) &msg);
> }
> 
> whereas I suggest this SCMI specific code should be part of
> transport/mapping shim layer of SCMI.
> 

Yes that's what I did with abstraction and few think including me that
it's unnecessary abstraction for such a generic use.

-- 
Regards,
Sudeep