linux-kernel - Re: [PATCH 0/6] mailbox: arm_mhu: add support for subchannels

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Date:   Fri, 5 May 2017 12:23:15 +0100
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>,
        Alexey Klimov <Alexey.Klimov@....com>
Subject: Re: [PATCH 0/6] mailbox: arm_mhu: add support for subchannels

On 05/05/17 12:12, Jassi Brar wrote:
> On Wed, May 3, 2017 at 2:51 PM, Sudeep Holla <sudeep.holla@....com> wrote:

[...]

>>
>> I don't quite get this. There are only 3 signals as you mentioned above.
>> Yes there are 2^32 possible values for the register, but how can that be
>> used ?
>>
> _Your_ protocol don't use more than 32 values, that doesn't mean other
> protocols don't either.
> 

Please read what I quoted from the spec.
"..the MHU drives the signal using a 32-bit register, with all 32 bits
logically ORed together. The MHU provides a set of registers to enable
software to set, clear, and check the status of each of the bits of this
register independently. The use of 32 bits for each interrupt
line enables software to provide more information about the source of
the interrupt. For example, each bit of the register can be associated
with a type of event that can contribute to raising the interrupt."

It is designed exactly for the above use-case. It was designed to fit
PCC in ACPI which allows what I am doing with this series.

It's very similar to many PMIC drivers we have. Though set of PMIC
events are bundled into single register and interrupt doesn't mean they
need to be support as event. E.g. look at mc13xxx, wm831x,...etc

I am not trying to fit the changes to our use-case. I would counter
argue that you did so when you push the driver. Since it's very clear
for the spec that the individual bits can be accessed atomically, it
was designed to be used as doorbell and not as a message carrier
register. I am fine if it can be used in that way but don't disagree to
support the use-case for which it was designed.

-- 
Regards,
Sudeep