linux-kernel - Re: [PATCHv8 2/2] mailbox: Introduce framework for mailbox

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Date:	Thu, 17 Jul 2014 12:55:41 +0530
From:	Jassi Brar <jaswinder.singh@...aro.org>
To:	Suman Anna <s-anna@...com>
Cc:	Arnd Bergmann <arnd@...db.de>, Sudeep Holla <sudeep.holla@....com>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ks.giri@...sung.com" <ks.giri@...sung.com>,
	"ijc+devicetree@...lion.org.uk" <ijc+devicetree@...lion.org.uk>,
	Mark Rutland <Mark.Rutland@....com>,
	"robh+dt@...nel.org" <robh+dt@...nel.org>,
	Pawel Moll <Pawel.Moll@....com>,
	"courtney.cavin@...ymobile.com" <courtney.cavin@...ymobile.com>,
	"mporter@...aro.org" <mporter@...aro.org>,
	"slapdau@...oo.com.au" <slapdau@...oo.com.au>,
	"lftan.linux@...il.com" <lftan.linux@...il.com>,
	"loic.pallardy@...com" <loic.pallardy@...com>,
	"ashwin.chaugule@...aro.org" <ashwin.chaugule@...aro.org>,
	"bjorn@...o.se" <bjorn@...o.se>,
	"patches@...aro.org" <patches@...aro.org>,
	"Mollie.Wu@...fujitsu.com" <Mollie.Wu@...fujitsu.com>,
	"t.takinishi@...fujitsu.com" <t.takinishi@...fujitsu.com>
Subject: Re: [PATCHv8 2/2] mailbox: Introduce framework for mailbox

On 16 July 2014 21:39, Suman Anna <s-anna@...com> wrote:
> Hi,
>
> On 07/16/2014 09:18 AM, Jassi Brar wrote:
>> On 16 July 2014 19:38, Arnd Bergmann <arnd@...db.de> wrote:
>>> On Wednesday 16 July 2014 18:42:22 Jassi Brar wrote:
>>>> On 16 July 2014 18:39, Arnd Bergmann <arnd@...db.de> wrote:
>>>>> On Wednesday 16 July 2014 18:35:33 Jassi Brar wrote:
>>>>>> On 16 July 2014 18:15, Arnd Bergmann <arnd@...db.de> wrote:
>>>>>>> On Wednesday 16 July 2014 18:07:04 Jassi Brar wrote:
>>>>>>>> On 16 July 2014 15:46, Arnd Bergmann <arnd@...db.de> wrote:
>>>>>>>>> On Wednesday 16 July 2014 10:40:19 Sudeep Holla wrote:
>>>>>>>>>>> +
>>>>>>>>>>> +Required property:
>>>>>>>>>>> +- mbox: List of phandle and mailbox channel specifier.
>>>>>>>>>>> +
>>>>>>>>>>> +- mbox-names: List of identifier strings for each mailbox channel
>>>>>>>>>>> +               required by the client.
>>>>>>>>>>> +
>>>>>>>>>>
>>>>>>>>>> IMO the mailbox names are more associated with the controller channels/
>>>>>>>>>> mailbox rather than the clients using it. Does it make sense to move
>>>>>>>>>> this under controller. It also avoid each client replicating the names.
>>>>>>>>>
>>>>>>>>> I think it would be best to just make the mbox-names property optional,
>>>>>>>>> like we have for other subsystems.
>>>>>>>>>
>>>>>>>> A very similar subsystem - DMAEngine also has 'dma-names' as a
>>>>>>>> required property.
>>>>>>>>
>>>>>>>> If a client is assigned only 1 mbox in DT, we can do without
>>>>>>>> mbox-names. But I am not sure what to do if a client needs two or more
>>>>>>>> differently capable mboxes? Simply allocating in order of mbox request
>>>>>>>> doesn't seem very robust.
>>>>>>>
>>>>>>> Traditionally, these things (regs, interrupts, ...) are just accessed
>>>>>>> by index. The reason why dmaengine requires the name is that some machines
>>>>>>> can use multiple DMA engine devices attached to the same request line,
>>>>>>> so the dmaengine subsystem can pick any of them that has a matching
>>>>>>> name.
>>>>>> And also, I think, when a client needs 2 different dma channels, say
>>>>>> for RX and TX each. The api can't assign the first channel specified
>>>>>> in 'dmas' property to the first channel request that comes to it,
>>>>>> unless we assume client driver always requests dma channels in the
>>>>>> order written in its DT node.  And this is the main reason I see for
>>>>>> having mbox-names property.
>>>>>
>>>>> Most subsystems require passing an explicit index in this case.
>>>>>
>>>>>>   If we make mbox-names optional, do we assume client driver must
>>>>>> request mbox in the order specified in its DT node?
>>>>>
>>>>> Correct.
>>>>>
>>>> OK. So how about we drop mbox-names altogether and expect client
>>>> driver to simply provide an index of the mbox needed?
>>>
>>> That would be fine with me, but I think a lot of people like
>>> the idea of identifying things by name, and are used to that
>>> from the other subsystems.
>>>
>>> Maybe you can leave the mbox-names property defined as 'optional'
>>> in the generic mbox binding but remove the code in Linux? That way
>>> we can always put it back at a later point without changing the
>>> binding in an incompatible way.
>
> I like this too, especially given that the framework uses this only to
> retrieve the index. The mbox_client structure currently requires the
> chan_name to be filled in, which means both the client driver and the
> framework end up having to parse the property.
>
Yeah let us discourage the use of mbox-names.

> While we are at this, how about we change the property name from "mbox"
> to a plural-form "mboxes" to be inline with gpios, dmas etc.
>
OK, will change mbox to mboxes.

-jassi
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