linux-kernel - Re: [PATCH v7 2/2] memory: atmel-ebi: add DT bindings documentation

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Date:	Wed, 4 May 2016 07:43:18 -0500
From:	Rob Herring <robh@...nel.org>
To:	Jean-Jacques Hiblot <jjhiblot@...phandler.com>
Cc:	Mark Rutland <mark.rutland@....com>,
	Boris Brezillon <boris.brezillon@...e-electrons.com>,
	Arnd Bergmann <arnd@...db.de>, Pawel Moll <pawel.moll@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Nicolas Ferre <nicolas.ferre@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Alexandre Belloni <alexandre.belloni@...e-electrons.com>,
	Kumar Gala <galak@...eaurora.org>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v7 2/2] memory: atmel-ebi: add DT bindings documentation

On Wed, May 4, 2016 at 5:06 AM, Jean-Jacques Hiblot
<jjhiblot@...phandler.com> wrote:
> 2016-05-03 21:11 GMT+02:00 Rob Herring <robh@...nel.org>:
>> On Tue, May 3, 2016 at 11:51 AM, Boris Brezillon
>> <boris.brezillon@...e-electrons.com> wrote:
>>> Hi Rob,
>>>
>>> On Tue, 3 May 2016 11:40:19 -0500
>>> Rob Herring <robh@...nel.org> wrote:
>>>
>>>> On Thu, Apr 28, 2016 at 02:03:27PM +0200, Boris Brezillon wrote:
>>>> > The EBI (External Bus Interface) is used to access external peripherals
>>>> > (NOR, SRAM, NAND, and other specific devices like ethernet controllers).
>>>> > Each device is assigned a CS line and an address range and can have its
>>>> > own configuration (timings, access mode, bus width, ...).
>>>> > This driver provides a generic DT binding to configure a device according
>>>> > to its requirements.
>>>> > For specific device controllers (like the NAND one) the SMC timings
>>>> > should be configured by the controller driver through the matrix and smc
>>>> > syscon regmaps.
>>>> >
>>>> > Signed-off-by: Boris Brezillon <boris.brezillon@...e-electrons.com>
>>>> > ---
>>>> >  .../bindings/memory-controllers/atmel,ebi.txt      | 136 +++++++++++++++++++++
>>>> >  1 file changed, 136 insertions(+)
>>>> >  create mode 100644 Documentation/devicetree/bindings/memory-controllers/atmel,ebi.txt
>>>> >
>>>> > diff --git a/Documentation/devicetree/bindings/memory-controllers/atmel,ebi.txt b/Documentation/devicetree/bindings/memory-controllers/atmel,ebi.txt
>>>> > new file mode 100644
>>>> > index 0000000..a6dca5c
>>>> > --- /dev/null
>>>> > +++ b/Documentation/devicetree/bindings/memory-controllers/atmel,ebi.txt
>>>> > @@ -0,0 +1,136 @@
>>>> > +* Device tree bindings for Atmel EBI
>>>> > +
>>>> > +The External Bus Interface (EBI) controller is a bus where you can connect
>>>> > +asynchronous (NAND, NOR, SRAM, ....) and synchronous memories (SDR/DDR SDRAMs).
>>>> > +The EBI provides a glue-less interface to asynchronous memories through the SMC
>>>> > +(Static Memory Controller).
>>>> > +
>>>> > +Required properties:
>>>> > +
>>>> > +- compatible:              "atmel,at91sam9260-ebi"
>>>> > +                   "atmel,at91sam9261-ebi"
>>>> > +                   "atmel,at91sam9263-ebi0"
>>>> > +                   "atmel,at91sam9263-ebi1"
>>>>
>>>> What are the differences between 0 and 1?
>>>
>>> Because this SoC has 2 EBI busses with different capabilities.
>>
>> Okay, correct answer. :)
>>
>> [...]
>>
>>>>
>>>> > +                   of the memory region requested by the device.
>>>> > +
>>>> > +EBI bus configuration associated with specific chip-select will be defined in
>>>> > +the configs subnode. This configs node will in turn contain several subnodes
>>>> > +named config-<cs-id>, each of them containing the following properties.
>>>>
>>>> This is a bit unusual. Why not just part of the child device nodes?
>>>
>>> Oh, come on! I reworked the binding because Mark complained about the
>>> previous binding which was doing exactly what you're suggesting. Can
>>> you please be consistent in your reviews...
>>
>> No, Mark and I both have our opinions. Which part of this patch
>> explains the history? If the revision history is not in the patch, I'm
>> not looking at it.
>>
>> My issue with it this way is that it has invented yet another way to
>> describe timings. I would like to be consistent across external bus
>> descriptions, but we're not very consistent to begin with though. The
>> most common seems to be the way you first did it. But I agree that it
>> is kind of screwy to have an intermediate node unless the controller
>> itself has sub-blocks within it and is not the established way to
>> describe a bus with chip selects. I would either put the properties
>> directly in the child nodes (e.g. flash@0,0) or put your config nodes
>> in the device node. I'd call it timings instead of config, but that's
>> just bikeshedding.
>>
>> memory-controller@...0 {
>>   ...
>>   flash@0,0 {
>>     timings {
>>       ...
>>     };
>>   };
>> };
>>
>
> I don't think the timings belong in the child node as they really are
> for the chip select and the chip select may select several devices at
> once. I'm thinking (again) of a FPGA here that could implement or
> example 4 serial ports at different addresses.

It is an established pattern already in i.MX WEIM and OMAP GPMC
bindings. The timings are a function of the device attached, so having
them in the device's node makes some sense. Arguably we should define
the timings in a generic way, but that's hard and no one wants to do
that.

Rob