Date:	Thu, 07 May 2015 12:18:30 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	"Dr. H. Nikolaus Schaller" <hns@...delico.com>
CC:	Mark Rutland <mark.rutland@....com>, Pavel Machek <pavel@....cz>,
	List for communicating with real GTA04 owners 
	<gta04-owner@...delico.com>, NeilBrown <neil@...wn.name>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Sebastian Reichel <sre@...nel.org>,
	"grant.likely@...aro.org" <grant.likely@...aro.org>,
	Jiri Slaby <jslaby@...e.cz>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Gta04-owner] [PATCH 0/3] tty slave device support - version
 3.

On 05/07/2015 11:11 AM, Dr. H. Nikolaus Schaller wrote:
> 
> Am 07.05.2015 um 16:30 schrieb Peter Hurley <peter@...leysoftware.com>:
> 
>> On 05/07/2015 08:46 AM, Dr. H. Nikolaus Schaller wrote:
>>> Am 06.05.2015 um 19:18 schrieb Mark Rutland <mark.rutland@....com>:
>>>> On Wed, May 06, 2015 at 05:09:20PM +0100, Dr. H. Nikolaus Schaller wrote:
>>>>> Am 06.05.2015 um 16:15 schrieb Mark Rutland <mark.rutland@....com>:
>>>>>
>>>>>>>>>>> No, I am not playing devil’s advocate (which would imply that I am doing this
>>>>>>>>>>> for fun to tease the dog), but I feel I have to be the advocate of future board
>>>>>>>>>>> designers who want to easily import an existing board DT and overwrite device
>>>>>>>>>>> tree nodes to describe design changes, i.e. what slave device is connected to
>>>>>>>>>>> which uart.
>>>>>>
>>>>>> [...]
>>>>>>
>>>>>>>> If this happens, you can move the slave device into a fragment that you
>>>>>>>> can include under the correct node. That's trivial.
>>>>>>>
>>>>>>> But less readable. And that is important as well.
>>>>>>
>>>>>> I disagree. The manipulation you have to perform to override properties
>>>>>> is at least as bad as including a file.
>>>>>
>>>>> What about:
>>>>>
>>>>> #include "omap3-beagle-xm.dts"
>>>>>
>>>>> / {
>>>>> 	/* HS USB Port 2 Power enable was inverted with the xM C */
>>>>> 	hsusb2_power: hsusb2_power_reg {
>>>>> 		enable-active-high;
>>>>> 	};
>>>>> };
>>>>>
>>>>> compared to 
>>>>>
>>>>> #include “board1.dts”
>>>>>
>>>>> / {
>>>>> 	/* slave was reconnected to uart4 */
>>>>> 	slave {
>>>>> 		uart = <&uart4>;
>>>>> 	};
>>>>> };
>>>>
>>>> As I mentioned, you can easily carve up your DTS to make that work with
>>>> includes if you really must:
>>>>
>>>> /* UART0 board variant */
>>>> #include "board.dtsi"
>>>> &uart0 {
>>>> 	#include "some-uart-slave.dtsi"
>>>> };
>>>>
>>>> /* UART1 board variant */
>>>> #include "board.dtsi"
>>>> &uart1 {
>>>> 	#include "some-uart-slave.dtsi"
>>>> };
>>>>
>>>> If you happen to find includes ugly then you can say it's ugly, but it's
>>>> functionally equivalent, and also means you can avoid having
>>>> disabled/partial nodes all over the place.
>>>
>>> Functionally equivalent would also be to copy the whole source file and
>>> s/&uart0/&uart1/.
>>>
>>> But this is not the best solution for the DT programmer since there is no
>>> automatic *reuse* of common parts.
>>>
>>> And your proposal requires 3 source files instead of 2 which deteriorates
>>> readibility and understanding what is really going on. And if you need to
>>> change the some-uart-slave, you have to touch a different file than for
>>> changing some other slave.
>>>
>>> Yes, it works, but IMHO other factors for a good design are also important.
>>>
>>> Maybe our main difference in PoV is that I specifically want to avoid that
>>> we force future DT programmers into “ugly” solutions (even if they work).
>>>
>>> If you think that DT programmers have to live with what they are
>>> given and do the best with it, we can end the discussion.
>>
>> The question of syntax is orthogonal to the discussion of the proper
>> devicetree representation.
>>
>> The awkwardness of expressing variants has nothing to do with the
>> appropriate device hierarchy (or whether there should be a hierarchy).
>> Describing variants is just as awkward when the parent-child relationship
>> is indisputable.
> 
> That is ok. But I still have not found the key rule when a parent-child relationship
> is indisputable. Candidates so far are “main interface” (which is still disputable)
> or “bus” (where we can dispute if a point-to-point connection is a “bus”).

Actually, at this point there is only one candidate, and that is the code
under review.


> But if both options are equally valid (maybe because there is no rule
> making either indisputable), I would chose the one with easier syntax.

There are many important criteria here.

1. Code complexity
2. Ease of adoption
3. Quality of abstraction
4. Extensibility

>> There was a recent discussion on devicetree ML regarding how best to
>> express and represent variance. Feel free to revive that discussion.
> 
> Does it help? Our core issue is not the syntax and variance per se. This are
> just examples to demonstrate differences in syntax of parent-child vs. phandle.
> 
> If we decide for either one, we have to live with syntactical and other
> implcationd.

The future implications of both the design and implementation need
careful consideration. That's why I'd like to see a v4.

Regards,
Peter Hurley

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