Date:	Mon, 26 Oct 2015 14:28:01 +0100
From:	Marc Gonzalez <marc_gonzalez@...madesigns.com>
To:	Mans Rullgard <mans@...sr.com>
CC:	DT <devicetree@...r.kernel.org>, Kumar Gala <galak@...eaurora.org>,
	Rob Herring <robh+dt@...nel.org>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	<linux-kernel@...r.kernel.org>,
	Sebastian Frias <sebastian_frias@...madesigns.com>,
	Thibaud Cornic <thibaud_cornic@...madesigns.com>,
	Mason <slash.tmp@...e.fr>
Subject: Re: [PATCH 2/3] devicetree: add binding for Aurora VLSI NB8800
 Ethernet controller

On 26/10/2015 13:05, Måns Rullgård wrote:

> Marc Gonzalez writes:
> 
>> On 23/10/2015 15:41, Måns Rullgård wrote:
>>
>>> Marc Gonzalez wrote:
>>>
>>>> On 22/10/2015 16:02, Mans Rullgard wrote:
>>>>
>>>>> This adds a binding for the Aurora VLSI NB8800 Ethernet controller
>>>>> using the "aurora,nb8800" compatible string.  When used in Sigma
>>>>> Designs chips a few additional control registers are available.
>>>>> This variant is indicated by the "sigma,smp8640-ethernet" compatible
>>>>> string.
>>>>
>>>> I've been meaning to ask a noob question to the devicetree group
>>>> about how names for compatible strings are chosen.
>>>>
>>>> Sigma Designs has two active SoC families, Tango3 (which consists of
>>>> about a dozen MIPS-based SoCs, typically named SMP86xx) and Tango4
>>>> (a few ARM-based SoCs, typically named SMP87xx). I should note that
>>>> there is no SMP8640 SoC AFAIK, rather SMP864x is a Tango3 sub-family
>>>> (I could locate 42,43,44,45,46).
>>
>> Just to make things a bit more confusing, I learned that Sigma made
>> one MIPS-based Tango4 SoC...
> 
> That explains the presence of tango4 mentions in a Sigma MIPS kernel
> tree I found.  Do you know what it was called?

It was called smp8910. (They broke the naming convention, but at
least it's clearly set apart from other "normal" chips.)

>>>> AFAIK, all our SoCs are using the same Aurora NB8800 Ethernet MAC,
>>>> along with the extra features. I find it odd to use a specific SoC
>>>> model to refer to this device, instead of a more generic name.
>>>> (It's weird having to mention smp8640 in the tango4 DT.)
>>>
>>> I picked 8640 since all 8640 or higher chips are compatible (863x chips
>>> (tango2) are not).  Some of the later versions have additional extra
>>> features, but they all work with the basic driver.
>>
>> According to my branch's FAEs, the first Tango3 was SMP8644.
> 
> I don't know which was first out the door, but judging by marketing
> materials, 8642, 8644, and 8646 were available around the same time.
> All of these also have an odd-numbered variant (8643 etc) without
> Macrovision features.

The only difference between chips '2N' and '2N+1' used to be
Macrovision support... But the rule was broken for smp8759,
which has more features than smp8758 (e.g. HDMI-in and PCIe)

>> I showed the DT to a colleague, and his reaction was: "Don't use
>> smp8640, it will confuse other engineers, Sigma didn't make a 8640
>> SoC."
>>
>> http://www.qobuz.com/info/IMG/pdf/SMP8643.pdf
>>
>> Would you be willing to change the compatible string to
>> "sigma,smp8644-foo" or "sigma,smp864x-foo" ?
>>
>> If it's not possible, I suppose I can add comments in the DT, to clear
>> the potential confusion for Sigma engineers.
> 
> My intent was certainly not to confuse.  Sigma product brochures talk
> about the "SMP8640 series," so I figured that would be a good way of
> referring to them as a group.  See e.g. this PDF, now gone from the
> main sigmadesigns.com site:
> http://wayback.archive.org/web/20150101024010/http://www.sigmadesigns.com/uploads/documents/selection_guide.pdf
> 
>>> There also appear to be some differences (bug fixes?) between 8643 and
>>> 8759 (the ones I have) not documented anywhere.
>>
>> Suppose you identify these differences, and you make the appropriate
>> changes to the driver. What compatible string would you use to refer
>> to the new features? I used "sigma,tango4-ethernet" but IIUC it must
>> be more specific, such as the first Tango4 chip to implement these
>> changes (I guess that would be the SMP8734).
> 
> There are differences even within the tango3 family.  The SMP867x chips
> have features not present on the earlier ones.  The tango3 and tango4
> codenames are too unspecific to be of use here.  It's better to use
> exact chip designations.

OK.

>> So I should write something like this in my DT?
>>
>>   eth0: ethernet@...00 {
>>     compatible = "sigma,smp8734-ethernet", "sigma,smp8640-ethernet", "aurora,nb8800";
>>
>> Hmmm, you mention this below, but you used "sigma,smp8759-ethernet".
>> What about earlier chips?
> 
> OK, how about this scheme then:
> 
> - Device trees list the exact chip, the chip family, the oldest
>   compatible family, and finally the basic "aurora,nb8800".  For the
>   SMP8759 that would look like this:
>   "sigma,smp8759-ethernet", "sigma,smp87xx-ethernet", "sigma,smp864x-ethernet",
>     "aurora,nb8800"

AFAICT, the list of tango4 chips is (in chronological order)

8910, 8734, 8756, 8758, 8759

The problem I see is that Sigma already has plans for non-tango4
87xx SoCs (two in fact: 8760 and 8762, as far as I've heard).
(What a mess.)

I would think the "chip family" needs to use the code-name like
tango3 or tango4 (for lack of a better discriminant).

Also, (purely hypothetical) suppose something changed in 8756 and up.
How would the 8758 pick up the improvement, but not the 8734?


I'm also confused, because I thought I read somewhere not to use
wildcards in compatible strings... <Looking> It was there:
http://devicetree.org/Device_Tree_Usage#Understanding_the_compatible_Property
(Sorry, some of this stuff is a bit hard for me to grok.)

Finally, I think what you decide to do can also be done for the
interrupt controller, right?

> - Drivers match against whatever they need to in order to safely
>   identify features.  If the driver finds a match for e.g.
>   "sigma,smp864x-ethernet" it is allowed to make use of any features
>   present in all chips of that family (even if the actual chip is a
>   later one, the DT says it's compatible).  If a specific chip is found
>   to need a bug workaround or whatever, the driver can enable that by
>   matching on the exact string.
> 
> This approach means that if the driver is updated to support newer
> features, no change is needed to the devicetree, and if a new chip shows
> up, say a hypothetical smp8764, a driver with support for the smp87xx
> family will automatically recognise it without changes.  Unfortunately
> the 863x (tango2) chips are not compatible with 864x and later, so it's
> not safe to use a catch-all smp86xx.
> 
> Speaking more generally about device trees, for chip families like this,
> it can be a good solution to have all the common parts in, say,
> smp87xx.dtsi which is included by chip-specific files, i.e. smp8734.dtsi
> and smp8759.dtsi, with overrides and additions as necessary.  These
> files can then be included by the actual board files such as
> smp8759-vantage.dts which can apply further tweaks and add nodes for
> off-chip peripherals.

I think my brain is having a device tree indigestion :-(
Maybe things will clear up in a few hours.

Regards.

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