Re: [RFC PATCH 5/6] arm64: dts: ti: Add Support for AM654 SoC

[Tony Lindgren]

* Nishanth Menon <nm@xxxxxx> [180614 13:07]:
> On 12:38-20180614, Tony Lindgren wrote:
> > Some comments on the ranges below.
> 
> Thanks for reviewing in detail (I understand we are in the middle of
> merge window, so thanks for the extra effort).
> 
> > 
> > * Nishanth Menon <nm@xxxxxx> [180607 16:41]:
> > > +	soc0: soc0 {
> > > +		compatible = "simple-bus";
> > > +		#address-cells = <2>;
> > > +		#size-cells = <2>;
> > > +		ranges;
> > 
> > I suggest you leave out the soc0, that's not real. Just make
> 
> Why is that so, on a more complex board representation with multiple
> SoCs, this is a clear node indicating what the main SoC is in the final
> dtb representation.

It does not have a real reg or range.

> > the cbass@0 the top level interconnect. It can then provide
> > ranges to mcu interconnect which can provide ranges to the wkup
> > interconnect. So just model it after what's in the hardware :)
> 
> That might blow up things quite a bit - it is like the comment in:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm/boot/dts/dra7.dtsi#n141

That comment at the link above not true I've found. What we have
there as "ocp" should be just "l3" and then the "l4" instances are
children of "l3". The direct ports from some "l4" devices are just
ranges at the parent "l3". And this will get changed slowly over
next few merge cycles.

> The trees are pretty deep with many interconnections (example main does
> have direct connections to wkup as well, which is simplified off in
> top level diagram) - basically it is not a direct one dimensional
> relationship. But then, the same is the case for other SoCs..

In the above example the connection from main to wkup is just a
range provided by main so not a problem.

> we can represent NAVSS as a bus segment as well.

Well ideally each module on the interconnects would be set up
separately to prevent drivers trying to ioremap ranges from
multiple modules. This is important as flushing posted write to
one module will not flush it for the other module.

> > I found the following ranges based on a quick look at the TRM,
> > they could be split further if needed for power domains for
> > genpd for example.
> 
> genpd is not really an issue, since it is handled in system firmware and
> OSes dont have a visibility into the permitted ranges that the OS is
> allowed to use.

There are other reasons beyond genpd too. Flushing posted writes
to modules is one. Getting rid of pointless deferred probe is
another one. Preventing device drivers trying to ioremap multiple
module is yet another one..

> I think it is just how accurate a representation is it worth.

The dts really is intended to describe the hardware :) So
let's not repeat the same mistake again with imaginary ranges.

> > 
> > main covers
> > 0x0000000000 - 0x5402000000
> > 
> > main provides at least the following ranges for mcu
> > 0x0028380000 - 0x002bc00000
> > 0x0040080000 - 0x0041c80000
> > 0x0045100000 - 0x0045180000
> > 0x0045600000 - 0x0045640000
> > 0x0045810000 - 0x0045860000
> > 0x0045950000 - 0x0045950400
> > 0x0045a50000 - 0x0045a50400
> > 0x0045b04000 - 0x0045b06400
> > 0x0045d10000 - 0x0045d24000
> > 0x0046000000 - 0x0060000000
> > 0x0400000000 - 0x0800000000
> > 0x4c3c020000 - 0x4c3c030000
> > 0x4c3e000000 - 0x4c3e040000
> > 0x5400000000 - 0x5402000000
> > 
> > then mcu provides the following ranges for wkup
> > 0x0042000000 - 0x0044410020
> > 0x0045000000 - 0x0045030000
> > 0x0045080000 - 0x00450a0000
> > 0x0045808000 - 0x0045808800
> > 0x0045b00000 - 0x0045b02400
> > 
> > This based on looking at "figure 1-1. device top-level
> > block diagram" and the memory map in TRM.
> 
> Thanks for researching. I did debate something like:
> 
> From A53 view, a more accurate view might be  - from an interconnect
> view of the world (still simplified - i have ignored the sub bus
> segments in the representations below):
> 
> msmc {
> 	navss_main {
> 		cbass_main{
> 			cbass_mcu {
> 				navss_mcu {
> 				};
> 				cbass_wkup{
> 				};
> 			};
> 		};
> 	};
> };
> 
> From R5 view, the view will be very different ofcourse:
> view of the world (still simplified):
> 
> cbass_mcu {
> 	navss_mcu {
> 	};
> 	cbass_wkup{
> 	};
> 	cbass_main{
> 		navss_main {
> 			msmc {
> 			};
> 		};
> 	};
> };

Well if we follow the hardware representation of the interconnects,
it should not matter from which processor view you're looking at things.
There are just different ranges provided.

> Do we really need this level of representation, I am not sure I had seen
> this detailed a representation in other aarch64 SoCs (I am sure they are
> as complex as TI SoCs as well).

Based on my experience yes. See also the reasons I listed above.

> I am trying to understand the direction and logic why we'd want to have
> such a detailed representation.
> 
> A more flatter representation of just the main segments allow for dts
> reuse between r5 and a53 as well (but that is minor).

Just model it based on the hardware, then there's no need to
debate it or rework it later on :)

Regards,

Tony