Message-ID: <fb559224-561c-3476-1146-96f7aea427f0@arm.com>
Date:	Mon, 1 Aug 2016 02:43:06 +0100
From:	André Przywara <andre.przywara@....com>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>,
	Rob Herring <robh+dt@...nel.org>, Chen-Yu Tsai <wens@...e.org>,
	Mike Turquette <mturquette@...libre.com>,
	Stephen Boyd <sboyd@...eaurora.org>
Cc:	devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-clk@...r.kernel.org
Subject: Re: [PATCH 00/13] arm64: Allwinner A64 support based on sunxi-ng

Hi Maxime,

On 26/07/16 21:30, Maxime Ripard wrote:
> Hi,
> 
> Here is the previous A64 patches made by Andre [1], reworked to use
> the new sunxi-ng clock framework.
> 
> This uses the current H3 clock code, as both are really similar. The
> first patches are just meant to rework slightly the H3 code, before
> introducing the A64-related patches.
> 
> Some WiP stuff have been removed, such as the MMC part, but this serie
> already has a decent amount of devices supported: uart, i2c, rsb, etc.

Thanks very much for looking into this and compiling this series!

In general this looks good to me - apart from the sunxi-ng clock usage.
I think I have some small fixes to the DT (have to compare against my
latest local branch), I will comment on this later.

As I think I never officially expressed my concerns about the new sunxi
clock system, so I use that opportunity here ;-)

As this became quite a long read, here a TL;DR:
- We consider using an SCPI based clock system for the A64, alongside
allwinner,simple-gates and fixed clocks. We try to avoid any Allwinner
specific clocks (apart from the simple-gates).
- ARM Trusted Firmware provides the SCPI implementation - for now, later
we may move this into a possible arisc firmware.
- We upstream some basic DT first, possibly omitting any controversial
clock parts at all.

Let me know what you think!


Now the long part ....

Basically I see those issues with the new clocks:
- sunxi-ng requires a complicated SoC specific source file in the
kernel. Although that makes the DT pretty easy (and avoids breaking it
the future), it ultimately requires an explicit code drop for every new
SoC, even if they share 95% of the clocks (as H3 and A64 do, for instance).
- This code file does not actually contain any code, instead it's just
data and looks like it should really be presented in DT - which brings
us back to something like the old sunxi clock scheme, which is
apparently not considered good enough. I still wonder if we could create
a generic sunxi-ng user, which explains the needed clocks in the DT
instead of in code. I admit that looks like quite some work.
- It makes it quite hard for any other DT user (*BSD, U-Boot) to use the
clocks, since they would have to copy quite verbatim the Linux
implementation choice. This is admittedly also true for the old clock
framework, but still unfortunate.

So as mentioned several times before, I am looking into a more firmware
driven clock system using the SCPI[1] framework.
The idea is:
- The basic clocks (OSC24M, OSC32K, AHB1, APB1, APB2, PERIPH0) are
expressed as fixed clocks. If I am not mistaken, Allwinner recommends
certain frequencies for them anyway, so we just use that and set them up
before booting Linux, for instance in ATF.
- The gates clocks are expressed as before, but by defining a generic DT
compatible fallback name. I have no idea why every SoC enters its name
into the simple_gates.c source file, while just mentioning the
compatible string in the DT bindings and using the SoC specific name
together with a generic fallback like "allwinner,sunxi-simple-gates"
would suffice. This means that we don't need to touch simple-gates.c
anymore most of the time.
- Any clock that can (and has to) be programmed with a variable
frequency is expressed as an SCPI clock. This interface allows basically
querying and setting a frequency - not very powerful, but sufficient for
the clocks I checked. Firmware then takes the burden of programming the
respective clock register - which is not rocket science if we lock the
base clocks to a certain frequency.

The advantage of this approach would be:
- The impact to Linux code is minimized. Normally there would be no need
to touch the kernel at all when we introduce a new SoC.
- Any other DT user can quite easily make use of the clock system
without adding tons of complicated Allwinner specific clock code. The
simple-gates driver is almost trivial to implement, and chances are SCPI
is already around anyway.
- If there are any peculiarities with a certain clock (implementation),
we can solve this in firmware. Fixes to code would immediately benefit
all users - existing kernels (from distributions), newer kernels and
other OSes.
- Having SCPI gives us simple regulators and sensors (temperature,
power) for free (in terms of no Linux code required). It also allows for
DVFS support, though this may require more work on the firmware side.
- This approach matches many of the more serious ARM64 machines out
there, which refrain from exposing all of their clock framework to Linux.

Also this opens the door to much easier support for new SoCs - up to a
point where any new chip would actually run out of the box on existing
distributions (thinking of LTS releases here). The pinctrl driver is a
nasty guy around here - but let's not make it worse and try to fix that
guy later ;-)

So I managed to finish a first prototype this weekend.
SCPI requires a mailbox and a shared memory region to work. The latter
is trivial, but we are lacking a proper mailbox driver for sunxi.
Besides that the Allwinner mailbox only works between the arisc and the
ARM cores, not between ARM cores or within one core in different
exception levels. So signalling a mailbox in EL1 and taking the IRQ in
EL3 does not work.
As a quick solution I implemented a synchronous "smc" mailbox, which
uses the ARM smc instruction to transfer control into firmware. Firmware
then reads the payload from the shared memory region, handles the
request and returns to EL1. We then advertise this mailbox in the SCPI
DT node, and the rest of the SCPI framework can happily work with that.
I described the clocks as mentioned above and put the MMC clocks under
SCPI control.
I put a draft DT here [2] to illustrate my ideas.

I know this is quite controversial, so what about these following steps:

- I polish my existing patches and send a prototype this week. We can
then discuss the merits and drawbacks of this approach based on the code.
- It it turns out that sunxi-ng is the way to go, I will test and
comment on this series.
- If we are not sure yet, we can try to go with an almost clock-less DT
for now (drop I2C and make UART0 use osc24M directly, for a start). This
would leave the door open for either approach. Or we use the
simple-gates only for now.

I am happy to discuss this here on the list.

Cheers,
Andre

[1]
http://infocenter.arm.com/help/topic/com.arm.doc.dui0922g/scp_message_interface_v1_2_DUI0922G_en.pdf
[2] https://gist.github.com/apritzel/faa3dc5cbe7591be8c55a439f725578e

> 
> Let me know what you think,
> Maxime
> 
> 1: http://lists.infradead.org/pipermail/linux-arm-kernel/2016-February/410338.html
> 
> Andre Przywara (5):
>   arm64: sunxi: Kconfig: add essential pinctrl driver
>   arm64: Kconfig: sunxi: add PINCTRL
>   Documentation: devicetree: add vendor prefix for Pine64
>   arm64: dts: add Allwinner A64 SoC .dtsi
>   arm64: dts: add Pine64 support
> 
> Maxime Ripard (8):
>   clk: sunxi-ng: mux: Rename mux macro to be consistent
>   clk: sunxi-ng: mux: Add mux table support
>   clk: sunxi-ng: sun8i: Rename DDR and video plls
>   clk: sunxi-ng: sun8i: Fix register offset
>   clk: sunxi-ng: sun8i: Rename H3 only clocks
>   clk: sunxi-ng: sun8i: Move fixed factors around
>   clk: sunxi-ng: sun8i: Prefix clock defines by SoC Name
>   clk: sunxi-ng: Add A64 clocks
> 
>  Documentation/devicetree/bindings/arm/sunxi.txt    |   1 +
>  .../devicetree/bindings/clock/sunxi-ccu.txt        |   1 +
>  .../devicetree/bindings/vendor-prefixes.txt        |   1 +
>  MAINTAINERS                                        |   1 +
>  arch/arm/boot/dts/sun8i-h3.dtsi                    |  62 +-
>  arch/arm64/Kconfig.platforms                       |   2 +
>  arch/arm64/boot/dts/Makefile                       |   1 +
>  arch/arm64/boot/dts/allwinner/Makefile             |   5 +
>  .../boot/dts/allwinner/sun50i-a64-pine64-plus.dts  |  50 ++
>  .../arm64/boot/dts/allwinner/sun50i-a64-pine64.dts |  70 ++
>  arch/arm64/boot/dts/allwinner/sun50i-a64.dtsi      | 273 +++++++
>  drivers/clk/sunxi-ng/Kconfig                       |  13 +-
>  drivers/clk/sunxi-ng/Makefile                      |   2 +-
>  drivers/clk/sunxi-ng/ccu-sun50i-a64.h              |  68 ++
>  drivers/clk/sunxi-ng/ccu-sun8i-h3.c                | 896 ++++++++++++++++-----
>  drivers/clk/sunxi-ng/ccu-sun8i-h3.h                |  44 +-
>  drivers/clk/sunxi-ng/ccu_div.h                     |   2 +-
>  drivers/clk/sunxi-ng/ccu_mp.h                      |   2 +-
>  drivers/clk/sunxi-ng/ccu_mux.c                     |  14 +
>  drivers/clk/sunxi-ng/ccu_mux.h                     |  29 +-
>  include/dt-bindings/clock/sun50i-a64-ccu.h         | 132 +++
>  include/dt-bindings/clock/sun8i-h3-ccu.h           | 188 ++---
>  include/dt-bindings/reset/sun50i-a64-ccu.h         |  97 +++
>  23 files changed, 1588 insertions(+), 366 deletions(-)
>  create mode 100644 arch/arm64/boot/dts/allwinner/Makefile
>  create mode 100644 arch/arm64/boot/dts/allwinner/sun50i-a64-pine64-plus.dts
>  create mode 100644 arch/arm64/boot/dts/allwinner/sun50i-a64-pine64.dts
>  create mode 100644 arch/arm64/boot/dts/allwinner/sun50i-a64.dtsi
>  create mode 100644 drivers/clk/sunxi-ng/ccu-sun50i-a64.h
>  create mode 100644 include/dt-bindings/clock/sun50i-a64-ccu.h
>  create mode 100644 include/dt-bindings/reset/sun50i-a64-ccu.h
> 

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