[RFC PATCH 0/5] Allwinner MMC firmware clocks implementation

Wed Aug 10 16:18:56 PDT 2016

On 10/08/16 16:10, Icenowy Zheng wrote:
> 
> 
> 09.08.2016, 19:57, "Andre Przywara" <andre.przywara at arm.com>:
>> Hi,
>>
>> this is a proof-of-concept series to demonstrate the usage of firmware
>> driven clocks using the SCPI protocol for Allwinner SoCs.
>> This aims to replace the tricky and highly SoC specific clocks drivers
>> that every new Allwinner SoC seems to require.
>> The idea is to abstract the internal clock specific code by the rather
>> generic SCPI interface, which support programming and reading clock
>> frequencies in an SoC agnostic manner. The actual clock access can be
>> put in firmware, which is by definition SoC specific and thus can
>> be easier and much quicker adapted to support a new SoC.
>> The SCPI interface requires a mailbox and a shared memory region to
>> send commands and receive results.
>>
>> This series introduces a new mailbox driver, which uses smc instructions
>> to trigger a mailbox. This allows the SCPI handlers to live as a runtime
>> service component in the existing ARM Trusted Firmware port. Beside
>> being able to easily add those handlers to the existing firmware code
>> this approach has the advantage of avoiding building, maintaining and
>> loading yet another firmware component. The management processor on the
>> A64 uses an OpenRISC core, which requires a not fully upstream supported
>> toolchain. Also this core can remain free to other tasks, like offloading
>> of realtime-sensitive tasks.
> 
> Here's a problem...
> 
> Although SCPI is a standard, SMC mailbox and ATF clock id is not.
> 
> And there's currently no generic clock framework in ATF...

And I don't see why there is a need for that.
More below.

> 
>> A future implementation may later revisit this decision, adding a proper
>> mailbox driver and implement the SCPI handler on the OpenRISC core, the
>> only change needed would be to swap the mailbox node in the DT then.
>>
>> Another added value is that SCPI brings us sensors and regulators
>> support, using this very same interface. All that's needed is to add
>> some firmware code to read the THS register or poke the AXP, for
>> instance (which ATF does already anyway) and advertise those via a DT
>> node. There would be no Linux changes needed for that.
>>
>> As an example this series adds MMC support on top of the basic support
>> bit posted recently [1].
>> We use the existing sunxi MMC driver for the purpose of this series,
>> although it is known to not fully support the new MMC controller in the
>> A64 chip. I found the support sufficient enough to drive SD cards, though.
>>
>> Patch 1/5 introduces the SMC mailbox driver, while patch 2/5 adds the
>> associated DT binding documentation. Patch 3/5 adds the basic SCPI nodes
>> to the SoC .dtsi, which patch 4/5 complements with the MMC nodes,
>> referencing the new firmware clocks.
>> The final patch 5/5 enables the SD card for the two boards which we
>> currently support.
>>
>> This series goes on top of the v4 A64 support series [1], which itself
>> is based on v4.8-rc1.
>> It allows booting and running a userland from a micro SD card on both
>> the Pine64 and the BananaPi M64 board.
>> The matching firmware implementation can be found in the allwinner-scpi
>> branch here [2]. The clock code in there is probably pretty stupid, but
>> works and is good enough to at least serve as a proof-of-concept for this
>> new clock approach.
>>
>> Please have a look and give comments, I am particularily interested in
>> how you like this idea. One motiviation is to save the kernel from
>> endless variations of clock driver code, also to be able to support
>> newer SoCs much easier without having to submit and maintain tedious
>> clock patches.
>>
>> Cheers,
>> Andre.
>>
>> [1]: http://lists.infradead.org/pipermail/linux-arm-kernel/2016-August/447512.html
>> [2]: https://github.com/apritzel/arm-trusted-firmware/commits/allwinner-scpi
> 
> The code seems to be difficult to extend...

This is just proof of concept code hacked together mostly in one night.
I just didn't want to delay this any more and so followed "Release
early, release often" here.
Once I add a second clock, I will probably rework it anyway.

> I hope you can make the SMC mailbox a standard interface,

I don't know if it needs to be some kind of "standard interface". We
follow the ARM SMC Calling Convention on one end and plug to the
existing Linux mailbox framework (which SCPI depends on) on the other,
advertising this via DT.
So all we need to take care of is upstreaming this.

> and add a common
> clock controlling system in the ATF...

Why is this needed? For now this is just another vendor specified SMC
call, advertised via DT. Other SoC vendors have those already in
upstream ATF (Mediatek, Rockchip).
If we confine these clock setups to one particular platform (or even
SoC), the code can be quite simple.

> (And maybe add A64 as part of the reference implemention)

Possibly, but right now I don't see a need for a bullet-proof and
upstream-accepted framework implementation - as long as we comply with a
standard interface like SCPI. Actually we can swap the implementation at
any time, as long as we stay standards compliant.
As mentioned above, this particular ATF implementation was more meant as
a proof of concept.

> (but I think it may be to difficult to you to make standards and change
> reference implemention...)

I don't have the impression that ATF is reluctant to those changes.
And which standards would I need to "make"? SCPI is already out there,
and adding platform specific code is normal and expected for a firmware
implementation. There is an extra design document [1] which describes
how to add vendor specific smc calls:
https://github.com/ARM-software/arm-trusted-firmware/blob/master/docs/rt-svc-writers-guide.md

Cheers,
Andre.

> 
>>
>> Andre Przywara (5):
>>   mailbox: introduce ARM SMC based mailbox
>>   DT: mailbox: add binding doc for the ARM SMC mailbox
>>   arm64: dts: sunxi: add SCPI node to sun50i-a64.dtsi
>>   arm64: dts: sunxi: add SCPI driven clocks and nodes for A64 MMC
>>   arm64: dts: sunxi: add MMC nodes to Pine64 and BPi-M64 .dts
>>
>>  .../devicetree/bindings/mailbox/arm-smc.txt | 53 ++++++++
>>  .../boot/dts/allwinner/sun50i-a64-bananapi-m64.dts | 29 +++++
>>  .../arm64/boot/dts/allwinner/sun50i-a64-pine64.dts | 20 +++
>>  arch/arm64/boot/dts/allwinner/sun50i-a64.dtsi | 87 +++++++++++++
>>  drivers/mailbox/Kconfig | 8 ++
>>  drivers/mailbox/Makefile | 2 +
>>  drivers/mailbox/smc-mailbox.c | 135 +++++++++++++++++++++
>>  7 files changed, 334 insertions(+)
>>  create mode 100644 Documentation/devicetree/bindings/mailbox/arm-smc.txt
>>  create mode 100644 drivers/mailbox/smc-mailbox.c
>>
>> --
>> 2.9.0
>>
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> 