[RFC PATCH 4/4] mfd: syscon: add ACPI support

[Hanjun Guo]

On 12/11/2015 06:51 PM, Arnd Bergmann wrote:
> On Friday 11 December 2015 18:35:17 Zhangfei Gao wrote:
>> On Wed, Dec 2, 2015 at 6:44 PM, Arnd Bergmann <arnd at arndb.de> wrote:
>>> On Wednesday 02 December 2015 17:09:28 Kefeng Wang wrote:
>>>
>>> This sounds like a bad idea:
>>>
>>> syscon is basically a hack to let us access register that the SoC designer
>>> couldn't fit in anywhere sane. We need something like this with devicetree
>>> because we decided not to have any interpreted bytecode to do this behind
>>> our back.
>>>
>>> With ACPI, the same thing is done with AML, which is actually nicer than
>>> syscon (once you have to deal with all the problems introduced by AML).
>>>
>>> Use that instead.
>>>
>>
>> Would you mind clarifying AML method, still not understand.
>> Is is means realize the operation in uefi/bootloader and call some
>> interface from kernel?
>>
>> The issue here is we want to access some common registers,
>> which we do not want to ioremap for many times in each driver.
>>
>> In hisi platforms, we have many common registers shared by many components.
>
> Sorry, you'd have to ask someone who is more familiar with ACPI.
>
> Basically, the idea of ACPI as I understand it is that you have a
> interpreted bytecode language that is provided by the BIOS as tables
> and executed in the context of the kernel. This byte code has access
> to stuff like GPIO, PCI config space, MMIO registers, IPMI or the
> embedded controller on a PC. This way, you can hide all the platform
> specific details behind a function call from a generic device driver,
> and the BIOS writer can provide the specific implementation for any
> registers that are shared across components.
>
> That's about all I know, but the ACPI specifications are available
> publicly, please have a look there for more details.

Thanks Arnd rise this up, I agree we need to abstract the syscon
inside UEFI.

For ACPI 6.0, there are method such as _ON, _OFF and _RST (reset)
to the device's power resources, I think it can be extended to
syscon to reset the device with access syscon in ACPI method,
those method will be implemented in UEFI which the kernel driver
don't need to care about the detail.

We are going to for that implementation for D02, and if problem
we meet, we will go back for this discussion.

Thanks
Hanjun