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

[wangkefeng.wang@huawei.com (Kefeng Wang)]

Hi Graeme, Arnd, and Lorenzo,

Firstly, we absolutely agree with the point which use AML to do some "special"
initialisation and configuration.

SAS and NIC driver were accepted by linux in hisilicon hip05 chip, and the drivers
reset the control by syscon, we want to use "_RST" method, which is introduced by
ACPI 6.0 spec in "7.3.25 _RST (Device Reset)", is it reasonable and standard for us?

But here is a scene, we can not find a suitable way to support ACPI. There is no
independent memory region in some module(the driver not upstreamed), that is,
when write and read the module's register, we must r/w by syscon. Any advice?

Thanks,
Kefeng

On 2015/12/3 18:41, Graeme Gregory wrote:
> On Wed, Dec 02, 2015 at 11:44:51AM +0100, Arnd Bergmann wrote:
>> On Wednesday 02 December 2015 17:09:28 Kefeng Wang wrote:
>>> This enables syscon with ACPI support.
>>> syscon_regmap_lookup_by_dev_property() function was added. With helper
>>> device_get_reference_node() and acpi_dev_find_plat_dev(), it can be used
>>> in both DT and ACPI.
>>>
>>> The device driver can obtain syscon using _DSD method in DSDT, an example
>>> is shown below.
>>>
>>>     Device(CTL0) {
>>>           Name(_HID, "HISI0061")
>>>           Name(_CRS, ResourceTemplate() {
>>>                   Memory32Fixed(ReadWrite, 0x80000000, 0x10000)
>>>           })
>>>     }
>>>
>>>     Device(DEV0) {
>>>           Name(_HID, "HISI00B1")
>>>           Name(_CRS, ResourceTemplate() {
>>>                   Memory32Fixed(ReadWrite, 0x8c030000, 0x10000)
>>>                   Interrupt(ResourceConsumer, Level, ActiveHigh, Exclusive){ 192 }
>>>           })
>>>
>>>           Name (_DSD, Package () {
>>>               ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
>>>               Package () {
>>>                   Package () {"syscon",Package() {\_SB.CTL0} }
>>>               }
>>>           })
>>>     }
>>>
>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>
>> 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.
>>
> 
> I have to agree with Arnd here, this is specifically why it was chosen
> to use ACPI on machines to move all these "hacks" to AML.
> 
> This leaves your driver being generic and any "special" initialisation
> can be supplied by the OEM through the ACPI tables.
> 
> Graeme
> 
> 
> .
>