[U-Boot] [PATCH 4/4] syscon: add Linux-compatible syscon API

[Simon Glass]

On 22 April 2018 at 22:58, Masahiro Yamada <yamada.masahiro at socionext.com>
wrote:
> Hi Simon,
>
>
> 2018-04-23 5:11 GMT+09:00 Simon Glass <sjg at chromium.org>:
>> Hi Masahiro,
>>
>> On 17 April 2018 at 20:38, Masahiro Yamada <yamada.masahiro at socionext.com
>
>> wrote:
>>> The syscon implementation in U-Boot is different from that in Linux.
>>> Thus, DT files imported from Linux do not work for U-Boot.
>>>
>>> In U-Boot driver model, each node is bound to a dedicated driver
>>> that is the most compatible to it.  This design gets along with the
>>> concept of DT, and the syscon in Linux originally worked like that.
>>>
>>> However, Linux commit bdb0066df96e ("mfd: syscon: Decouple syscon
>>> interface from platform devices") changed the behavior because it is
>>> useful to let a device bind to another driver, but still works as a
>>> syscon provider.
>>>
>>> That change had happened before U-Boot initially supported the syscon
>>> driver by commit 6f98b7504f70 ("dm: Add support for generic system
>>> controllers (syscon)").  So, the U-Boot's syscon works differently
>>> from the beginning.  I'd say this is mis-implementation given that
>>> DT is not oriented to a particular project, but Linux is the canon
>>> of DT in practice.
>>>
>>> The problem typically arises in the combination of "syscon" and
>>> "simple-mfd" compatibles.
>>>
>>> In Linux, they are orthogonal, i.e., the order between "syscon" and
>>> "simple-mfd" does not matter at all.
>>>
>>> Assume the following compatible.
>>>
>>>    compatible = "foo,bar-syscon", "syscon", "simple-mfd";
>>>
>>> In U-Boot, this device node is bound to the syscon driver
>>> (driver/core/syscon-uclass.c) since the "syscon" is found to be the
>>> most compatible.  Then, syscon_get_regmap() succeeds.
>>>
>>> However,
>>>
>>>    compatible = "foo,bar-syscon", "simple-mfd", "syscon";
>>>
>>> does not work because this node is bound to the simple-bus driver
>>> (drivers/core/simple-bus.c) in the favor of "simple-mfd" compatible.
>>> The compatible string "syscon" is just dismissed.
>>>
>>> Moreover,
>>>
>>>    compatible = "foo,bar-syscon", "syscon";
>>>
>>> works like the first case because the syscon driver populates the
>>> child devices.  This is wrong because populating children is the job
>>> of "simple-mfd" (or "simple-bus").
>>>
>>> This commit ports syscon_node_to_regmap() from Linux.  This API
>>> does not require the given node to be bound to a driver in any way.
>>>
>>> Reported-by: Kunihiko Hayashi <hayashi.kunihiko at socionext.com>
>>> Signed-off-by: Masahiro Yamada <yamada.masahiro at socionext.com>
>>> ---
>>>
>>>  drivers/core/syscon-uclass.c | 63
>> ++++++++++++++++++++++++++++++++++++++++++++
>>>  include/syscon.h             |  8 ++++++
>>>  2 files changed, 71 insertions(+)
>>
>> I was not aware of this change in how Linux worked, but it makes sense.
>>
>> Please can you add a test for this?
>
>
> I do not believe a missing test would block this patch,
> but I volunteered to add it (as a separate patch).
>
> http://patchwork.ozlabs.org/patch/902733/

Thanks!