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SubjectRe: [PATCH v4 02/11] mfd: Add support for Kontron sl28cpld management controller
Am 2020-06-08 12:02, schrieb Andy Shevchenko:
> +Cc: some Intel people WRT our internal discussion about similar
> problem and solutions.
>
> On Mon, Jun 8, 2020 at 11:30 AM Lee Jones <lee.jones@linaro.org> wrote:
>> On Sat, 06 Jun 2020, Michael Walle wrote:
>> > Am 2020-06-06 13:46, schrieb Mark Brown:
>> > > On Fri, Jun 05, 2020 at 10:07:36PM +0200, Michael Walle wrote:
>> > > > Am 2020-06-05 12:50, schrieb Mark Brown:
>
> ...
>
>> Right. I'm suggesting a means to extrapolate complex shared and
>> sometimes intertwined batches of register sets to be consumed by
>> multiple (sub-)devices spanning different subsystems.
>>
>> Actually scrap that. The most common case I see is a single Regmap
>> covering all child-devices.
>
> Yes, because often we need a synchronization across the entire address
> space of the (parent) device in question.
>
>> It would be great if there was a way in
>> which we could make an assumption that the entire register address
>> space for a 'tagged' (MFD) device is to be shared (via Regmap) between
>> each of the devices described by its child-nodes. Probably by picking
>> up on the 'simple-mfd' compatible string in the first instance.
>>
>> Rob, is the above something you would contemplate?
>>
>> Michael, do your register addresses overlap i.e. are they intermingled
>> with one another? Do multiple child devices need access to the same
>> registers i.e. are they shared?

No they don't overlap, expect for maybe the version register, which is
just there once and not per function block.

>>
>> > > > But, there is more in my driver:
>> > > > (1) there is a version check
>>
>> If we can rid the Regmap dependency, then creating an entire driver to
>> conduct a version check is unjustifiable. This could become an inline
>> function which is called by each of the sub-devices instead, for
>> example.

sounds good to me. (although there would then be a probe fail per
sub-device
if the version is not supported)

>> > > > (2) there is another function for which there is no suitable linux
>> > > > subsystem I'm aware of and thus which I'd like to us sysfs
>> > > > attributes for: This controller supports 16 non-volatile
>> > > > configuration bits. (this is still TBD)
>>
>> There is a place for everything in Linux.
>>
>> What do these bits configure?

- hardware strappings which have to be there before the board powers up,
like clocking mode for different SerDes settings
- "keep-in-reset" bits for onboard peripherals if you want to save power
- disable watchdog bits (there is a watchdog which is active right from
the start and supervises the bootloader start and switches to failsafe
mode if it wasn't successfully started)
- special boot modes, like eMMC, etc.

Think of it as a 16bit configuration word.

>> > > TBH I'd also say that the enumeration of the subdevices for this
>> > > device should be in the device rather than the DT, they don't
>> > > seem to be things that exist outside of this one device.
>> >
>> > We're going circles here, formerly they were enumerated in the MFD.
>> > Yes, they are devices which aren't likely be used outside a
>> > "sl28cpld", but there might there might be other versions of the
>> > sl28cpld with other components on different base addresses. I
>> > don't care if they are enumerated in DT or MFD, actually, I'd
>> > prefer the latter. _But_ I would like to have the device tree
>> > properties for its subdevices, e.g. the ones for the watchdog or
>> > whatever components there might be in the future.
>>
>> [...]
>>
>> > MFD core can
>> > match a device tree node today; but only one per unique compatible
>> > string. So what should I use to differentiate the different
>> > subdevices?
>>
>> Right. I have been aware of this issue. The only suitable solution
>> to this would be to match on 'reg'.

see below (1)

>>
>> FYI: I plan to fix this.
>>
>> If your register map needs to change, then I suggest that this is
>> either a new device or at least a different version of the device and
>> would also have to be represented as different (sub-)mfd_cell.
>>
>> > Rob suggested the internal offset, which I did here.
>>
>> FWIW, I don't like this idea. DTs should not have to be modified
>> (either in the first instance or subsequently) or specifically
>> designed to patch inadequacies in any given OS.

How does (1) play together with this? What do you propose the "reg"
property should contain?

>>
>> > But then, there is less use in duplicating the offsets in the MFD
>> > just to have the MFD enumerate the subdevices and then match
>> > the device tree nodes against it. I can just use
>> > of_platform_populate() to enumerate the children and I won't
>> > have to duplicate the base addresses.
>>
>> Which is fine. However this causes a different issue for you. By
>> stripping out the MFD code you render the MFD portion seemingly
>> superfluous. Another issue driver authors commonly contend with.

Yeah, I agree.

-michael

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