Re: [PATCH 2/2] dt-bindings: mfd: syscon: Add support for regmap fast-io

From: AngeloGioacchino Del Regno
Date: Tue Apr 05 2022 - 07:07:06 EST

Il 05/04/22 08:48, Krzysztof Kozlowski ha scritto:
On 05/04/2022 00:26, Nícolas F. R. A. Prado wrote:
On Mon, Apr 04, 2022 at 11:39:49AM +0200, AngeloGioacchino Del Regno wrote:
Il 04/04/22 10:55, Krzysztof Kozlowski ha scritto:
On 04/04/2022 10:40, AngeloGioacchino Del Regno wrote:
Il 02/04/22 13:38, Krzysztof Kozlowski ha scritto:
On 01/04/2022 15:50, AngeloGioacchino Del Regno wrote:
The syscon driver now enables the .fast_io regmap configuration when
the 'fast-io' property is found in a syscon node.

Keeping in mind that, in regmap, fast_io is checked only if we are
not using hardware spinlocks, allow the fast-io property only if
there is no hwlocks reference (and vice-versa).

I have doubts you need a property for this. "fast" is subjective in
terms of hardware, so this looks more like a software property, not

I think most of MMIOs inside a SoC are considered fast. Usually also the
syscon/regmap consumer knows which regmap it gets, so knows that it is
fast or not.

Hello Krzysztof,

well yes, this property is changing how software behaves - specifically,
as you've correctly understood, what regmap does.

It's true that most of MMIOs inside a SoC are considered fast.. the word "most" is
the exact reason why I haven't proposed simply hardcoding '.fast_io = true' in
syscon, or in regmap-mmio...
There are too many different SoCs around, and I didn't want to end up breaking
anything (even if it should be unlikely, since MMIO is fast by principle).

Hi Angelo,

I think I can see what Krzysztof means by saying this looks more like a software

This property isn't simply saying whether the hardware is fast or not by itself,
since that's relative. Rather, it means that this hardware is fast relative to
the time overhead of using a mutex for locking in regmap. Since this is a
software construct, the property as a whole is software-dependent. If for some
reason the locking in regmap were to be changed and was now a lot faster or
slower, the same hardware could now be considered "fast" or "slow". This seems
to me a good reason to avoid making "fastness" part of the ABI for each

Thanks, that very good explanation!

Okay now that's clearing up my mind, makes me see the issue that was raised in
a different perspective. I've finally got it, and Krzysztof is definitely right
about that, as in the proposed solution raises potential future issues and
definitely cannot be considered for merging.

I should've sent this as a RFC instead; this topic is becoming larger than I
could foresee in the short term.

Thanks Krzysztof for raising this flag, and thank you Nic for finding a way to
wake up the hamster in my brain :-P

What I am proposing, is the regmap consumer knows whether access is fast
or not, so it could call get_regmap() or
syscon_regmap_lookup_by_phandle() with appropriate argument.

Even if we stay with a DT property, I am not sure if this is an
attribute of syscon but rather of a bus.

Best regards,

I'm sorry for sending a v2 so fast - apparently, I initially didn't fully
understand your comment, but now it's clear.

Actually, since locking in regmap's configuration does not use DT at all
in any generic case, maybe bringing this change purely in code may be a
good one... and I have evaluated that before proposing this kind of change.

My concerns about that kind of approach are:
- First of all, there are * a lot * of drivers, in various subsystems, that
are using syscon, so changing some function parameter in syscon.c would
result in a commit that would be touching hundreds of them... and some of
them would be incorrect, as the default would be no fast-io, while they
should indeed enable that. Of course this would have to be changed later
by the respective driver maintainer(s), potentially creating a lot of
commit noise with lots of Fixes tags, which I am trying to avoid;
- Not all drivers are using the same syscon exported function to get a
handle to regmap and we're looking at 6 of them; changing only one of
the six would be rather confusing, and most probably logically incorrect
as well...

Of course you know, but for the sake of making this easily understandable
for any casual developers reading this, functions are:
- device_node_to_regmap()
- syscon_node_to_regmap()
- syscon_regmap_lookup_by_compatible()
- syscon_regmap_lookup_by_phandle()
- syscon_regmap_lookup_by_phandle_args()
- syscon_regmap_lookup_by_phandle_optional().

What if a separate function was added with the additional regmap configuration
argument? That way setting the "fast_io" would be opt-in much like a DT property
would. The other drivers wouldn't need to be changed.

Exactly, there is no need to change all callers now.
1. You just add rename existing code and add argument:

syscon_regmap_lookup_by_compatible_mmio(...., unsigned long flags);

2. Add a wrappers (with old names):
syscon_regmap_lookup_by_compatible() {
syscon_regmap_lookup_by_compatible_mmio(..., SYSCON_IO_DEFAULT);

3. and finally slowly convert the users where it is relevant.

Just one more thing. I was rather thinking out loud, instead of
proposing a proper solution about clients defining speed. I am still not
sure if this is correct approach, because actually the regmap provider
knows the best whether it is slow or fast. Clients within SoC should
know it, but what if one client asks for fast regmap, other for slow? Or
not every client is updated to new API?

I didn't consider that kind of solution (adding yet one more function just for
the fast_io case, or wrapping the current ones) exactly because of the reasons
that you just explained...

...if we do that, we're going to also see issues like:
- Client #1 calls syscon fast-io, Client #2 is not converted
- Client #1 is expected to probe earlier than Client #2
- Probe order changes
- Now Client #1 also isn't fast-io

...unless reinitializing regmap mmio (regmap_init_mmio), but this would obviously
be a no-go, because races are under the *nearest* corner at that point. So.. no.

Another solution would be to add such property to the bus on which the
syscon device is sitting. Although this is also not complete. Imagine:

syscon <--ahb-fast-bus--> some bus bridge <--apb-slow-bus--> syscon consumer

Although your original case also would not be accurate here.

...but then that would still be a software construct and we'd be talking about
the same thing again, just in a different context.

I've also been thinking of adding a way to change the regmap locking on-the-fly
without having to re-init fully, but that also looks like being a no-go, as I
can foresee racing with that solution as well.

By the way, Let me give you some full-er context here, so that you can fully
understand, without any doubt, what I'm trying to do...

Here, on MediaTek SoCs, the HW ABI is in "full IP blocks" (warning: this "term"
may be wrong, but bear with me), for which we see each MMIO range having clocks,
resets, and other IP-specific registers, and sometimes these are "mixed" together.
There's no way to declare a small MMIO sub-range for just clocks, hence syscon is
really the only (relatively?) clean solution that can be used.

Thing is, MTK platforms are continuously locking/unlocking mutexes for simple and
fast operations, even "turning on/off" a clock (actually, due to the nature of the
clock tree for these SoCs, it's always more than one clock at once, so it's always
more than one mutex lock/unlock...), or operating on XHCI registers, etc etc etc.
This gives *a lot* of overhead and you can really experience the stutter sometimes,
and I mean experience with your own eyes, even on more performant chips...

I then checked some other non-MTK platforms, and I saw that it's not really just a
MTK thing (even though it's the heaviest user of syscon between the ones that I
checked)... so I saw a nice improvement pointer for lots of platforms... and I'm
sure that you immediately got that part.

I'm starting to get confused about what could be a good and acceptable solution
here, because at this point there are a lot of blockers around for any different
thing I can think of.