Message-ID: <df2fc253-b16a-3d93-7682-513abdf967d5@linaro.org>
Date:   Tue, 5 Apr 2022 08:48:32 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Nícolas F. R. A. Prado 
        <nfraprado@...labora.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
Cc:     lee.jones@...aro.org, robh+dt@...nel.org, krzk+dt@...nel.org,
        arnd@...db.de, matthias.bgg@...il.com, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel@...labora.com
Subject: Re: [PATCH 2/2] dt-bindings: mfd: syscon: Add support for regmap
 fast-io

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
>>>>> hardware.
>>>>>
>>>>> 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
> property.
> 
> 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
> hardware.

Thanks, that very good explanation!

> 
>>>
>>> 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,
>>> Krzysztof
>>
>> 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?

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.

Best regards,
Krzysztof

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