linux-kernel - Re: [PATCH v2 2/2] reset: Reset controller driver for Intel LGM SoC

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Date:   Mon, 14 Oct 2019 17:41:00 +0800
From:   Dilip Kota <eswara.kota@...ux.intel.com>
To:     Philipp Zabel <p.zabel@...gutronix.de>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>,
        Philipp Zabel <pza@...gutronix.de>
Cc:     "Chuan Hua, Lei" <chuanhua.lei@...ux.intel.com>,
        cheol.yong.kim@...el.com, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, qi-ming.wu@...el.com,
        robh@...nel.org, Hauke Mehrtens <hauke@...ke-m.de>
Subject: Re: [PATCH v2 2/2] reset: Reset controller driver for Intel LGM SoC

Hi Philipp,

On 10/8/2019 11:56 PM, Philipp Zabel wrote:
> Hi Martin,
>
> On Mon, 2019-10-07 at 21:53 +0200, Martin Blumenstingl wrote:
>> Hi Philipp,
>>
>> On Thu, Oct 3, 2019 at 4:19 PM Philipp Zabel <pza@...gutronix.de> wrote:
>> [...]
>>>> because the register layout was greatly simplified for the newer SoCs
>>>> (for which there is reset-intel) compared to the older ones
>>>> (reset-lantiq).
>>>> Dilip's suggestion (in my own words) is that you take his new
>>>> reset-intel driver, then we will work on porting reset-lantiq over to
>>>> that so in the end we can drop the reset-lantiq driver.
>>> Just to be sure, you are suggesting to add support for the current
>>> lantiq,reset binding to the reset-intel driver at a later point? I
>>> see no reason not to do that, but I'm also not quite sure what the
>>> benefit will be over just keeping reset-lantiq as is?
>> according to Chuan and Dilip the current reset-lantiq implementation
>> is wrong [0].
> The only issue seems to be the .reset callback, which doesn't have any
> users anway.
The DT binding of reset-lantiq driver is also having issue. I have 
explained here [1].
>
>> my understanding is that the Lantiq and Intel LGM reset controllers
>> are identical except:
>> - the Lantiq variant uses a weird register layout (reset and status
>> registers not at consecutive offsets)
>> - the bits of the reset and status registers sometimes don't match on
>> the Lantiq variant
> Thank you, so these are a good explanation for why the DT bindings
> should be different.
>
>> - the Intel variant has a dedicated registers area for the reset
>> controller registers, while the Lantiq variant mixes them with various
>> other functionality (for example: USB2 PHYs)
> I'm not quite sure I understand why the intel driver is using syscon,
> then. Either way, it shouldn't make a big difference if regmap is used
> anyway.
Yes, we decided to remove the syscon and use the regmap.[2]
>
>>>> This approach means more work for me (as I am probably the one who
>>>> then has to do the work to port reset-lantiq over to reset-intel).
>>> More work than what alternative?
>> compared to "fixing" the existing reset-lantiq driver (reset callback)
> That is still something you could do, or just drop the .reset callback
> because there are no reset consumers using it anyway.
>
> One correct thing to do would be to identify those self-clearing reset
> bits and to disallow calling assert/deassert on them.
>
>> and then (instead of adding a new driver) integrating Intel LGM
>> support into reset-lantiq
> Since at this point I'm not even sure whether merging the two at all is
> better than keeping them separate, I have no opinion on whether merging
> intel support into the lantiq driver or the other way around is
> preferable.
>
>>>> I'm happy to do that work if you think that it's worth following this
>>>> approach.  So I want your opinion on this before I spend any effort on
>>>> porting reset-lantiq over to reset-intel.
>>> Reset drivers are typically so simple, I'm not quite sure whether it is
>>> worth to integrate multiple drivers if it complicates matters too much.
>>> In this case though I expect it would just be adding support for a
>>> custom .of_xlate and lantiq specific register property parsing?
>> yes, that's how I understand the Lantiq and Intel reset controllers:
>> - reset/status/assert/deassert callbacks would be shared across all variants
>> - register parsing and of_xlate are SoC specific
> Ok. If that turns out to be less rather than more boilerplate than two
> separate drivers, that should be fine.

Thanks Philipp for your time and briefly explaining your view.

Regards,
Dilip

[1]: https://www.spinics.net/lists/devicetree/msg308930.html
[2]: https://lkml.org/lkml/2019/9/2/289

> regards
> Philipp