linux-kernel - Re: [PATCH v3 2/2] i2c: exynos5: add support for ExynosAutov9 SoC

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Message-ID: <da9bd8cc-9415-6db7-024e-8d50b5f666f7@canonical.com>
Date:   Tue, 16 Nov 2021 10:31:58 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Chanho Park <chanho61.park@...sung.com>,
        'Sam Protsenko' <semen.protsenko@...aro.org>,
        'Jaewon Kim' <jaewon02.kim@...sung.com>
Cc:     'Wolfram Sang' <wsa@...nel.org>,
        'Rob Herring' <robh+dt@...nel.org>,
        linux-samsung-soc@...r.kernel.org, linux-i2c@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 2/2] i2c: exynos5: add support for ExynosAutov9 SoC

On 16/11/2021 02:12, Chanho Park wrote:
>> With this patch the Exynos850 HSI2C becomes functional. The only nit-pick
>> from my side (just a food for thought): do we want to configure USI
>> related config inside of particular drivers (SPI, I2C, UART)? Or it would
>> be better design to implement some platform driver for that, so we can
>> choose USI configuration (SPI/I2C/UART) in device tree? I think this
>> series is good to be merged as is, but we should probably consider all
>> upsides and downsides of each option, for the future work.
> 
> I'm also considering how to support this USI configuration gracefully.
> Current version of USI is v2 which means there is a v1 version as well. It might be a non-upstream SoC so we don't need to consider it so far.
> But, there is a possibility that the USI hw version can be bumped for future SoCs.
> 
> As you probably know, earlier version of the product kernel has a USI SoC driver[1] and it was designed to be configured the USI settings by device tree.
> 
> Option1) Make a USI driver under soc/samsung/ like [1].
> Option2) Use more generic driver such as "reset driver"? This might be required to extend the reset core driver.
> Option3) Each USI driver(uart/i2c/spi) has its own USI configurations respectively and expose some configurations which can be variable as device tree.
> 
> [1]: https://github.com/ianmacd/d2s/blob/master/drivers/soc/samsung/usi_v2.c

I don't have user manuals, so all my knowledge here is based on
Exynos9825 vendor source code, therefore it is quite limited. In
devicetree the USI devices have their own nodes - but does it mean it's
separate SFR range dedicated to USI? Looks like that, especially that
address space is just for one register (4 bytes).

In such case having separate dedicated driver makes sense and you would
only have to care about driver ordering (e.g. via device links or phandles).

Option 2 looks interesting - reusing reset framework to set proper USI
mode, however this looks more like a hack. As you said Chanho, if there
is a USI version 3, this reset framework might not be sufficient.

In option 3 each driver (UART/I2C/SPI) would need to receive second IO
range and toggle some registers, which could be done via shared
function. If USI v3 is coming, all such drivers could get more complicated.

I think option 1 is the cleanest and extendable in future. It's easy to
add usi-v3 or whatever without modifying the UART/I2C/SPI drivers. It
also nicely encapsulates USI-related stuff in separate driver. Probe
ordering should not be a problem now.

But as I said, I don't have even the big picture here, so I rely on your
opinions more.

Best regards,
Krzysztof