linux-kernel - Re: [PATCH v2 3/4] memory: tegra124-emc: Fix compilation warnings on 64bit platforms

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Message-ID: <1cde1d43-f139-cb90-395e-8f8fceb41bce@canonical.com>
Date:   Mon, 17 May 2021 10:04:21 -0400
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>
To:     Dmitry Osipenko <digetx@...il.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Mikko Perttunen <mperttunen@...dia.com>
Cc:     linux-kernel@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-clk@...r.kernel.org
Subject: Re: [PATCH v2 3/4] memory: tegra124-emc: Fix compilation warnings on
 64bit platforms

On 17/05/2021 09:47, Dmitry Osipenko wrote:
> 17.05.2021 16:39, Krzysztof Kozlowski пишет:
>>>>>  #define DRAM_DEV_SEL_ALL			0
>>>>> -#define DRAM_DEV_SEL_0				(2 << 30)
>>>>> -#define DRAM_DEV_SEL_1				(1 << 30)
>>>>> +#define DRAM_DEV_SEL_0				(2u << 30)
>>>>> +#define DRAM_DEV_SEL_1				(1u << 30)
>>>>
>>>> Why not using BIT()? This would make even this 2<<30 less awkard...
>>>
>>> The bitfield 31:30 is a enum, 3 is a wrong value. Formally it's
>>> incorrect to use the BIT() macro here.
>>
>> Why "3"? BIT(31) is the same as 2<<30.
> 
> By 3 I meant BIT(31)|BIT(30). This bitfield is explicitly designated as
> a enum in the hardware documentation.

I understand it and using BIT() here does not mean someone has to set
both of them. BIT() is a helper pointing out that you want to toggle one
bit. It does not mean that it is allowed to do so always!

> 
>> It's common to use BIT for
>> register fields which do not accept all possible values. Now you
>> basically reimplement BIT() which is error-prone.
> 
> Could you please show couple examples? The common practice today is to
> use FIELD_PREP helpers, but this driver was written before these helpers
> existed.


There are plenty of such examples so I guess it would be easier to ask
you to provide counter ones. Few IT for enum-like registers found within 2 minutes:

https://elixir.bootlin.com/linux/latest/C/ident/MAX77620_CNFG_GPIO_INT_MASK
https://elixir.bootlin.com/linux/v5.13-rc2/source/drivers/regulator/max77650-regulator.c#L18
https://elixir.bootlin.com/linux/v5.13-rc2/source/drivers/regulator/tps6524x-regulator.c#L62
https://elixir.bootlin.com/linux/v5.13-rc2/source/drivers/regulator/tps80031-regulator.c#L39
https://elixir.bootlin.com/linux/v5.13-rc2/source/drivers/regulator/da9121-regulator.h#L200
https://elixir.bootlin.com/linux/v5.13-rc2/source/drivers/regulator/da9121-regulator.h#L231

Best regards,
Krzysztof