linux-kernel - Re: [PATCH] i2c: iproc: Change driver to use 'BIT' macro

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Date:   Wed, 17 Apr 2019 16:48:32 -0700
From:   Ray Jui <ray.jui@...adcom.com>
To:     Peter Rosin <peda@...ntia.se>, Wolfram Sang <wsa@...-dreams.de>
Cc:     "linux-i2c@...r.kernel.org" <linux-i2c@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "bcm-kernel-feedback-list@...adcom.com" 
        <bcm-kernel-feedback-list@...adcom.com>,
        Rayagonda Kokatanur <rayagonda.kokatanur@...adcom.com>
Subject: Re: [PATCH] i2c: iproc: Change driver to use 'BIT' macro



On 4/14/2019 11:56 PM, Peter Rosin wrote:
> On 2019-04-13 00:59, Peter Rosin wrote:
>> On 2019-04-03 23:05, Ray Jui wrote:
>>> Change the iProc I2C driver to use the 'BIT' macro from all '1 << XXX'
>>> bit operations to get rid of compiler warning and improve readability of
>>> the code
>>
>> All? I see lots more '1 << XXX_SHIFT' matches. I might be behind though?
> 
> I verified that, and yes indeed, I was behind. That said, see below...
> 

Right. Previous change that this change depends on is already queued in
i2c/for-next.

>> Anyway, if you are cleaning up, I'm just flagging that BIT(XXX_SHIFT) looks
>> a bit clunky to me. You might consider renaming all those single-bit
>> XXX_SHIFT macros to simple be
>>
>> #define XXX BIT(<xxx>)
>>
>> instead of
>>
>> #define XXX_SHIFT <xxx>
>>
>> but that triggers more churn, so is obviously more error prone. You might
>> not dare it?
>>

With the current code, I don't see how that is cleaner. With XXX_SHIFT
specified, it makes it very clear to the user that the define a for a
bit location within a register. You can argue and say it makes the
define longer, but not less clear.

>> Cheers,
>> Peter
>>
>>> Signed-off-by: Ray Jui <ray.jui@...adcom.com>
>>> ---
>>>  drivers/i2c/busses/i2c-bcm-iproc.c | 6 +++---
>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/i2c/busses/i2c-bcm-iproc.c b/drivers/i2c/busses/i2c-bcm-iproc.c
>>> index 562942d0c05c..a845b8decac8 100644
>>> --- a/drivers/i2c/busses/i2c-bcm-iproc.c
>>> +++ b/drivers/i2c/busses/i2c-bcm-iproc.c
>>> @@ -717,7 +717,7 @@ static int bcm_iproc_i2c_xfer_single_msg(struct bcm_iproc_i2c_dev *iproc_i2c,
>>>  
>>>  			/* mark the last byte */
>>>  			if (i == msg->len - 1)
>>> -				val |= 1 << M_TX_WR_STATUS_SHIFT;
>>> +				val |= BIT(M_TX_WR_STATUS_SHIFT);
>>>  
>>>  			iproc_i2c_wr_reg(iproc_i2c, M_TX_OFFSET, val);
>>>  		}
>>> @@ -844,7 +844,7 @@ static int bcm_iproc_i2c_cfg_speed(struct bcm_iproc_i2c_dev *iproc_i2c)
>>>  
>>>  	iproc_i2c->bus_speed = bus_speed;
>>>  	val = iproc_i2c_rd_reg(iproc_i2c, TIM_CFG_OFFSET);
>>> -	val &= ~(1 << TIM_CFG_MODE_400_SHIFT);
>>> +	val &= ~BIT(TIM_CFG_MODE_400_SHIFT);
>>>  	val |= (bus_speed == 400000) << TIM_CFG_MODE_400_SHIFT;
> 
> These two statements now no longer "match". One uses BIT and the other open
> codes the shift. I think that's bad. Losing the _SHIFT suffix and including
> BIT in the macro expansion, as suggested above, yields:
> 
> 	val &= ~TIM_CFG_MODE_400;
> 	if (bus_speed == 400000)
> 		val |= TIM_CFG_MODE_400;
> 
> which is perhaps one more line, but also more readable IMO.
> 

A single line with evaluation embedded is nice and clean to me. I guess
this is subjective.

I'll leave the decision to Wolfram. If he also prefers the above change
to be made, sure. Otherwise, I'll leave it as it is.

> But all this is of course in deep nit-pick-territory...
> 
> Cheers,
> Peter
> 

Thanks,

Ray