linux-kernel - Re: [PATCH -next v2] staging: comedi: Remove unused variable ‘min_full_scale’ and function 'get_min_full

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Message-ID: <25e0e925-cc90-3e88-a3bd-15b70cb7b4ec@mev.co.uk>
Date:   Fri, 14 May 2021 12:16:11 +0100
From:   Ian Abbott <abbotti@....co.uk>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Bixuan Cui <cuibixuan@...wei.com>
Cc:     linux-kernel@...r.kernel.org, hsweeten@...ionengravers.com,
        grandmaster@...klimov.de
Subject: Re: [PATCH -next v2] staging: comedi: Remove unused variable ‘min_full_scale’ and function 'get_min_full_scales'

On 14/05/2021 08:52, Greg KH wrote:
> On Fri, May 14, 2021 at 04:52:14PM +0800, Bixuan Cui wrote:
>> The variable ‘min_full_scale’ and function 'get_min_full_scales' are
>> not used, So delete them.
>>
>> Signed-off-by: Bixuan Cui <cuibixuan@...wei.com>
>> ---
>> Changes from v2:
>> * Delete function 'get_min_full_scales'
>>
>>   drivers/comedi/drivers/jr3_pci.c | 15 ---------------
>>   1 file changed, 15 deletions(-)
>>
>> diff --git a/drivers/comedi/drivers/jr3_pci.c b/drivers/comedi/drivers/jr3_pci.c
>> index 7a02c4fa3cda..f963080dd61f 100644
>> --- a/drivers/comedi/drivers/jr3_pci.c
>> +++ b/drivers/comedi/drivers/jr3_pci.c
>> @@ -186,19 +186,6 @@ static void set_full_scales(struct jr3_sensor __iomem *sensor,
>>   	set_s16(&sensor->command_word0, 0x0a00);
>>   }
>>   
>> -static struct six_axis_t get_min_full_scales(struct jr3_sensor __iomem *sensor)
>> -{
>> -	struct six_axis_t result;
>> -
>> -	result.fx = get_s16(&sensor->min_full_scale.fx);
>> -	result.fy = get_s16(&sensor->min_full_scale.fy);
>> -	result.fz = get_s16(&sensor->min_full_scale.fz);
>> -	result.mx = get_s16(&sensor->min_full_scale.mx);
>> -	result.my = get_s16(&sensor->min_full_scale.my);
>> -	result.mz = get_s16(&sensor->min_full_scale.mz);
>> -	return result;
> 
> Are you _SURE_ that this is ok to do?  You are reading from the
> hardware, and that is sometimes required depending on the
> internal-to-the-device state machine.
> 
> Have you tested to verify that not doing these reads does not break
> something?
> 
> This is a constant theme in these "remove variables that are not being
> used" patches, please be aware that how hardware works is very specific
> and when you mess with how it is accessed, that has the chance of
> causing problems.

I don't think removing this would do any harm.  The `get_s16` function 
is just a simple wrapper around `readl` that converts the type of the 
returned value to `s16`.  The registers for minimum full scale and 
maximum full scale are just informational, and there is nothing in the 
manual[1] about reading registers in a particular order.  The driver 
used to print these values to the kernel log, but that code was removed 
in commit a1d16659538a ("staging: comedi: jr3_pci: remove noisy printk").

[1] 5907E - Legacy PCI Receiver Manual
<https://www.jr3.com/component/phocadownload/category/6-product-manuals?download=36:5907e-legacy-pci-receiver-manual>

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