linux-kernel - Re: [PATCH v2 2/3] soundwire: SDCA: add helper macro to access controls

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Message-ID: <d717f109-7ffd-11cb-f44d-f6c0b48d4985@linux.intel.com>
Date:   Wed, 16 Sep 2020 08:11:34 -0500
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Vinod Koul <vkoul@...nel.org>
Cc:     Guennadi Liakhovetski <guennadi.liakhovetski@...ux.intel.com>,
        alsa-devel@...a-project.org,
        Kai Vehmanen <kai.vehmanen@...ux.intel.com>, tiwai@...e.de,
        gregkh@...uxfoundation.org,
        open list <linux-kernel@...r.kernel.org>, broonie@...nel.org,
        Sanyog Kale <sanyog.r.kale@...el.com>,
        Bard liao <yung-chuan.liao@...ux.intel.com>,
        Rander Wang <rander.wang@...ux.intel.com>
Subject: Re: [PATCH v2 2/3] soundwire: SDCA: add helper macro to access
 controls



On 9/16/20 7:35 AM, Vinod Koul wrote:
> On 14-09-20, 09:44, Pierre-Louis Bossart wrote:
>>> For LSB bits, I dont think this is an issue. I expect it to work, for example:
>>> #define CONTROL_LSB_MASK  GENMASK(2, 0)
>>>           foo |= u32_encode_bits(control, CONTROL_LSB_MASK);
>>>
>>> would mask the control value and program that in specific bitfeild.
>>>
>>> But for MSB bits, I am not sure above will work so, you may need to extract
>>> the bits and then use, for example:
>>> #define CONTROL_MSB_BITS        GENMASK(5, 3)
>>> #define CONTROL_MSB_MASK        GENMASK(17, 15)
>>>
>>>           control = FIELD_GET(CONTROL_MSB_BITS, control);
>>>           foo |= u32_encode_bits(control, CONTROL_MSB_MASK);
>>>
>>>> If you have a better suggestion that the FIELD_PREP/FIELD_GET use, I am all
>>>> ears. At the end of the day, the mapping is pre-defined and we don't have
>>>> any degree of freedom. What I do want is that this macro/inline function is
>>>> shared by all codec drivers so that we don't have different interpretations
>>>> of how the address is constructed.
>>>
>>> Absolutely, this need to be defined here and used by everyone else.
>>
>> Compare:
>>
>> #define SDCA_CONTROL_MSB_BITS        GENMASK(5, 3)
>> #define SDCA_CONTROL_MSB_MASK        GENMASK(17, 15)
>> #define SDCA_CONTROL_LSB_MASK        GENMASK(2, 0)
>>
>> foo |= u32_encode_bits(control, SDCA_CONTROL_LSB_MASK);
>> control = FIELD_GET(SDCA_CONTROL_MSB_BITS, control);
>> foo |= u32_encode_bits(control, SDCA_CONTROL_MSB_MASK);
>>
>> with the original proposal:
>>
>> foo |= FIELD_GET(GENMASK(2, 0), control))	
>> foo |= FIELD_PREP(GENMASK(17, 15), FIELD_GET(GENMASK(5, 3), control))	
>>
>> it gets worse when the LSB positions don't match, you need another variable
>> and an additional mask.
>>
>> I don't see how this improves readability? I get that hard-coding magic
>> numbers is a bad thing in general, but in this case there are limited
>> benefits to the use of additional defines.
> 
> I think it would be prudent to define the masks and use them rather than
> magic values. Also it makes it future proof

I don't see your point at all. The values cannot be modified, a 
different macro would be needed for a standard change.

Anyways, I am not going to argue further, I'll use your code example as 
is and move on.