linux-kernel - Re: [alsa-devel] [PATCH 1/3] soundwire: remove bitfield for unique

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Date:   Thu, 24 Oct 2019 07:42:13 -0500
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Vinod Koul <vkoul@...nel.org>
Cc:     alsa-devel@...a-project.org, tiwai@...e.de,
        gregkh@...uxfoundation.org, linux-kernel@...r.kernel.org,
        Ranjani Sridharan <ranjani.sridharan@...ux.intel.com>,
        broonie@...nel.org, srinivas.kandagatla@...aro.org,
        jank@...ence.com, slawomir.blauciak@...el.com,
        Sanyog Kale <sanyog.r.kale@...el.com>,
        Bard liao <yung-chuan.liao@...ux.intel.com>,
        Rander Wang <rander.wang@...ux.intel.com>
Subject: Re: [alsa-devel] [PATCH 1/3] soundwire: remove bitfield for
 unique_id, use u8

On 10/24/19 6:29 AM, Vinod Koul wrote:
> On 22-10-19, 18:48, Pierre-Louis Bossart wrote:
>> There is no good reason why the unique_id needs to be stored as 4
>> bits. The code will work without changes with a u8 since all values
> 
> Well this was due to the fact the slave id defined by MIPI has unique id
> as 4 bits. In fact if you look closely there are other fields in
> sdw_slave_id doing this

it's not because we extract 4 bits that we need to store the information 
in 4 bits.

> 
>> are already filtered while parsing the ACPI tables and Slave devID
>> registers.
>>
>> Use u8 representation. This will allow us to encode a
>> "IGNORE_UNIQUE_ID" value to account for firmware/BIOS creativity.
> 
> Why are we shoving firmware/BIOS issues into the core?

The core uses a matching formula which is too strict and does not work 
on multiple platforms.

You can argue that the BIOS should be fixed, but the counter argument is 
that the practice of ignoring the unique ID is allowed by the MIPI standard.

> 
>>
>> Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
>> ---
>>   include/linux/soundwire/sdw.h | 4 +++-
>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/soundwire/sdw.h b/include/linux/soundwire/sdw.h
>> index 688b40e65c89..28745b9ba279 100644
>> --- a/include/linux/soundwire/sdw.h
>> +++ b/include/linux/soundwire/sdw.h
>> @@ -403,6 +403,8 @@ int sdw_slave_read_prop(struct sdw_slave *slave);
>>    * SDW Slave Structures and APIs
>>    */
>>   
>> +#define SDW_IGNORED_UNIQUE_ID 0xFF
>> +
>>   /**
>>    * struct sdw_slave_id - Slave ID
>>    * @mfg_id: MIPI Manufacturer ID
>> @@ -418,7 +420,7 @@ struct sdw_slave_id {
>>   	__u16 mfg_id;
>>   	__u16 part_id;
>>   	__u8 class_id;
>> -	__u8 unique_id:4;
>> +	__u8 unique_id;
>>   	__u8 sdw_version:4;
>>   };
>>   
>> -- 
>> 2.20.1
>