linux-kernel - Re: [PATCH 2/3] soundwire: qcom: add auto enumeration support

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Date:   Fri, 5 Mar 2021 10:19:02 -0600
From:   Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        vkoul@...nel.org
Cc:     sanyog.r.kale@...el.com, yung-chuan.liao@...ux.intel.com,
        linux-kernel@...r.kernel.org, alsa-devel@...a-project.org
Subject: Re: [PATCH 2/3] soundwire: qcom: add auto enumeration support




>>>>>>> +        if (!val1 && !val2)
>>>>>>> +            break;
>>>>>>> +
>>>>>>> +        addr = buf2[1] | (buf2[0] << 8) | (buf1[3] << 16) |
>>>>>>> +            ((u64)buf1[2] << 24) | ((u64)buf1[1] << 32) |
>>>>>>> +            ((u64)buf1[0] << 40);
>>>>>>> +
>>>>>>> +        sdw_extract_slave_id(bus, addr, &id);
>>>>>>> +        /* Now compare with entries */
>>>>>>> +        list_for_each_entry_safe(slave, _s, &bus->slaves, node) {
>>>>>>> +            if (sdw_compare_devid(slave, id) == 0) {
>>>>>>> +                u32 status = qcom_swrm_get_n_device_status(ctrl, 
>>>>>>> i);
>>>>>>> +                if (status == SDW_SLAVE_ATTACHED) {
>>>>>>> +                    slave->dev_num = i;
>>>>>>> +                    mutex_lock(&bus->bus_lock);
>>>>>>> +                    set_bit(i, bus->assigned);
>>>>>>> +                    mutex_unlock(&bus->bus_lock);
>>>>>>> +
>>>>>>> +                }
>>>>>>
>>>>>> And that part is strange as well. The bus->assigned bit should be 
>>>>>> set even if the Slave is not in the list provided by platform 
>>>>>> firmware. It's really tracking the state of the hardware, and it 
>>>>>> should not be influenced by what software knows to manage.
>>>>>
>>>>> Am not 100% sure If I understand the concern here, but In normal 
>>>>> (non auto enum) cases this bit is set by the bus code while its 
>>>>> doing enumeration to assign a dev number from the assigned bitmap!
>>>>>
>>>>> However in this case where auto enumeration happens it makes sense 
>>>>> to set this here with matching dev number!
>>>>>
>>>>> AFAIU from code, each bit in this bitmap corresponds to slave dev 
>>>>> number!
>>>>
>>>> Yes, but the point was "why do you compare with information coming 
>>>> from platform firmware"? if the hardware reports the presence of 
>>>> devices on 
>>>
>>> This is the logic that hardware IP document suggests to use to get 
>>> get the correct the device number associated with the slave!
>>>
>>>
>>>> the link, why not use the information as is?
>>>>
>>>> You recently added code that helps us deal with devices that are not 
>>>> listed in DT or ACPI tables, so why would we filter in this specific 
>>>> loop?
>>
>> I don't think my point was understood, so let me try to explain it 
>> differently.
>>
>> it's my understanding that the hardware reads the DevID registers and 
>> writes a Device Number - so that the entire enumeration sequence 
>> started by reading DevID0 and finished by a successful write to 
>> SCP_DevNum is handled in hardware.
>>
>> The question is: what happens if that device is NOT described in the 
>> Device Tree data? The loop over bus->slaves will not find this device 
>> by comparing with known devID values, so the set_bit(i, bus->assigned) 
>> will not happen.
> 
> yes, that is true, There is no way we can assign a dev_number to the 
> device which is not enumerated on the bus!
> 
> Am sure this is the same behavior with soundwire core too, atleast form 
> the code I can see it sets the assigned bit for only the devices that 
> are enumerated on the bus! Not all the devices specified in DT!
> Unless I missed something!

I am talking about the other way around, where a device is present and 
enumerated on the bus but not listed in DT. In that case the hardware 
did assign a device number but bus->assigned will not be set.