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

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Message-ID: <c5fe182d-1769-73ed-0268-5353fd030521@linux.intel.com>
Date:   Wed, 3 Mar 2021 10:35:02 -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



On 3/3/21 3:38 AM, Srinivas Kandagatla wrote:
> 
> 
> On 02/03/2021 14:34, Pierre-Louis Bossart wrote:
>>
>>
>>>>> +        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.