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

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Message-ID: <5b067c8d-0d90-1531-a1b1-118fa73c0078@linaro.org>
Date:   Fri, 5 Mar 2021 10:39:50 +0000
From:   Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        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 03/03/2021 16:35, Pierre-Louis Bossart wrote:
> 
> 
> 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.

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!

--srini


--srini
> 
>