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

[Pierre-Louis Bossart]

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.