Re: [PATCH 5/6] soundwire: qcom: update register read/write routine

[Pierre-Louis Bossart]

On 2/1/21 9:50 AM, Srinivas Kandagatla wrote:
> On 29/01/2021 19:33, Pierre-Louis Bossart wrote:
> > On 1/29/21 11:32 AM, Srinivas Kandagatla wrote:
> > > In the existing code every soundwire register read and register write
> > > are kinda blocked. Each of these are using a special command id that
> >
> > what does 'kinda blocked' mean?
>
> I meant read/writes are waiting for completion interrupt!
>
> > > generates interrupt after it successfully finishes. This is really
> > > overhead, limiting and not really necessary unless we are doing
> > > something special.
> > >
> > > We can simply read/write the fifo that should also give exactly
> > > what we need! This will also allow to read/write registers in
> > > interrupt context, which was not possible with the special
> > > command approach.
> >
> > This is really unclear, sorry.
>
> If read/writes are waiting for an interrupt, it becomes difficult to 
> read or write to any registers from same interrupt handler!

Well, yes, you need to handle the complete() at a lower level than the 
code that initiates the transactions otherwise you self-deadlock.

IIRC in the Intel initial code, the complete was in the handler and the 
register IOs in the thread.

> > > +    if (id != SWR_BROADCAST_CMD_ID) {
> > > +        if (id < 14)
> > > +            id += 1;
> > > +        else
> > > +            id = 0;
> >
> > that is really odd. if id=13 (group2) then id becomes 14 (master 
> > address). A comment is really needed here.
>
> This is magic value for each fifo read or write, so that we can verify 
> that them by comparing with this magic value!
>
> This has nothing to do with device number!

You should probably add a comment here then, or use a #define instead of 
the 14 which threw me off.

> > > +    if (cmd_id == SWR_BROADCAST_CMD_ID) {
> > > +        /*
> > > +         * sleep for 10ms for MSM soundwire variant to allow broadcast
> > > +         * command to complete.
> >
> > that's also super-odd. There is nothing in SoundWire that makes any 
> > difference between a regular and a broadcast command. they all 
> > complete in the same time (a frame).
> > > +         */
> > > +        ret = wait_for_completion_timeout(&swrm->broadcast, (2 * 
> > > HZ/10));
> >
> > is this 10ms really or dependent on CONFIG_HZ?

comment missed?

> > > +        if (!ret)
> > > +            ret = SDW_CMD_IGNORED;
> > > +        else
> > > +            ret = SDW_CMD_OK;
> >
> > no CMD_FAILED support?
>
> Qcom controllers does not provide that information if the command is 
> ignored or failed by any means!
>
> That was the behavior from the starting of this driver.

ah yes, now I remember this.