RE: I3C Mastership RFC

[Vitor Soares <Vitor.Soares@synopsys.com>]

From: Boris Brezillon <boris.brezillon@collabora.com>
Date: Mon, Nov 25, 2019 at 13:09:36

> On Mon, 25 Nov 2019 13:00:17 +0000
> Vitor Soares <Vitor.Soares@synopsys.com> wrote:
> 
> > > > > > > I'm pretty sure we solved that already (that's what
> > > > > > > i3c_master_acquire_bus_ownership() calls are supposed to take care of).
> > > > > > > Can you be a bit more specific? What makes you think the master might
> > > > > > > not be in control of the bus when i3c_dev_do_priv_xfers_locked() is
> > > > > > > called?    
> > > > > > 
> > > > > > You are assuming that after i3c_master_acquire_bus_ownership() return, 
> > > > > > secondary master already owns the bus. Main master can ack the MR request 
> > > > > > and not send the CETACCMST immediately.
> > > > > >    
> > > > > 
> > > > > In Cadence HC driver, I'm waiting for GETACCMST longer, polling the
> > > > > status and after I exit from ->request_mastership(), I'm the bus owner.
> > > > > If not, error exit code is returned and we can't make the transfers.
> > > > > Are you able to implement the same behavior?    
> > > > 
> > > > You can assume everyone will do in that way. What happen if you receive a 
> > > > request or an information from current master?  
> > > 
> > > We have this ->request_mastership() method so controllers that have
> > > this logic (MR+wait(GETACCMST) automated can still interface with the
> > > subsystem.   
> > 
> > I can also poll the GETACCMST but we are assuming nothing will happen 
> > between MR and GETACCMST.
> 
> Nothing coming from the master that tries to acquire the bus, yes.
> Nothing coming from the current master, no, and that shouldn't be a
> problem as long as those operations don't involve acquiring bus->lock.
> And if some of those operation involve acquiring the lock (I'd still
> need to understand which operation that would be) they'll just be
> delayed/rejected.

You are assuming this is straight forward which is not the case. Between 
MR and GETACCMST may happen everything as in a Master-Slave topology.
For me, poll the controller to check when GETACCMST arrive and lock 
everything is not a solution.

> 
> > 
> > > If your controller handles the MR/GETACCMST separately, it
> > > shouldn't be hard to implement, and we can even provide an helper if
> > > people end up duplicating the code.  
> > 
> > I already implement a callback in my code so each controller be able to 
> > do their stuff in that in request/deliver mastership.
> 
> Can you share some details here? What would those callbacks supposed to
> do and when would they be called?

My understanding is that different controllers may have different ways to 
deliver the bus ownership hence I implemented that.

Best regards,
Vitor Soares

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