Re: [PATCH v5 2/8] firmware: add support for ARM System Control and Power Interface(SCPI) protocol

[Sudeep Holla]

On 31/07/15 14:45, Jassi Brar wrote:
> On Fri, Jul 31, 2015 at 6:38 PM, Sudeep Holla <sudeep.holla@xxxxxxx> wrote:
> > On 31/07/15 11:43, Sudeep Holla wrote:
> > > On 31/07/15 11:38, Jassi Brar wrote:
> > > > On Fri, Jul 31, 2015 at 3:10 PM, Sudeep Holla <sudeep.holla@xxxxxxx>
> > > > wrote:
> > > > > I forgot to mention, we have a the following description in
> > > > > mbox_client_txdone which is misleading:
> > > > >
> > > > > "The client/protocol had received some 'ACK' packet and it notifies the
> > > > > API that the last packet was sent successfully. This only works if the
> > > > > controller can't sense TX-Done."
> > > > >
> > > > > which is clearly not the case in SCPI. IMO we may have to reword that.
> > > > Yes. And also see whether it could race with polling driven tx_tick.
> > >
> > > Yes I am also looking at that now while I am trying to check if
> > > TXDONE_BY_ACK works on Juno, will keep you posted.
> >
> > OK, I recollect the racy condition now which I had in my mind from the
> > beginning convincing myself why we can't use that option. I was not good
> > in words to explain it so far but let me try with the ASCII art this
> > time. Note Tx ACK below means the remote setting the register flag and
> > not to be confused with the ACK packet. For simplicity Rx can be assumed
> > to be Tx ACK packet
> >
> >       Time       MHU/SCPI                        Remote SCP
> >                     |                                |
> >          1          |------------ Tx1 -------------->|
> >                     |                                |
> >          2          |<----------- Tx1 ACK -----------|
> >                     |                                |
> >          3          |------------ Tx2 -------------->|
> >                     |                                |
> >          4          |<----------- Rx1 ---------------|
> >                     |                                |
> >          5          |<----------- Tx2 ACK -----------|
> >                     |                                |
> >          6          |------------ Tx3 -------------->|
> >                     |                                |
> >          7          |<----------- Rx2 ---------------|
> >
> > Now lets consider the above scenario, suppose we have TXDONE_BY_ACK
> > and use mbox_client_txdone in Rx interrupt(i.e. response packet), we end
> > up in the race easily IIUC.
> >
> > E.g. A client would have sent Tx2(3) before Rx1 interrupt(4) and if we
> > ACK Tx1 now in (3), we will end up acknowledging Tx2(5) as the mailbox
> > core assumes only one Tx request at a time which is clearly not the case
> > in our setup. The client can then go ahead and send Tx3(6) overwriting
> > the payload while remote was processing or even result in remote missing
> > the request completely. Does that make sense or am I missing something ?
> Yeah, that could happen. But the race can be fixed by checking
> last_tx_done if the controller provides that. If last_tx_done is not
> implemented, polling won't race.
>
> Please try the following...

Looks good to me. Sometimes it takes very long time(days) to hit race
conditions(esp. in firmware), so I need some time to think before
I cook up a patch to start stress test this on Juno so that I don't
waste time waiting for result.

Regards,
Sudeep
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