Re: [PATCH v4 3/5] soc: qcom: Introduce APCS IPC driver

[Jeffrey Hugo]

On 5/5/2017 1:22 PM, Jassi Brar wrote:
> On Sat, May 6, 2017 at 12:07 AM, Bjorn Andersson
> <bjorn.andersson@xxxxxxxxxx> wrote:
> > On Fri 05 May 03:26 PDT 2017, Jassi Brar wrote:
> >
> > > On Fri, May 5, 2017 at 1:35 AM, Bjorn Andersson
> > > <bjorn.andersson@xxxxxxxxxx> wrote:
> > >
> > > > +
> > > > +static int qcom_apcs_ipc_send_data(struct mbox_chan *chan, void *data)
> > > > +{
> > > > +       struct qcom_apcs_ipc *apcs = container_of(chan->mbox,
> > > > +                                                 struct qcom_apcs_ipc, mbox);
> > > > +       unsigned long idx = (unsigned long)chan->con_priv;
> > > > +
> > > > +       writel(BIT(idx), apcs->base + apcs->offset);
> > > > +
> > > When/how does this bit get ever cleared again?
> > > You may want to add last_tx_done() callback to check if this bit is
> > > cleared before you can send the next interrupt. And set
> > > txdone_poll/irq accordingly.
> >
> > It's a write-only register, writing a bit fires off an edge triggered
> > interrupt on the specific remote processor, which will ack the
> > associated IRQ status and handle the interrupt.
> >
> > As the "message" is just a notification to the other side that it needs
> > to act on "something", there's no harm in notifying it multiple times
> > before it has a chance to ack the IRQ and a write after that will be
> > seen as a separate interrupt.
> What causes it to return to '0'?

Technically nothing.  This is not a traditional register.  Its an 
address that is modeled as a register that the hardware knows, when 1 is 
written to the specific bit, the hardware is to take a specific action 
(generate the interrupt).

As Bjorn stated, its a write only address.  The hardware documentation 
states not to read it.  On most platforms, if you attempt to read the 
address, you will always get 0, although that is not guaranteed, and 
there have been platforms where attempting to read the address will 
result in a bus hang.

Conceptually, the hardware automatically resets it to "0" in the next 
clock cycle.  Software doesn't need to care.

> I think the driver should wait for it to become 0 before writing 1.
> For example, the protocol has a command that says to remote cpu to
> increase the voltage supply by 0.1v. This command is filled in a
> structure and laid out in the shared memory before you ring the
> 'doorbell'.  In this situation you don't want the remote cpu to act
> twice on the same command. Also for a new command, you don't want to
> overwrite the last command packet before remote cpu has consumed it.

That doesn't apply here.  The doorbell in this case is a signal for the 
remote processor to go look at the FIFO in shared memory.  Since 
interrupts can be lost (ie the local processor sends multiple interrupts 
before the remote processor can service them, so the GIC drops them 
since the interrupt is already pending), the remote processor is 
required to drain the FIFO when it processes the interrupt.

There is no way to determine if the remote processor has observed a 
message, that does not involve pretty trivial race conditions.

I invented this protocol.  From what I've seen (granted I did not do a 
thorough review), Bjorn is doing the correct thing here.

--
Jeffrey Hugo
Qualcomm Datacenter Technologies as an affiliate of Qualcomm 
Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.