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

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Message-ID: <CABb+yY1u9mspX1Zu0mg21JMYxpKxsXsvBa9LnF6VNr1_42UJUQ@mail.gmail.com>
Date:   Sat, 6 May 2017 10:18:09 +0530
From:   Jassi Brar <jassisinghbrar@...il.com>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Jeffrey Hugo <jhugo@...eaurora.org>,
        Andy Gross <andy.gross@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Ohad Ben-Cohen <ohad@...ery.com>,
        linux-arm-msm@...r.kernel.org, linux-soc@...r.kernel.org,
        Devicetree List <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-remoteproc@...r.kernel.org
Subject: Re: [PATCH v4 3/5] soc: qcom: Introduce APCS IPC driver

On Sat, May 6, 2017 at 6:49 AM, Bjorn Andersson
<bjorn.andersson@...aro.org> wrote:
> On Fri 05 May 13:22 PDT 2017, Jassi Brar wrote:

>> How is it supposed to work if a client queues more than one request?
>
> One such example is found in patch 5 in this series. There are two FIFOs
> in shared memory, one in each direction. Each fifo has a index-pair
> associated; a write-index is used by the writer to inform the reader
> where the valid data in the ring buffer ends and a read-index indicates
> to the writer how far behind the read is.
>
> The writer will just push data into the FIFO, each time firing off an
> APCS IPC interrupt and when the remote interrupt handler runs it will
> consume all the messages from the read-index to the write-index. All
> without the need for the reader to signal the writer that it has
> received the interrupts.
>
> In the event that the write-index catches up with the read-index a
> dedicated flag is set which will cause the reader to signal that the
> read-index is updated - allowing the writer to sleep waiting for room in
> the FIFO.
>
Interesting.Just for my enlightenment...

  Where does the writer sleep in the driver? I see it simply sets the
bit and leave.
Such a flag (or head and tail pointers matching) should be checked in
last_tx_done()

If you think RPM will _always_ be ready to accept new messages (though
we have seen that doesn't hold in some situations), then you don't
need last_tx_done. The client should call mbox_client_txdone() after
mbox_send_message().

thnx