linux-kernel - Re: [PATCH v8 2/4] soc: qcom: Add AOSS QMP driver

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Date:   Fri, 31 May 2019 17:31:14 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     Andy Gross <agross@...nel.org>,
        David Brown <david.brown@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Arun Kumar Neelakantam <aneela@...eaurora.org>,
        Vinod Koul <vkoul@...nel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        devicetree@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 2/4] soc: qcom: Add AOSS QMP driver

Hi,

On Fri, May 31, 2019 at 5:09 PM Bjorn Andersson
<bjorn.andersson@...aro.org> wrote:
>
> On Fri 31 May 15:24 PDT 2019, Doug Anderson wrote:
>
> > Hi,
> >
> > On Thu, May 30, 2019 at 8:01 PM Bjorn Andersson
> > <bjorn.andersson@...aro.org> wrote:
> > >
> > > +/**
> > > + * qmp_send() - send a message to the AOSS
> > > + * @qmp: qmp context
> > > + * @data: message to be sent
> > > + * @len: length of the message
> > > + *
> > > + * Transmit @data to AOSS and wait for the AOSS to acknowledge the message.
> > > + * @len must be a multiple of 4 and not longer than the mailbox size. Access is
> > > + * synchronized by this implementation.
> > > + *
> > > + * Return: 0 on success, negative errno on failure
> > > + */
> > > +static int qmp_send(struct qmp *qmp, const void *data, size_t len)
> > > +{
> > > +       int ret;
> > > +
> > > +       if (WARN_ON(len + sizeof(u32) > qmp->size))
> > > +               return -EINVAL;
> > > +
> > > +       if (WARN_ON(len % sizeof(u32)))
> > > +               return -EINVAL;
> > > +
> > > +       mutex_lock(&qmp->tx_lock);
> > > +
> > > +       /* The message RAM only implements 32-bit accesses */
> > > +       __iowrite32_copy(qmp->msgram + qmp->offset + sizeof(u32),
> > > +                        data, len / sizeof(u32));
> > > +       writel(len, qmp->msgram + qmp->offset);
> > > +       qmp_kick(qmp);
> > > +
> > > +       ret = wait_event_interruptible_timeout(qmp->event,
> > > +                                              qmp_message_empty(qmp), HZ);
> > > +       if (!ret) {
> > > +               dev_err(qmp->dev, "ucore did not ack channel\n");
> > > +               ret = -ETIMEDOUT;
> > > +
> > > +               /* Clear message from buffer */
> > > +               writel(0, qmp->msgram + qmp->offset);
> > > +       } else {
> > > +               ret = 0;
> > > +       }
> >
> > Just like Vinod said in in v7, the "ret = 0" is redundant.
> >
>
> If the condition passed to wait_event_interruptible_timeout() evaluates
> true the remote side has consumed the message and ret will be 1. We end
> up in the else block (i.e. not timeout) and we want the function to
> return 0, so we set ret to 0.
>
> Please let me know if I'm reading this wrong.

Ah, it's me that's confused.  I missed the "!" on ret.  Maybe it'd be
less confusing if you did:

time_left = wait_event_interruptible_timeout(...)
if (!time_left)

Even though you _can_ use "ret", it's less confusing to use a
different variable since (often) ret is an error code.

Speaking of which: do you actually handle the case where you get an
interrupt?  Should the above just be wait_event_timeout()?