linux-kernel - Re: [PATCH v5 2/8] i2c: iproc: Add slave mode support

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Date:   Tue, 2 Apr 2019 13:54:56 +0530
From:   Rayagonda Kokatanur <rayagonda.kokatanur@...adcom.com>
To:     Ray Jui <ray.jui@...adcom.com>
Cc:     Wolfram Sang <wsa@...-dreams.de>, Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>, linux-i2c@...r.kernel.org,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org,
        bcm-kernel-feedback-list@...adcom.com,
        Shreesha Rajashekar <shreesha.rajashekar@...adcom.com>,
        Michael Cheng <ccheng@...adcom.com>
Subject: Re: [PATCH v5 2/8] i2c: iproc: Add slave mode support

Hi Ray/Wolfram,

On Tue, Apr 2, 2019 at 3:03 AM Ray Jui <ray.jui@...adcom.com> wrote:
>
> Hi Wolfram/Rayagonda,
>
> On 3/27/2019 3:14 PM, Wolfram Sang wrote:
> >
> >> +static void bcm_iproc_i2c_slave_init(
> >> +    struct bcm_iproc_i2c_dev *iproc_i2c, bool need_reset)
> >> +{
> >> +    u32 val;
> >> +
> >> +    if (need_reset) {
> >> +            /* put controller in reset */
> >> +            val = readl(iproc_i2c->base + CFG_OFFSET);
> >> +            val |= BIT(CFG_RESET_SHIFT);
> >> +            writel(val, iproc_i2c->base + CFG_OFFSET);
> >> +
> >> +            /* wait 100 usec per spec */
> >> +            udelay(100);
> >> +
> >> +            /* bring controller out of reset */
> >> +            val &= ~(BIT(CFG_RESET_SHIFT));
> >> +            writel(val, iproc_i2c->base + CFG_OFFSET);
> >> +    }
> >> +
> >> +    /* flush TX/RX FIFOs */
> >> +    val = (BIT(S_FIFO_RX_FLUSH_SHIFT) | BIT(S_FIFO_TX_FLUSH_SHIFT));
> >> +    writel(val, iproc_i2c->base + S_FIFO_CTRL_OFFSET);
> >
> > Will flushing FIFOs work when a slave is register while a master
> > transfer is on-going at the same time?
> >
>
> Okay, as you pointed out in a subsequent email, this can't happen.
>
> >> +
> >> +    /* RANDOM SLAVE STRETCH time - 20ms*/
> >
> > What is a "random stretch time"? 20ms sounds like a lot. Also, missing
> > space before comment terminator.
> >
>
> Rayagonda,
>
> Could you please help to comment on the choice of the 20 ms to allow
> clock stretch from the slave? In probably all cases, the slave should
> not need more than 1 ms? 20 ms does seem way too long as Wolfram pointed
> out.

In fact we are programming max slave stretch time  ie 25ms, comment
should be correcting.
Its maximum time for slave to complete read/write operation, if slave
is done with read/write then clock will not be stretched further, it
will be released immediately.
Hence I feel no harm in programming max timeout value.
Also determining correct slave stretch is very subjective and depends
on slave type as well.

Best regards,
Rayagonda

>
> Will fix the missing space before comment terminator.
>
> >> @@ -224,22 +473,25 @@ static int bcm_iproc_i2c_init(struct bcm_iproc_i2c_dev *iproc_i2c)
> >>
> >>      /* put controller in reset */
> >>      val = readl(iproc_i2c->base + CFG_OFFSET);
> >> -    val |= 1 << CFG_RESET_SHIFT;
> >> -    val &= ~(1 << CFG_EN_SHIFT);
> >> +    val |= BIT(CFG_RESET_SHIFT);
> >> +    val &= ~(BIT(CFG_EN_SHIFT));
> >>      writel(val, iproc_i2c->base + CFG_OFFSET);
> >>
> >>      /* wait 100 usec per spec */
> >>      udelay(100);
> >>
> >>      /* bring controller out of reset */
> >> -    val &= ~(1 << CFG_RESET_SHIFT);
> >> +    val &= ~(BIT(CFG_RESET_SHIFT));
> >>      writel(val, iproc_i2c->base + CFG_OFFSET);
> >>
> >>      /* flush TX/RX FIFOs and set RX FIFO threshold to zero */
> >> -    val = (1 << M_FIFO_RX_FLUSH_SHIFT) | (1 << M_FIFO_TX_FLUSH_SHIFT);
> >> +    val = (BIT(M_FIFO_RX_FLUSH_SHIFT) | BIT(M_FIFO_TX_FLUSH_SHIFT));
> >>      writel(val, iproc_i2c->base + M_FIFO_CTRL_OFFSET);
> >>      /* disable all interrupts */
> >> -    writel(0, iproc_i2c->base + IE_OFFSET);
> >> +    val = readl(iproc_i2c->base + IE_OFFSET);
> >> +    val &= ~(IE_M_ALL_INTERRUPT_MASK <<
> >> +                    IE_M_ALL_INTERRUPT_SHIFT);
> >> +    writel(val, iproc_i2c->base + IE_OFFSET);
> >
> > This block looks unrelated, but I won't be too strict here...
> >
> >> +    case M_CMD_STATUS_FIFO_UNDERRUN:
> >> +            dev_dbg(iproc_i2c->device, "FIFO under-run\n");
> >> +            return -ENXIO;
> >> +
> >> +    case M_CMD_STATUS_RX_FIFO_FULL:
> >> +            dev_dbg(iproc_i2c->device, "Master Rx FIFO full > 10ms\n");
> >> +            return -ETIMEDOUT;
> >> +
> >
> > ... however, this looks really unrelated to me. This is about master
> > transmission, or?
>
> This should be submitted in a separate commit. Will do that in the next
> iteration of patch series.
>
> >
> > Rest looks OK.
> >
>
> Thanks,
>
> Ray