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From:   Baolin Wang <baolin.wang@linaro.org>
Date:   Wed, 8 Aug 2018 11:19:07 +0800
Message-ID: <CAMz4kuJrzaZn9560tF73a-WPNhhRj5S2ezTKr0HHpcGRXMw-Ew@mail.gmail.com>
Subject: Re: [PATCH 2/2] spi: sprd: Add SPI driver for Spreadtrum SC9860
To:     Trent Piepho <tpiepho@impinj.com>
Cc:     "mark.rutland@arm.com" <mark.rutland@arm.com>,
        "broonie@kernel.org" <broonie@kernel.org>,
        "robh+dt@kernel.org" <robh+dt@kernel.org>,
        "linux-spi@vger.kernel.org" <linux-spi@vger.kernel.org>,
        "orsonzhai@gmail.com" <orsonzhai@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "lanqing.liu@spreadtrum.com" <lanqing.liu@spreadtrum.com>,
        "zhang.lyra@gmail.com" <zhang.lyra@gmail.com>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>
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Hi Trent,

On 8 August 2018 at 01:10, Trent Piepho <tpiepho@impinj.com> wrote:
> On Tue, 2018-08-07 at 18:43 +0800, Baolin Wang wrote:
>>
>> +static u32 sprd_spi_transfer_max_timeout(struct sprd_spi *ss,
>> +                                      struct spi_transfer *t)
>> +{
>> +     /*
>> +      * The time spent on transmission of the full FIFO data is the maximum
>> +      * SPI transmission time.
>> +      */
>> +     u32 size = t->bits_per_word * SPRD_SPI_FIFO_SIZE;
>> +     u32 bit_time_us = SPRD_SPI_HZ / t->speed_hz + 1;
>> +     u32 total_time_us = size * bit_time_us;
>> +     /*
>> +      * There is an interval between data and the data in our SPI hardware,
>> +      * so the total transmission time need add the interval time.
>> +      *
>> +      * The inteval calculation formula:
>> +      * interval time (source clock cycles) = interval * 4 + 10.
>> +      */
>> +     u32 interval_cycle = SPRD_SPI_FIFO_SIZE * ((ss->interval << 2) + 10);
>> +     u32 interval_time_us = interval_cycle * SPRD_SPI_HZ / ss->src_clk + 1;
>
> If interval is greater than 31, this will overflow.

Usually we will not set such a big value for interval,  but this is a
risk like you said. So we will check and limit the interval value to
make sure the formula will not overflow.

>
> Also SPRD_SPI_HZ is not the speed anything runs at, as one might think
> from the name.  It's the number of microseconds in a second.  The is
> already a Linux macro for that, USEC_PER_SEC, that you should use
> instead.

Right, will use USEC_PER_SEC instead.

>
>> +
>> +     return total_time_us + interval_time_us;
>> +}
>> +
>
>
>> +static void sprd_spi_set_speed(struct sprd_spi *ss, u32 speed_hz)
>> +{
>> +     /*
>> +      * From SPI datasheet, the prescale calculation formula:
>> +      * prescale = SPI source clock / (2 * SPI_freq) - 1;
>> +      */
>> +     u32 clk_div = ss->src_clk / (speed_hz << 1) - 1;
>
> You should probably round up here.  The convention is to use the
> closest speed less than equal to the requested speed, but since this is
> a divider, rounding it down will select the next highest speed.

Per the datasheet, we do not need round up/down the speed. Since our
hardware can handle the clock calculated by the above formula if the
requested speed is in the normal region (less than ->max_speed_hz).

>> +
>> +     writel_relaxed(clk_div, ss->base + SPRD_SPI_CLKD);
>> +}
>> +
>
>> +
>> +static int sprd_spi_probe(struct platform_device *pdev)
>> +{
>> +     struct device_node *np = pdev->dev.of_node;
>> +     struct spi_controller *sctlr;
>> +     struct resource *res;
>> +     struct sprd_spi *ss;
>> +     int ret;
>> +
>> +     pdev->id = of_alias_get_id(pdev->dev.of_node, "spi");
>> +     sctlr = spi_alloc_master(&pdev->dev, sizeof(*ss));
>> +     if (!sctlr)
>> +             return -ENOMEM;
>> +
>> +     ss = spi_controller_get_devdata(sctlr);
>> +     if (of_property_read_u32(np, "sprd,spi-interval", &ss->interval))
>> +             ss->interval = SPRD_SPI_ITVL_NUM;
>> +
>> +     res = platform_get_resource(pdev, IORESOURCE_MEM, 0);
>> +     ss->base = devm_ioremap_resource(&pdev->dev, res);
>> +     if (IS_ERR(ss->base)) {
>> +             ret = PTR_ERR(ss->base);
>> +             goto free_controller;
>> +     }
>> +
>> +     ss->dev = &pdev->dev;
>> +     sctlr->dev.of_node = pdev->dev.of_node;
>> +     sctlr->mode_bits = SPI_CPOL | SPI_CPHA | SPI_3WIRE | SPI_TX_DUAL;
>> +     sctlr->bus_num = pdev->id;
>> +     sctlr->setup = sprd_spi_setup;
>> +     sctlr->set_cs = sprd_spi_chipselect;
>> +     sctlr->transfer_one = sprd_spi_transfer_one;
>> +     sctlr->max_speed_hz = (ss->src_clk / 2) < SPRD_SPI_MAX_SPEED_HZ ?
>> +             ss->src_clk / 2 : SPRD_SPI_MAX_SPEED_HZ;
>
> You can write this as:
>         sctlr->max_speed_hz = min(ss->src_clk / 2, SPRD_SPI_MAX_SPEED_HZ);

Right. Thanks for your comments.

-- 
Baolin Wang
Best Regards