linux-kernel - Re: [PATCH v5] drm/sun4i: hdmi: Implement I2C adapter for A10s DDC bus

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Date:   Wed, 28 Jun 2017 20:39:33 +1000
From:   Jonathan Liu <net147@...il.com>
To:     Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:     David Airlie <airlied@...ux.ie>, Chen-Yu Tsai <wens@...e.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-sunxi <linux-sunxi@...glegroups.com>
Subject: Re: [PATCH v5] drm/sun4i: hdmi: Implement I2C adapter for A10s DDC bus

Hi Maxime,

On 28 June 2017 at 19:20, Maxime Ripard
<maxime.ripard@...e-electrons.com> wrote:
> On Wed, Jun 28, 2017 at 12:36:52AM +1000, Jonathan Liu wrote:
>> +#define SUN4I_HDMI_DDC_INT_STATUS_ERROR_MASK ( \
>> +     SUN4I_HDMI_DDC_INT_STATUS_ILLEGAL_FIFO_OPERATION | \
>> +     SUN4I_HDMI_DDC_INT_STATUS_DDC_RX_FIFO_UNDERFLOW | \
>> +     SUN4I_HDMI_DDC_INT_STATUS_DDC_TX_FIFO_OVERFLOW | \
>> +     SUN4I_HDMI_DDC_INT_STATUS_ARBITRATION_ERROR | \
>> +     SUN4I_HDMI_DDC_INT_STATUS_ACK_ERROR | \
>> +     SUN4I_HDMI_DDC_INT_STATUS_BUS_ERROR \
>> +)
>> +
>> +static bool is_err_status(u32 int_status)
>> +{
>> +     return !!(int_status & SUN4I_HDMI_DDC_INT_STATUS_ERROR_MASK);
>> +}
>> +
>> +static bool is_fifo_flag_unset(struct sun4i_hdmi *hdmi, u32 *fifo_status,
>> +                            u32 flag)
>> +{
>> +     *fifo_status = readl(hdmi->base + SUN4I_HDMI_DDC_FIFO_STATUS_REG);
>> +     return !(*fifo_status & flag);
>> +}
>> +
>> +static int fifo_transfer(struct sun4i_hdmi *hdmi, u8 *buf, int len, bool read)
>> +{
>> +     /* 1 byte takes 9 clock cycles (8 bits + 1 ACK) */
>> +     unsigned long byte_time = DIV_ROUND_UP(USEC_PER_SEC,
>> +                                            clk_get_rate(hdmi->ddc_clk)) * 9;
>
> There's no real need for it to be dynamic. The clock rate will not
> change, and the order of magnitude is roughly 100us, so let's just use
> that (and make a comment).
>

Ok.

>> +     u32 int_status;
>> +     u32 fifo_status;
>> +     /* Read needs empty flag unset, write needs full flag unset */
>> +     u32 flag = read ? SUN4I_HDMI_DDC_FIFO_STATUS_EMPTY :
>> +                       SUN4I_HDMI_DDC_FIFO_STATUS_FULL;
>> +     int ret;
>> +
>> +     /* Wait until error or FIFO ready */
>> +     ret = readl_poll_timeout(hdmi->base + SUN4I_HDMI_DDC_INT_STATUS_REG,
>> +                              int_status,
>> +                              is_err_status(int_status) ||
>> +                              is_fifo_flag_unset(hdmi, &fifo_status, flag),
>> +                              min(len, SUN4I_HDMI_DDC_FIFO_SIZE) * byte_time,
>> +                              100000);
>> +
>> +     if (is_err_status(int_status))
>> +             return -EIO;
>> +     if (ret)
>> +             return -ETIMEDOUT;
>
> Why not just have
> ret = readl_poll_timeout(hdmi->base + SUN4I_HDMI_DDC_FIFO_STATUS_REG, reg,
>                          !(reg & flag), 100, 100000);
>
> if (ret < 0)
>         if (is_err_status())
>                 return -EIO;
>         return ret;
>
>

If I check error status after readl_poll_timeout and there is an error
(e.g. the I2C address does not have a corresponding device connected
or nothing connected to HDMI port) it will keep checking the fifo
status even though error bit is set in the int status and then timeout
after 100 ms. If it checks the int status register at the same time,
it will error after 100 nanoseconds. I don't want to introduce
unnecessary delays considering part of the reason for adding this
driver to make it more usable for non-standard use cases.

>> +
>> +     /* Read FIFO level */
>> +     int level = (int)(fifo_status & SUN4I_HDMI_DDC_FIFO_STATUS_LEVEL_MASK);
>
> and explicitly read the fifo status here. That will make you remove
> that function that does two things while claiming that it does only
> one, and it will be more obvious.
>

I will fix the is_fifo_flag_unset function so it only does one thing.

> You can also just use reg at this point, instead of reading it once
> again.
>
> Maxime
>
> --
> Maxime Ripard, Free Electrons
> Embedded Linux and Kernel engineering
> http://free-electrons.com

Regards,
Jonathan