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

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Date:   Thu, 29 Jun 2017 17:56:20 +0200
From:   Maxime Ripard <maxime.ripard@...e-electrons.com>
To:     Jonathan Liu <net147@...il.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

On Wed, Jun 28, 2017 at 08:39:33PM +1000, Jonathan Liu wrote:
> >> +     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.

Well, polling for 100ms doesn't seem great either. What was the
rationale behind that timeout?

And we can also reverse the check and look at the INT_STATUS
register. The errors will be there, and we can program the threshold
we want in both directions and use the
DDC_FIFO_Request_Interrupt_Status bit.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com

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