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

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Message-ID: <20170630093422.om7b75q5wyuby6r7@flea>
Date:   Fri, 30 Jun 2017 11:34:22 +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

Hi,

On Fri, Jun 30, 2017 at 08:22:13AM +1000, Jonathan Liu wrote:
> Hi Maxime,
> 
> On 30 June 2017 at 01:56, Maxime Ripard
> <maxime.ripard@...e-electrons.com> wrote:
> > 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?
> >
> 
> When an error occurs one of the error bits will be set in the
> INT_STATUS register so this is detected very quickly if I check the
> INT_STATUS and FIFO_STATUS at the same time. The 100 ms timeout is in
> case the I2C slave does clock stretching in which case the transfer
> may take longer than the predicted time.

100ms isn't stretching anymore, it's worse than that.

> > 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.
> >
> 
> I did try that when I was doing the v3 patch but I couldn't get it to
> work as mentioned previously in the v3 patch discussion. I programmed
> the FIFO_RX_TRIGGER_THRES and FIFO_TX_TRIGGER_THRES in DDC_FIFO_Ctrl
> register at the same time as setting FIFO_Address_Clear but the
> request interrupt status bit did not get updated to the appropriate
> state that is consistent with the FIFO level and the thresholds. I did
> try this several times for subsequent patch versions without success.

What values did you set it to?

Maxime

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

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