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

[Jonathan Liu]

Hi Maxime,

On 30 June 2017 at 19:58, Jonathan Liu <net147 at gmail.com> wrote:
> Hi Maxime,
>
> On 30 June 2017 at 19:34, Maxime Ripard
> <maxime.ripard at free-electrons.com> wrote:
>> 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 at free-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.
>>
>
> What would you suggest?
>

You need to consider the fact that there are I2C devices such as
sensors that may do I2C clock stretching up to several tens of
milliseconds. For example the HTU21D humidity sensor takes up to 50 ms
max to perform measurement of temperature during which it is holding
down the clock to do I2C clock stretching.

>>> > 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?
>>
>
> FIFO_RX_TRIGGER_THRES: 0
> FIFO_TX_TRIGGER_THRES: 15
>
>> Maxime
>>
>> --
>> Maxime Ripard, Free Electrons
>> Embedded Linux and Kernel engineering
>> http://free-electrons.com
>
> Regards,
> Jonathan

Regards,
Jonathan