[Intel-gfx] [RESEND PATCHv3] drm/i915/dp: Change aux_ctl reg read to polling read

[Jani Nikula]

On Fri, 09 Dec 2022, Arun R Murthy <arun.r.murthy at intel.com> wrote:
> The busy timeout logic checks for the AUX BUSY, then waits for the
> timeout period and then after timeout reads the register for BUSY or
> Success.
> Instead replace interrupt with polling so as to read the AUX CTL
> register often before the timeout period. Looks like there might be some
> issue with interrupt-on-read. Hence changing the logic to polling read.
>
> v2: replace interrupt with polling read
> v3: use usleep_rang instead of msleep, updated commit msg
>
> Signed-off-by: Arun R Murthy <arun.r.murthy at intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_dp_aux.c | 24 ++++++++++++---------
>  1 file changed, 14 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux.c b/drivers/gpu/drm/i915/display/intel_dp_aux.c
> index 91c93c93e5fc..230f27d75846 100644
> --- a/drivers/gpu/drm/i915/display/intel_dp_aux.c
> +++ b/drivers/gpu/drm/i915/display/intel_dp_aux.c
> @@ -41,21 +41,25 @@ intel_dp_aux_wait_done(struct intel_dp *intel_dp)
>  	i915_reg_t ch_ctl = intel_dp->aux_ch_ctl_reg(intel_dp);
>  	const unsigned int timeout_ms = 10;
>  	u32 status;
> -	bool done;
> +	int try;
>  
> -#define C (((status = intel_de_read_notrace(i915, ch_ctl)) & DP_AUX_CH_CTL_SEND_BUSY) == 0)
> -	done = wait_event_timeout(i915->display.gmbus.wait_queue, C,
> -				  msecs_to_jiffies_timeout(timeout_ms));
> +	for (try = 0; try < 10; try++) {
> +		status = intel_uncore_read_notrace(&i915->uncore, ch_ctl);
> +		if ((status & DP_AUX_CH_CTL_SEND_BUSY) == 0)
> +			break;
> +		usleep_range(400, 500);
> +	}
> +	if (try == 3) {
> +		status = intel_uncore_read_notrace(&i915->uncore, ch_ctl);
> +		if ((status & DP_AUX_CH_CTL_SEND_BUSY) != 0)
> +			drm_err(&i915->drm,
> +				"%s: did not complete or timeout within %ums (status 0x%08x)\n",
> +				intel_dp->aux.name, timeout_ms, status);
> +	}
>  
>  	/* just trace the final value */
>  	trace_i915_reg_rw(false, ch_ctl, status, sizeof(status), true);

Okay, so there are still a bunch of issues above. For example, try < 10
vs. try == 3, reverting back to intel_uncore_* functions after conflict
resolution, having duplicated reads and conditions.

Now, I should've taken a step back earlier and realized you should use
the helper we already have for this: intel_de_wait_for_register().

All of the above shrinks to just a few lines:

	ret = intel_de_wait_for_register(i915, ch_ctl, DP_AUX_CH_CTL_SEND_BUSY, 0, timeout_ms);
	if (ret)
		drm_err(...);

Sorry for missing this earlier.

BR,
Jani.

>  
> -	if (!done)
> -		drm_err(&i915->drm,
> -			"%s: did not complete or timeout within %ums (status 0x%08x)\n",
> -			intel_dp->aux.name, timeout_ms, status);
> -#undef C
> -
>  	return status;
>  }

-- 
Jani Nikula, Intel Open Source Graphics Center