[Intel-gfx] [PATCH] drm/i915/display: Reset message bus after each read/write operation

[Rodrigo Vivi]

On Wed, Oct 04, 2023 at 01:25:04PM +0300, Mika Kahola wrote:
> Every know and then we receive the following error when running
> for example IGT test kms_flip.
> 
> [drm] *ERROR* PHY G Read 0d80 failed after 3 retries.
> [drm] *ERROR* PHY G Write 0d81 failed after 3 retries.
> 
> Since the error is sporadic in nature, the patch proposes
> to reset the message bus after every successful or unsuccessful
> read or write operation. However, testing revealed that this
> alone is not sufficient method an additiona delay is also
> introduces anything from 200us to 300us. This delay is experimental
> value and has no specification to back it up.

have you tried the delays without the bus_reset?
have you talked to hw architects about this?

I wonder if we should add the delay inside the bus_reset itself?
although the bit 15 clear check should be enough by itself and
it doesn't look like it is a hw/fw reset involved to justify the
extra delay.

well, at least some /* FIXME: */ or /* XXX: */ comments is
desired along with the messages if we are going with this
hack without understanding why...

> 
> Signed-off-by: Mika Kahola <mika.kahola at intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_cx0_phy.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_cx0_phy.c b/drivers/gpu/drm/i915/display/intel_cx0_phy.c
> index abd607b564f1..a71b8a29d6b0 100644
> --- a/drivers/gpu/drm/i915/display/intel_cx0_phy.c
> +++ b/drivers/gpu/drm/i915/display/intel_cx0_phy.c
> @@ -220,9 +220,12 @@ static u8 __intel_cx0_read(struct drm_i915_private *i915, enum port port,
>  	/* 3 tries is assumed to be enough to read successfully */
>  	for (i = 0; i < 3; i++) {
>  		status = __intel_cx0_read_once(i915, port, lane, addr);
> +		intel_cx0_bus_reset(i915, port, lane);
>  
>  		if (status >= 0)
>  			return status;
> +
> +		usleep_range(200, 300);
>  	}
>  
>  	drm_err_once(&i915->drm, "PHY %c Read %04x failed after %d retries.\n",
> @@ -299,9 +302,12 @@ static void __intel_cx0_write(struct drm_i915_private *i915, enum port port,
>  	/* 3 tries is assumed to be enough to write successfully */
>  	for (i = 0; i < 3; i++) {
>  		status = __intel_cx0_write_once(i915, port, lane, addr, data, committed);
> +		intel_cx0_bus_reset(i915, port, lane);
>  
>  		if (status == 0)
>  			return;
> +
> +		usleep_range(200, 300);
>  	}
>  
>  	drm_err_once(&i915->drm,
> -- 
> 2.34.1
>