Re: [PATCH 1/4] drm/i915/dp: retry i2c-over-aux seven times on AUX DEFER

[Todd Previte <tprevite@gmail.com>]

On 09/20/2013 06:42 AM, Jani Nikula wrote:
> Per DP1.2 spec.
>
> Signed-off-by: Jani Nikula <jani.nikula@intel.com>
> ---
>   drivers/gpu/drm/i915/intel_dp.c |    7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index 9770160..6626514 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -654,7 +654,12 @@ intel_dp_i2c_aux_ch(struct i2c_adapter *adapter, int mode,
>   		break;
>   	}
>   
> -	for (retry = 0; retry < 5; retry++) {
> +	/*
> +	 * DP1.2 sections 2.7.7.1.5.6.1 and 2.7.7.1.6.6.1: A DP Source device is
> +	 * required to retry at least seven times upon receiving AUX_DEFER
> +	 * before giving up the AUX transaction.
> +	 */
> +	for (retry = 0; retry < 7; retry++) {
>   		ret = intel_dp_aux_ch(intel_dp,
>   				      msg, msg_bytes,
>   				      reply, reply_bytes);
>
Hey Jani,

Although it's not explicitly stated in the specification (I went through 
both the 1.1a and 1.2a specifications and couldn't find it), I think the 
the retry count of 7 applies to all AUX transactions, both native and 
I2C. That's alluded to in the description of the link training sequence 
(pg 382, 2nd paragraph from the bottom) where the sink may defer up to 
seven times before the source can abort training.

With that in mind, it might be a good idea to use a defined constant for 
the retry count for both native and I2C AUX transactions. That would 
keep it consistent throughout the code and be easier to update moving 
forward should the specification change.


-T