Re: [PATCH v2 1/4] drm/dp: Add helper to set LTTPRs in transparent mode

[neil.armstrong@linaro.org]

On 11/12/2024 15:42, Johan Hovold wrote:
> On Wed, Dec 11, 2024 at 03:04:12PM +0200, Abel Vesa wrote:
>   
>> +/**
>> + * drm_dp_lttpr_set_transparent_mode - set the LTTPR in transparent mode
>> + * @aux: DisplayPort AUX channel
>> + * @enable: Enable or disable transparent mode
>> + *
>> + * Returns 0 on success or a negative error code on failure.
>> + */
>> +int drm_dp_lttpr_set_transparent_mode(struct drm_dp_aux *aux, bool enable)
>> +{
>> +	u8 val = enable ? DP_PHY_REPEATER_MODE_TRANSPARENT :
>> +			  DP_PHY_REPEATER_MODE_NON_TRANSPARENT;
>> +	int ret = drm_dp_dpcd_writeb(aux, DP_PHY_REPEATER_MODE, val);
>> +
>> +	return ret == 1 ? 0 : ret;
> 
> This looks correct, but I had to go look at drm_dp_dpcd_writeb() to make
> sure it never returns 0 (for short transfers).
> 
>> +}
>> +EXPORT_SYMBOL(drm_dp_lttpr_set_transparent_mode);
> 
> This appears to be what the driver currently uses, but why not
> EXPORT_SYMBOL_GPL?

drivers/gpu/drm/display/drm_dp_helper.c is not GPL licenced, so
this is the right macro to use.

Neil

> 
>> +
>> +/**
>> + * drm_dp_lttpr_init - init LTTPR transparency mode according to DP standard
>> + *
>> + * @aux: DisplayPort AUX channel
>> + * @lttpr_count: Number of LTTPRs
>> + *
>> + * Returns 0 on success or a negative error code on failure.
>> + */
>> +int drm_dp_lttpr_init(struct drm_dp_aux *aux, int lttpr_count)
>> +{
>> +	if (!lttpr_count)
>> +		return 0;
>> +
>> +	/*
>> +	 * See DP Standard v2.0 3.6.6.1 about the explicit disabling of
>> +	 * non-transparent mode and the disable->enable non-transparent mode
>> +	 * sequence.
>> +	 */
>> +	drm_dp_lttpr_set_transparent_mode(aux, true);
> 
> Error handling?
> 
>> +
>> +	if (lttpr_count > 0 && !drm_dp_lttpr_set_transparent_mode(aux, false))
> 
> No need to check lttpr_count again here.
> 
>> +		return 0;
> 
> I'd check for errors instead of success here and do the rollback before
> returning -EINVAL.
> 
>> +
>> +	/*
>> +	 * Roll-back to tranparent mode if setting non-tranparent mode failed or
>> +	 * the number of LTTPRs is invalid
>> +	 */
>> +	drm_dp_lttpr_set_transparent_mode(aux, true);
>> +
>> +	return -EINVAL;
> 
> And return 0 explicitly here.
> 
>> +}
>> +EXPORT_SYMBOL(drm_dp_lttpr_init);
> 
> In any case this works well and is needed for external display on the
> Lenovo ThinkPad T14s, while not breaking the X13s which does not need
> it:
> 
> Tested-by: Johan Hovold <johan+linaro@kernel.org>
> 
> Johan
>