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

[Johan Hovold <johan@kernel.org>]

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?

> +
> +/**
> + * 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