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+lin...@kernel.org>
Johan
