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

[Abel Vesa <abel.vesa@linaro.org>]

On 24-12-11 15:42:27, 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).

Will follow Dmitry's proposal here.

	if (ret < 0)
        	return ret;

	return (ret == 1) ? 0 : -EIO;


> 
> > +}
> > +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?

Yes, this makes sense. But other than throwing an error I don't think
there is much to be done. I'll add an drm_err here just in case. 

> 
> > +
> > +	if (lttpr_count > 0 && !drm_dp_lttpr_set_transparent_mode(aux, false))
> 
> No need to check lttpr_count again here.

So the logic behind lttpr_count and this transparency mode changing, as
specified in the DP standard, is as follows:

- If there are 0 LTTPRs counted, then nothing to be done, otherwise set to
transparent mode.

- Then, if there are between 0 and 8 LTTPRs counted, set non-transparent
mode successfully.

- Otherwise, rollback to transparent mode.

This last rollback might result in two transparent mode settings without
a non-transparent one in between, but AFAIU, that is OK. Making sure this
doesn't happen would just make the implementation more ugly without any
benefit, IMO.

> 
> > +		return 0;
> 
> I'd check for errors instead of success here and do the rollback before
> returning -EINVAL.
> 

Yes, I think it would be more cleaner. Will do that.

> > +
> > +	/*
> > +	 * 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.

Yes. Will do that.

> 
> > +}
> > +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

Thanks for reviewing and testing!
Abel