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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Z21VD82XCOVgVJUh@linaro.org>
Date: Thu, 26 Dec 2024 15:07:27 +0200
From: Abel Vesa <abel.vesa@...aro.org>
To: Johan Hovold <johan@...nel.org>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Karol Herbst <kherbst@...hat.com>, Lyude Paul <lyude@...hat.com>,
	Danilo Krummrich <dakr@...hat.com>,
	Jani Nikula <jani.nikula@...ux.intel.com>,
	Rodrigo Vivi <rodrigo.vivi@...el.com>,
	Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
	Tvrtko Ursulin <tursulin@...ulin.net>,
	Rob Clark <robdclark@...il.com>,
	Abhinav Kumar <quic_abhinavk@...cinc.com>,
	Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
	Sean Paul <sean@...rly.run>,
	Marijn Suijten <marijn.suijten@...ainline.org>,
	Bjorn Andersson <andersson@...nel.org>,
	Konrad Dybcio <konradybcio@...nel.org>,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	nouveau@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
	intel-xe@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
	freedreno@...ts.freedesktop.org
Subject: Re: [PATCH v2 1/4] drm/dp: Add helper to set LTTPRs in transparent
 mode

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@...nel.org>
> 
> Johan

Thanks for reviewing and testing!
Abel