linux-kernel - Re: [PATCH] drm/fb-helper: Blacklist writeback when adding connectors to fbdev

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Date:   Thu, 15 Nov 2018 17:42:39 +0100
From:   Boris Brezillon <boris.brezillon@...tlin.com>
To:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...ux.ie>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        Eric Anholt <eric@...olt.net>
Subject: Re: [PATCH] drm/fb-helper: Blacklist writeback when adding
 connectors to fbdev

On Thu, 15 Nov 2018 17:32:48 +0100
Paul Kocialkowski <paul.kocialkowski@...tlin.com> wrote:

> Writeback connectors do not produce any on-screen output and require
> special care for use. Such connectors are hidden from enumeration in
> DRM resources by default, but they are still picked-up by fbdev.
> This makes rather little sense since fbdev is not really adapted for
> dealing with writeback.
> 
> Moreover, this is also a source of issues when userspace disables the
> CRTC (and associated plane) without detaching the CRTC from the
> connector (which is hidden by default). In this case, the connector is
> still using the CRTC, leading to am "enabled/connectors mismatch" and

				   ^an

> eventually the failure of the associated atomic commit. This situation
> happens with VC4 testing under IGT GPU Tools.
> 
> Filter out writeback connectors in the fbdev helper to solve this.
> 
> Signed-off-by: Paul Kocialkowski <paul.kocialkowski@...tlin.com>

Reviewed-by: Boris Brezillon <boris.brezillon@...tlin.com>

> ---
>  drivers/gpu/drm/drm_fb_helper.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index a502f3e519fd..dd852a25d375 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -219,6 +219,9 @@ int drm_fb_helper_single_add_all_connectors(struct drm_fb_helper *fb_helper)
>  	mutex_lock(&fb_helper->lock);
>  	drm_connector_list_iter_begin(dev, &conn_iter);
>  	drm_for_each_connector_iter(connector, &conn_iter) {
> +		if (connector->connector_type == DRM_MODE_CONNECTOR_WRITEBACK)
> +			continue;
> +
>  		ret = __drm_fb_helper_add_one_connector(fb_helper, connector);
>  		if (ret)
>  			goto fail;