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Date:   Mon, 11 Mar 2019 07:22:00 +0100
From:   Gerd Hoffmann <kraxel@redhat.com>
To:     Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
Cc:     David Airlie <airlied@linux.ie>, Daniel Vetter <daniel@ffwll.ch>,
        virtualization@lists.linux-foundation.org,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] drm/bochs: Fix NULL dereference on atomic_disable helper
Message-ID: <20190311062200.4mtel3j5fjxtxrcf@sirius.home.kraxel.org>
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On Sun, Mar 10, 2019 at 06:21:50PM -0300, Rodrigo Siqueira wrote:
> When the subtest basic-plain-flip from IGT project was executed on Bochs
> driver, the following bug appears:
> 
> BUG: unable to handle kernel NULL pointer dereference at 0000000000000000

>  ? drm_atomic_helper_commit_modeset_disables+0x396/0x3e0 [drm_kms_helper]

> +static void  bochs_crtc_atomic_disable(struct drm_crtc *crtc,
> +				       struct drm_crtc_state *old_state)
> +{
> +}
> +
>  static void bochs_crtc_atomic_flush(struct drm_crtc *crtc,
>  				    struct drm_crtc_state *old_crtc_state)
>  {
> @@ -66,6 +71,7 @@ static const struct drm_crtc_funcs bochs_crtc_funcs = {
>  static const struct drm_crtc_helper_funcs bochs_helper_funcs = {
>  	.mode_set_nofb = bochs_crtc_mode_set_nofb,
>  	.atomic_enable = bochs_crtc_atomic_enable,
> +	.atomic_disable = bochs_crtc_atomic_disable,

Shouldn't we make the callback optional instead of adding empty dummy
functions to drivers?

cheers,
  Gerd