Re: [PATCH v6 4/4] drm/i915: Fix error handling in intelfb_create

[Daniel Vetter <daniel@ffwll.ch>]

On Sat, Oct 24, 2015 at 12:27:57AM +0200, Lukas Wunner wrote:
> intelfb_create() is called once on driver initialization. It either
> uses the fb inherited from BIOS or allocates a new one by calling
> intelfb_alloc(). Afterwards, it calls two functions which can fail:
> 
> - drm_fb_helper_alloc_fbi() can fail with -ENOMEM.
> - ioremap_wc() can fail on alignment issues etc.
> 
> In the case where we allocated the fb with intelfb_alloc(), if either
> of the two functions fail we currently unpin and unref the bo,
> but leak the fb. We need to unref the fb instead of the bo here.
> 
> In the case where the fb was inherited from BIOS, the fb struct was
> allocated by dev_priv->display.get_initial_plane_config() and is in
> active use by a crtc, so it seems wrong to unpin and unref its bo.
> We could treat failure of the above two functions as a fatal error
> and call i915_driver_unload(), or try to limp on (fbcon won't work,
> but X11 might). Let's do the latter.
> 
> However we should at least log an error message. Currently a failure
> of the above two functions is not reported at all: The negative return
> value is passed up the call stack to intel_fbdev_initial_config()
> which ignores it.
> 
> To be fair, this is a corner case which few users probably ever
> experience, nevertheless we should try to get it right.
> 
> Signed-off-by: Lukas Wunner <lukas@wunner.de>

I don't think there's a leak here really. We always assign ifbdev->fb in
intelfb_alloc, which means the fbdev teardown code will take care of it.
The correct approach is probably to not unref anything at all, or if we
unref then we have to clear ifbdev->fb (since it's the reference that very
pointer is holding that we're clearing up).
-Daniel

> ---
>  drivers/gpu/drm/i915/intel_fbdev.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/intel_fbdev.c b/drivers/gpu/drm/i915/intel_fbdev.c
> index 12597b5..b8c11a1 100644
> --- a/drivers/gpu/drm/i915/intel_fbdev.c
> +++ b/drivers/gpu/drm/i915/intel_fbdev.c
> @@ -227,6 +227,7 @@ static int intelfb_create(struct drm_fb_helper *helper,
>  
>  	info = drm_fb_helper_alloc_fbi(helper);
>  	if (IS_ERR(info)) {
> +		DRM_ERROR("Failed to allocate fb_info\n");
>  		ret = PTR_ERR(info);
>  		goto out_unpin;
>  	}
> @@ -253,6 +254,7 @@ static int intelfb_create(struct drm_fb_helper *helper,
>  		ioremap_wc(dev_priv->gtt.mappable_base + i915_gem_obj_ggtt_offset(obj),
>  			   size);
>  	if (!info->screen_base) {
> +		DRM_ERROR("Failed to remap framebuffer into virtual memory\n");
>  		ret = -ENOSPC;
>  		goto out_destroy_fbi;
>  	}
> @@ -284,9 +286,14 @@ static int intelfb_create(struct drm_fb_helper *helper,
>  out_destroy_fbi:
>  	drm_fb_helper_release_fbi(helper);
>  out_unpin:
> -	i915_gem_object_ggtt_unpin(obj);
> -	drm_gem_object_unreference(&obj->base);
> -	mutex_unlock(&dev->struct_mutex);
> +	/* If fb was preallocated by BIOS, try to limp on. Else free it. */
> +	if (prealloc)
> +		mutex_unlock(&dev->struct_mutex);
> +	else {
> +		i915_gem_object_ggtt_unpin(obj);
> +		mutex_unlock(&dev->struct_mutex);
> +		drm_framebuffer_unreference(&intel_fb->base);
> +	}
>  	return ret;
>  }
>  
> -- 
> 1.8.5.2 (Apple Git-48)
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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