drm/vgem: Use devm_drm_dev_alloc

This means we also need to slightly restructure the exit code, so that
final cleanup of the drm_device is triggered by unregistering the
platform device. Note that devres is both clean up when the driver is
unbound (not the case for vgem, we don't bind), and also when unregistering
the device (very much the case for vgem). Therefore we can rely on devres
even though vgem isn't a proper platform device driver.

This also somewhat untangles the load code, since the drm and platform device
setup are no longer interleaved, but two distinct steps.

v2: use devres_open/release_group so we can use devm without real
hacks in the driver core or having to create an entire fake bus for
testing drivers. Might want to extract this into helpers eventually,
maybe as a mock_drm_dev_alloc or test_drm_dev_alloc.

v3: Fix error code handling (Melissa)

Cc: Melissa Wen <melissa.srw@gmail.com>
Reviewed-by: Melissa Wen <melissa.srw@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Rob Clark <robdclark@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200909120745.716178-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/vgem/vgem_drv.c

@@ -401,16 +401,8 @@ static int vgem_prime_mmap(struct drm_gem_object *obj,
 	return 0;
 }
 
-static void vgem_release(struct drm_device *dev)
-{
-	struct vgem_device *vgem = container_of(dev, typeof(*vgem), drm);
-
-	platform_device_unregister(vgem->platform);
-}
-
 static struct drm_driver vgem_driver = {
 	.driver_features		= DRIVER_GEM | DRIVER_RENDER,
-	.release			= vgem_release,
 	.open				= vgem_open,
 	.postclose			= vgem_postclose,
 	.gem_free_object_unlocked	= vgem_gem_free_object,
@@ -442,48 +434,49 @@ static struct drm_driver vgem_driver = {
 static int __init vgem_init(void)
 {
 	int ret;
+	struct platform_device *pdev;
 
-	vgem_device = kzalloc(sizeof(*vgem_device), GFP_KERNEL);
-	if (!vgem_device)
-		return -ENOMEM;
+	pdev = platform_device_register_simple("vgem", -1, NULL, 0);
+	if (IS_ERR(pdev))
+		return PTR_ERR(pdev);
 
-	vgem_device->platform =
-		platform_device_register_simple("vgem", -1, NULL, 0);
-	if (IS_ERR(vgem_device->platform)) {
-		ret = PTR_ERR(vgem_device->platform);
-		goto out_free;
+	if (!devres_open_group(&pdev->dev, NULL, GFP_KERNEL)) {
+		ret = -ENOMEM;
+		goto out_unregister;
 	}
 
-	dma_coerce_mask_and_coherent(&vgem_device->platform->dev,
+	dma_coerce_mask_and_coherent(&pdev->dev,
 				     DMA_BIT_MASK(64));
-	ret = drm_dev_init(&vgem_device->drm, &vgem_driver,
-			   &vgem_device->platform->dev);
-	if (ret)
-		goto out_unregister;
-	drmm_add_final_kfree(&vgem_device->drm, vgem_device);
+
+	vgem_device = devm_drm_dev_alloc(&pdev->dev, &vgem_driver,
+					 struct vgem_device, drm);
+	if (IS_ERR(vgem_device)) {
+		ret = PTR_ERR(vgem_device);
+		goto out_devres;
+	}
+	vgem_device->platform = pdev;
 
 	/* Final step: expose the device/driver to userspace */
 	ret = drm_dev_register(&vgem_device->drm, 0);
 	if (ret)
-		goto out_put;
+		goto out_devres;
 
 	return 0;
 
-out_put:
-	drm_dev_put(&vgem_device->drm);
-	platform_device_unregister(vgem_device->platform);
-	return ret;
+out_devres:
+	devres_release_group(&pdev->dev, NULL);
 out_unregister:
-	platform_device_unregister(vgem_device->platform);
-out_free:
-	kfree(vgem_device);
+	platform_device_unregister(pdev);
 	return ret;
 }
 
 static void __exit vgem_exit(void)
 {
+	struct platform_device *pdev = vgem_device->platform;
+
 	drm_dev_unregister(&vgem_device->drm);
-	drm_dev_put(&vgem_device->drm);
+	devres_release_group(&pdev->dev, NULL);
+	platform_device_unregister(pdev);
 }
 
 module_init(vgem_init);