[PATCH v2 2/2] drm/aperture: Prevent conflicting framebuffers removal if option is set

[Javier Martinez Canillas]

The simpledrm driver allows to use the frame buffer that was set-up by the
firmware. This gives early video output before the platform DRM driver is
probed and takes over.

But it would be useful to have a way to disable this take over by native
DRM drivers. For example, there may be bugs in the DRM drivers that could
cause the display output to not work correctly.

For those cases, it would be good to keep the simpledrm driver instead and
at least get a working display as set-up by the firmware.

Let's add a drm.disable_native_drivers kernel command line parameter, that
when set to true it prevents the conflicting framebuffers to being removed.

Since the drivers call drm_aperture_remove_conflicting_framebuffers() very
early in their probe callback, this will cause the drivers' probe to fail.

Thanks to Neal Gompa for the suggestion and Thomas Zimmermann for the idea
on how this could be implemented.

Suggested-by: Neal Gompa <ngompa13@xxxxxxxxx>
Signed-off-by: Javier Martinez Canillas <javierm@xxxxxxxxxx>
---

Changes in v2:
- Rename command line parameter to drm.disable_native_drivers.
- Return -EBUSY instead of -EINVAL when the function fails.
- Invert the parameter logic and make it false by default.

 drivers/gpu/drm/drm_aperture.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/drivers/gpu/drm/drm_aperture.c b/drivers/gpu/drm/drm_aperture.c
index 1a8ed0c616d6..6ae20b6d6499 100644
--- a/drivers/gpu/drm/drm_aperture.c
+++ b/drivers/gpu/drm/drm_aperture.c
@@ -14,6 +14,11 @@
 #include <drm/drm_drv.h>
 #include <drm/drm_print.h>
 
+static bool drm_disable_native_drivers;
+module_param_named(disable_native_drivers, drm_disable_native_drivers, bool, 0600);
+MODULE_PARM_DESC(disable_native_drivers,
+		 "Disable native DRM drivers probing [default=false]");
+
 /**
  * DOC: overview
  *
@@ -307,6 +312,9 @@ static int drm_aperture_remove_conflicting_fbdev_framebuffers(resource_size_t ba
  * This function removes graphics device drivers which use memory range described by
  * @base and @size.
  *
+ * The conflicting framebuffers removal does not happen when drm.disable_native_drivers=1 is
+ * set. When this option is enabled, the function will return an -EBUSY errno code instead.
+ *
  * Returns:
  * 0 on success, or a negative errno code otherwise
  */
@@ -315,6 +323,9 @@ int drm_aperture_remove_conflicting_framebuffers(resource_size_t base, resource_
 {
 	int ret;
 
+	if (drm_disable_native_drivers)
+		return -EBUSY;
+
 	ret = drm_aperture_remove_conflicting_fbdev_framebuffers(base, size, primary,
 								 req_driver);
 	if (ret)
@@ -335,6 +346,9 @@ EXPORT_SYMBOL(drm_aperture_remove_conflicting_framebuffers);
  * for any of @pdev's memory bars. The function assumes that PCI device with
  * shadowed ROM drives a primary display and so kicks out vga16fb.
  *
+ * The conflicting framebuffers removal does not happen when drm.disable_native_drivers=1 is
+ * set. When this option is enabled, the function will return an -EBUSY errno code instead.
+ *
  * Returns:
  * 0 on success, or a negative errno code otherwise
  */
@@ -344,6 +358,9 @@ int drm_aperture_remove_conflicting_pci_framebuffers(struct pci_dev *pdev,
 	resource_size_t base, size;
 	int bar, ret = 0;
 
+	if (drm_disable_native_drivers)
+		return -EBUSY;
+
 	for (bar = 0; bar < PCI_STD_NUM_BARS; ++bar) {
 		if (!(pci_resource_flags(pdev, bar) & IORESOURCE_MEM))
 			continue;
-- 
2.31.1