drm/panthor: Fix the user MMIO offset logic for emulators

Currently, we pick the MMIO offset based on the size of the pgoff_t
type seen by the process that manipulates the FD, such that a 32-bit
process can always map the user MMIO ranges. But this approach doesn't
work well for emulators like FEX, where the emulator is a 64-bit binary
which might be executing 32-bit code. In that case, the kernel thinks
it's the 64-bit process and assumes DRM_PANTHOR_USER_MMIO_OFFSET_64BIT
is in use, but the UMD library expects DRM_PANTHOR_USER_MMIO_OFFSET_32BIT,
because it can't mmap() anything above the pgoff_t size.

In order to solve that, we need a way to explicitly set the user MMIO
offset from the UMD, such that the kernel doesn't have to guess it
from the TIF_32BIT flag set on user thread. We keep the old behavior
if DRM_PANTHOR_SET_USER_MMIO_OFFSET is never called.

Changes in v2:
- Drop the lock/immutable fields and allow SET_USER_MMIO_OFFSET
  requests to race with mmap() requests
- Don't do the is_user_mmio_offset test twice in panthor_mmap()
- Improve the uAPI docs

Changes in v3:
- Bump to version 1.5 instead of 1.4 after rebasing
- Add R-bs
- Fix/rephrase comment as suggested by Liviu

Reviewed-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Link: https://lore.kernel.org/r/20250606080932.4140010-3-boris.brezillon@collabora.com
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>

1 files changed, 38 insertions, 0 deletions

diff --git a/include/uapi/drm/panthor_drm.h b/include/uapi/drm/panthor_drm.h
index 3a76c4f2882b..e1f43deb7eca 100644
--- a/include/uapi/drm/panthor_drm.h
+++ b/include/uapi/drm/panthor_drm.h
@@ -130,6 +130,20 @@ enum drm_panthor_ioctl_id {
 
 	/** @DRM_PANTHOR_BO_SET_LABEL: Label a BO. */
 	DRM_PANTHOR_BO_SET_LABEL,
+
+	/**
+	 * @DRM_PANTHOR_SET_USER_MMIO_OFFSET: Set the offset to use as the user MMIO offset.
+	 *
+	 * The default behavior is to pick the MMIO offset based on the size of the pgoff_t
+	 * type seen by the process that manipulates the FD, such that a 32-bit process can
+	 * always map the user MMIO ranges. But this approach doesn't work well for emulators
+	 * like FEX, where the emulator is an 64-bit binary which might be executing 32-bit
+	 * code. In that case, the kernel thinks it's the 64-bit process and assumes
+	 * DRM_PANTHOR_USER_MMIO_OFFSET_64BIT is in use, but the UMD library expects
+	 * DRM_PANTHOR_USER_MMIO_OFFSET_32BIT, because it can't mmap() anything above the
+	 * pgoff_t size.
+	 */
+	DRM_PANTHOR_SET_USER_MMIO_OFFSET,
 };
 
 /**
@@ -1002,6 +1016,28 @@ struct drm_panthor_bo_set_label {
 };
 
 /**
+ * struct drm_panthor_set_user_mmio_offset - Arguments passed to
+ * DRM_IOCTL_PANTHOR_SET_USER_MMIO_OFFSET
+ *
+ * This ioctl is only really useful if you want to support userspace
+ * CPU emulation environments where the size of an unsigned long differs
+ * between the host and the guest architectures.
+ */
+struct drm_panthor_set_user_mmio_offset {
+	/**
+	 * @offset: User MMIO offset to use.
+	 *
+	 * Must be either DRM_PANTHOR_USER_MMIO_OFFSET_32BIT or
+	 * DRM_PANTHOR_USER_MMIO_OFFSET_64BIT.
+	 *
+	 * Use DRM_PANTHOR_USER_MMIO_OFFSET (which selects OFFSET_32BIT or
+	 * OFFSET_64BIT based on the size of an unsigned long) unless you
+	 * have a very good reason to overrule this decision.
+	 */
+	__u64 offset;
+};
+
+/**
  * DRM_IOCTL_PANTHOR() - Build a Panthor IOCTL number
  * @__access: Access type. Must be R, W or RW.
  * @__id: One of the DRM_PANTHOR_xxx id.
@@ -1045,6 +1081,8 @@ enum {
 		DRM_IOCTL_PANTHOR(WR, TILER_HEAP_DESTROY, tiler_heap_destroy),
 	DRM_IOCTL_PANTHOR_BO_SET_LABEL =
 		DRM_IOCTL_PANTHOR(WR, BO_SET_LABEL, bo_set_label),
+	DRM_IOCTL_PANTHOR_SET_USER_MMIO_OFFSET =
+		DRM_IOCTL_PANTHOR(WR, SET_USER_MMIO_OFFSET, set_user_mmio_offset),
 };
 
 #if defined(__cplusplus)