[PATCH 2/2] drm/tyr: allow userspace buffers to be mismatched

[Deborah Brouwer <deborah.brouwer@collabora.com>]

Allow userspace to provide a buffer that is smaller or larger than the
object that the kernel expects to write into it.

Set a minimum buffer size to match those in PANTHOR_UOBJ_MIN_SIZE. If
userspace provides at least the minimum buffer size, then simply write
until the buffer is full.

If userspace provides a larger buffer than necessary, then zero out the
remaining buffer.

This ensures UAPI compatibility with Panthor.

Signed-off-by: Deborah Brouwer <deborah.brouwer@collabora.com>
---
 drivers/gpu/drm/tyr/file.rs | 45 +++++++++++++++++++++++++++++++++++++--------
 1 file changed, 37 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/tyr/file.rs b/drivers/gpu/drm/tyr/file.rs
index 9f60a90d4948..a5f96abe4d2e 100644
--- a/drivers/gpu/drm/tyr/file.rs
+++ b/drivers/gpu/drm/tyr/file.rs
@@ -1,11 +1,17 @@
 // SPDX-License-Identifier: GPL-2.0 or MIT
 
+use core::mem::{
+    offset_of,
+    size_of_val, //
+};
+
 use kernel::{
     drm::{
         self,
         Registered, //
     },
     prelude::*,
+    transmute::AsBytes,
     uaccess::UserSlice,
     uapi, //
 };
@@ -16,6 +22,31 @@
     TyrDrmRegistrationData, //
 };
 
+trait UobjMinSize {
+    fn min_size(&self) -> usize;
+}
+
+impl UobjMinSize for uapi::drm_panthor_gpu_info {
+    fn min_size(&self) -> usize {
+        offset_of!(Self, tiler_present) + size_of_val(&self.tiler_present)
+    }
+}
+
+// If the user buffer is at least the minimum required size, then
+// copy as much of the kernel object as will fit into the user buffer.
+// Don't fail if the user buffer is smaller than the kernel object.
+// If the user buffer is bigger than needed, zero out the remainder.
+fn set_uobj<T: AsBytes>(usr_dst: u64, size: u32, min_size: usize, src_obj: &T) -> Result {
+    if (size as usize) < min_size {
+        return Err(EINVAL);
+    }
+
+    let mut writer = UserSlice::new(UserPtr::from_addr(usr_dst as usize), size as usize).writer();
+    writer.write_truncated(src_obj)?;
+    writer.clear()?;
+    Ok(())
+}
+
 #[pin_data]
 pub(crate) struct TyrDrmFileData {}
 
@@ -48,14 +79,12 @@ pub(crate) fn dev_query(
         } else {
             match devquery.type_ {
                 uapi::drm_panthor_dev_query_type_DRM_PANTHOR_DEV_QUERY_GPU_INFO => {
-                    let mut writer = UserSlice::new(
-                        UserPtr::from_addr(devquery.pointer as usize),
-                        devquery.size as usize,
-                    )
-                    .writer();
-
-                    writer.write(&reg_data.gpu_info)?;
-
+                    set_uobj(
+                        devquery.pointer,
+                        devquery.size,
+                        reg_data.gpu_info.min_size(),
+                        &reg_data.gpu_info,
+                    )?;
                     Ok(0)
                 }
                 _ => Err(EINVAL),

-- 
2.56.0