[PATCH v3 23/23] rust: drm: kms: expose a connector's requested link depth

[Mike Lothian]

`max bpc` is how userspace asks for the number of bits the link
carries, and it is deliberately independent of the framebuffer's
format: scanning out an eight-bit surface over a ten-bit link is the
ordinary case. A driver could attach the property but had no way to
read back what was asked for, so the request could only be ignored.

Add the accessor, and make the linear format modifier nameable outside
the crate so a driver that accepts only linear scanout can say so in
its plane's modifier list rather than leaving userspace to infer it.

Assisted-by: Claude:claude-opus-5
Signed-off-by: Mike Lothian <mike@xxxxxxxxxxxxxx>
---
 rust/kernel/drm/fourcc.rs        |  5 ++++-
 rust/kernel/drm/kms/connector.rs | 16 ++++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/rust/kernel/drm/fourcc.rs b/rust/kernel/drm/fourcc.rs
index 010823c4c86c..3a68af66f236 100644
--- a/rust/kernel/drm/fourcc.rs
+++ b/rust/kernel/drm/fourcc.rs
@@ -13,7 +13,10 @@ const fn fourcc_code(a: u8, b: u8, c: u8, d: u8) -> u32 {
 // function-like macros in bindgen yet.
 pub(crate) const FORMAT_MOD_INVALID: u64 = 0xffffffffffffff;
 /// Linear framebuffer layout (`DRM_FORMAT_MOD_LINEAR`).
-pub(crate) const FORMAT_MOD_LINEAR: u64 = 0;
+///
+/// A driver that accepts only linear scanout has to say so through the plane's format-modifier
+/// list, or userspace sees no `IN_FORMATS` property and has to guess what the plane will take.
+pub const FORMAT_MOD_LINEAR: u64 = 0;
 
 /// 32 bpp RGB with unused alpha.
 pub const XRGB8888: u32 = fourcc_code(b'X', b'R', b'2', b'4');
diff --git a/rust/kernel/drm/kms/connector.rs b/rust/kernel/drm/kms/connector.rs
index 231857cc1f20..633f4f76a610 100644
--- a/rust/kernel/drm/kms/connector.rs
+++ b/rust/kernel/drm/kms/connector.rs
@@ -935,6 +935,22 @@ fn colorspace(&self) -> u32 {
         self.as_raw().colorspace
     }
 
+    /// The bits per colour channel userspace has asked the link to carry, through the `max bpc`
+    /// property.
+    ///
+    /// This is a property of the *link*, not of the framebuffer: userspace routinely scans out an
+    /// eight-bit surface over a ten-bit link, and a driver that derives its output depth from the
+    /// framebuffer format alone silently ignores what was asked for.
+    ///
+    /// Meaningful only on a connector that
+    /// [`UnregisteredConnector::attach_max_bpc_property`] was called for; everything else leaves
+    /// it at zero.
+    fn max_requested_bpc(&self) -> u32 {
+        // `max_requested_bpc` is an `unsigned int` clamped by DRM to the range the driver gave
+        // `drm_connector_attach_max_bpc_property()`, so it needs no validation here.
+        self.as_raw().max_requested_bpc as u32
+    }
+
     /// The electro-optical transfer function from the `HDR_OUTPUT_METADATA` blob, or [`None`] if
     /// userspace has not set one.
     ///