drm/i915/dsb: Implement intel_dsb_gosub()

Add support for the new GOSUB DSB instruction (available on ptl+),
which instructs the DSB to jump to a different buffer, execute
the commands there, and then return execution to the next
instruction in the original buffer.

There are a few alignment related workarounds that need to
be dealt with when emitting GOSUB instruction.

v2: Right shift head and tail pointer passed to gosub command (chaitanya)
v3: Add macro for right shifting head/tail pointers (Animesh)
v4: Fix typo in commit message (Uma)
    Add comments explaining why right shifting htp is needed (Animesh)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Animesh Manna <animesh.manna@intel.com>
Signed-off-by: Animesh Manna <animesh.manna@intel.com>
Link: https://lore.kernel.org/r/20250523062041.166468-5-chaitanya.kumar.borah@intel.com

2 files changed, 75 insertions, 0 deletions

diff --git a/drivers/gpu/drm/i915/display/intel_dsb.c b/drivers/gpu/drm/i915/display/intel_dsb.c
index 9b2de4e7e681..cd32b59adf18 100644
--- a/drivers/gpu/drm/i915/display/intel_dsb.c
+++ b/drivers/gpu/drm/i915/display/intel_dsb.c
@@ -93,6 +93,10 @@ struct intel_dsb {
 /* see DSB_REG_VALUE_MASK */
 #define DSB_OPCODE_POLL			0xA
 /* see DSB_REG_VALUE_MASK */
+#define DSB_OPCODE_GOSUB		0xC /* ptl+ */
+#define   DSB_GOSUB_HEAD_SHIFT		26
+#define   DSB_GOSUB_TAIL_SHIFT		0
+#define   DSB_GOSUB_CONVERT_ADDR(x)	((x) >> 6)
 
 static bool pre_commit_is_vrr_active(struct intel_atomic_state *state,
 				     struct intel_crtc *crtc)
@@ -533,6 +537,75 @@ static void intel_dsb_align_tail(struct intel_dsb *dsb)
 	dsb->free_pos = aligned_tail / 4;
 }
 
+static void intel_dsb_gosub_align(struct intel_dsb *dsb)
+{
+	u32 aligned_tail, tail;
+
+	intel_dsb_ins_align(dsb);
+
+	tail = dsb->free_pos * 4;
+	aligned_tail = ALIGN(tail, CACHELINE_BYTES);
+
+	/*
+	 * "The GOSUB instruction cannot be placed in
+	 *  cacheline QW slot 6 or 7 (numbered 0-7)"
+	 */
+	if (aligned_tail - tail <= 2 * 8)
+		intel_dsb_buffer_memset(&dsb->dsb_buf, dsb->free_pos, 0,
+					aligned_tail - tail);
+
+	dsb->free_pos = aligned_tail / 4;
+}
+
+void intel_dsb_gosub(struct intel_dsb *dsb,
+		     struct intel_dsb *sub_dsb)
+{
+	struct intel_crtc *crtc = dsb->crtc;
+	struct intel_display *display = to_intel_display(crtc->base.dev);
+	unsigned int head, tail;
+	u64 head_tail;
+
+	if (drm_WARN_ON(display->drm, dsb->id != sub_dsb->id))
+		return;
+
+	if (!assert_dsb_tail_is_aligned(sub_dsb))
+		return;
+
+	intel_dsb_gosub_align(dsb);
+
+	head = intel_dsb_head(sub_dsb);
+	tail = intel_dsb_tail(sub_dsb);
+
+	/*
+	 * The GOSUB instruction has the following memory layout.
+	 *
+	 * +------------------------------------------------------------+
+	 * |  Opcode  |   Rsvd    |      Head Ptr     |     Tail Ptr    |
+	 * |   0x0c   |           |                   |                 |
+	 * +------------------------------------------------------------+
+	 * |<- 8bits->|<- 4bits ->|<--   26bits    -->|<--  26bits   -->|
+	 *
+	 * We have only 26 bits each to represent the head and  tail
+	 * pointers even though the addresses itself are of 32 bit. However, this
+	 * is not a problem because the addresses are 64 bit aligned and therefore
+	 * the last 6 bits are always Zero's. Therefore, we right shift the address
+	 * by 6 before embedding it into the GOSUB instruction.
+	 */
+
+	head_tail = ((u64)(DSB_GOSUB_CONVERT_ADDR(head)) << DSB_GOSUB_HEAD_SHIFT) |
+		((u64)(DSB_GOSUB_CONVERT_ADDR(tail)) << DSB_GOSUB_TAIL_SHIFT);
+
+	intel_dsb_emit(dsb, lower_32_bits(head_tail),
+		       (DSB_OPCODE_GOSUB << DSB_OPCODE_SHIFT) |
+		       upper_32_bits(head_tail));
+
+	/*
+	 * "NOTE: the instructions within the cacheline
+	 *  FOLLOWING the GOSUB instruction must be NOPs."
+	 */
+	intel_dsb_align_tail(dsb);
+}
+
 void intel_dsb_finish(struct intel_dsb *dsb)
 {
 	struct intel_crtc *crtc = dsb->crtc;
diff --git a/drivers/gpu/drm/i915/display/intel_dsb.h b/drivers/gpu/drm/i915/display/intel_dsb.h
index e843c52bf97c..8b2cf0a7b7e6 100644
--- a/drivers/gpu/drm/i915/display/intel_dsb.h
+++ b/drivers/gpu/drm/i915/display/intel_dsb.h
@@ -57,6 +57,8 @@ void intel_dsb_vblank_evade(struct intel_atomic_state *state,
 void intel_dsb_poll(struct intel_dsb *dsb,
 		    i915_reg_t reg, u32 mask, u32 val,
 		    int wait_us, int count);
+void intel_dsb_gosub(struct intel_dsb *dsb,
+		     struct intel_dsb *sub_dsb);
 void intel_dsb_chain(struct intel_atomic_state *state,
 		     struct intel_dsb *dsb,
 		     struct intel_dsb *chained_dsb,