[PATCH 13/16] hw/misc: add i.MX 95 DPU command-sequencer stub (headless)

[Kyle Fox <kylefoxaustin.github@gmail.com>]

A minimal stub for the i.MX 95 DPU (display controller) command
sequencer. This machine is headless, but Linux's dpu95 driver busy-waits
with no timeout on the sequencer status register during probe, so a
reads-as-zero stub would hang userspace bring-up. The stub reports the
sequencer idle with FIFO space so the probe completes. Real display
scanout is left to a follow-on series.

Signed-off-by: Kyle Fox <kylefoxaustin.github@gmail.com>
---
 hw/misc/Kconfig      |   3 ++
 hw/misc/imx95_dpu.c  | 117 +++++++++++++++++++++++++++++++++++++++++++
 hw/misc/meson.build  |   1 +
 hw/misc/trace-events |   1 +
 4 files changed, 122 insertions(+)
 create mode 100644 hw/misc/imx95_dpu.c

diff --git a/hw/misc/Kconfig b/hw/misc/Kconfig
index e1b8c390312..59c55cb5b83 100644
--- a/hw/misc/Kconfig
+++ b/hw/misc/Kconfig
@@ -290,3 +290,6 @@ config IMX95_PMIC
 
 config IMX95_XCACHE
     bool
+
+config IMX95_DPU
+    bool
diff --git a/hw/misc/imx95_dpu.c b/hw/misc/imx95_dpu.c
new file mode 100644
index 00000000000..4e09b399d85
--- /dev/null
+++ b/hw/misc/imx95_dpu.c
@@ -0,0 +1,117 @@
+/*
+ * NXP i.MX 95 DPU (Display Processing Unit) command-sequencer status stub
+ *
+ * Copyright (c) 2026, Kyle Fox
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ *
+ * The DPU is not modelled: this base machine is headless, and real display
+ * scanout is a follow-on series. With the real System Manager powering the
+ * display mix, however, Linux's dpu95 driver stops deferring and probes - and
+ * its blit-engine bring-up busy-waits with NO
+ * timeout on the command sequencer status register:
+ *   dpu95_cs_wait_idle()       spins until CMDSEQ_STATUS.IDLE      is set
+ *   dpu95_cs_wait_fifo_space() spins until CMDSEQ_STATUS.FIFOSPACE >= 192
+ * A plain zero-returning stub never satisfies either, so the probe kthread
+ * hangs forever, which in turn wedges wait_for_device_probe() in
+ * kernel_init() and userspace never starts.
+ *
+ * This stub returns CMDSEQ_STATUS with IDLE set and a full FIFOSPACE so both
+ * polls pass instantly; every other register reads back zero and writes are
+ * dropped. That is enough to let the probe complete (the actual command
+ * submission / fence path runs only at runtime, which we never reach). A
+ * follow-on series that models the DPU for real (adding display scanout and
+ * Wayland output) removes this stub.
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/module.h"
+#include "hw/core/sysbus.h"
+#include "qom/object.h"
+#include "trace.h"
+
+#define TYPE_IMX95_DPU "imx95.dpu"
+OBJECT_DECLARE_SIMPLE_TYPE(IMX95DPUState, IMX95_DPU)
+
+#define IMX95_DPU_REG_SIZE 0x400000
+
+struct IMX95DPUState {
+    SysBusDevice    parent_obj;
+    MemoryRegion    iomem;
+};
+
+/* Command-sequencer status, from the dpu95 blit register map. */
+#define DPU_CMDSEQ_STATUS           0x1019c
+#define DPU_CMDSEQ_STATUS_IDLE      0x40000000u
+#define DPU_CMDSEQ_STATUS_FIFOSPACE 0x0001ffffu
+
+static uint64_t imx95_dpu_read(void *opaque, hwaddr offset, unsigned size)
+{
+    if (offset == DPU_CMDSEQ_STATUS) {
+        /*
+         * Report the sequencer idle with a full FIFO. This is deliberately
+         * NOT the reset value (0x41000080: IDLE set but FIFOSPACE = 128, below
+         * the driver's threshold of 192), because the dpu95 probe polls
+         * FIFOSPACE before the enable handshake that would raise it on real
+         * silicon. Returning a constant full FIFO is sound only because the
+         * model is probe-time-only: no actual command sequence is ever
+         * submitted. If real DPU command submission is ever modelled,
+         * FIFOSPACE must track the command FIFO occupancy instead.
+         */
+        uint32_t status = DPU_CMDSEQ_STATUS_IDLE | DPU_CMDSEQ_STATUS_FIFOSPACE;
+        trace_imx95_dpu_cmdseq_status(status);
+        return status;
+    }
+    return 0;
+}
+
+static void imx95_dpu_write(void *opaque, hwaddr offset,
+                            uint64_t value, unsigned size)
+{
+}
+
+static const MemoryRegionOps imx95_dpu_ops = {
+    .read = imx95_dpu_read,
+    .write = imx95_dpu_write,
+    .endianness = DEVICE_LITTLE_ENDIAN,
+    .impl = {
+        .min_access_size = 4,
+        .max_access_size = 4,
+    },
+    .valid = {
+        .min_access_size = 4,
+        .max_access_size = 4,
+    },
+};
+
+static void imx95_dpu_init(Object *obj)
+{
+    IMX95DPUState *s = IMX95_DPU(obj);
+
+    memory_region_init_io(&s->iomem, obj, &imx95_dpu_ops, s,
+                          TYPE_IMX95_DPU, IMX95_DPU_REG_SIZE);
+    sysbus_init_mmio(SYS_BUS_DEVICE(obj), &s->iomem);
+}
+
+static void imx95_dpu_class_init(ObjectClass *klass, const void *data)
+{
+    DeviceClass *dc = DEVICE_CLASS(klass);
+
+    set_bit(DEVICE_CATEGORY_DISPLAY, dc->categories);
+    dc->desc = "NXP i.MX 95 DPU (status stub)";
+}
+
+static const TypeInfo imx95_dpu_info = {
+    .name           = TYPE_IMX95_DPU,
+    .parent         = TYPE_SYS_BUS_DEVICE,
+    .instance_size  = sizeof(IMX95DPUState),
+    .instance_init  = imx95_dpu_init,
+    .class_init     = imx95_dpu_class_init,
+};
+
+static void imx95_dpu_register_types(void)
+{
+    type_register_static(&imx95_dpu_info);
+}
+
+type_init(imx95_dpu_register_types)
diff --git a/hw/misc/meson.build b/hw/misc/meson.build
index d7d283a191b..9d629c8efef 100644
--- a/hw/misc/meson.build
+++ b/hw/misc/meson.build
@@ -180,3 +180,4 @@ system_ss.add(when: 'CONFIG_IMX95_GPC', if_true: files('imx95_gpc.c'))
 system_ss.add(when: 'CONFIG_IMX95_SRC', if_true: files('imx95_src.c'))
 system_ss.add(when: 'CONFIG_IMX95_PMIC', if_true: files('imx95_pmic.c'))
 system_ss.add(when: 'CONFIG_IMX95_XCACHE', if_true: files('imx95_xcache.c'))
+system_ss.add(when: 'CONFIG_IMX95_DPU', if_true: files('imx95_dpu.c'))
diff --git a/hw/misc/trace-events b/hw/misc/trace-events
index 0555b15a1f3..e921b5f290a 100644
--- a/hw/misc/trace-events
+++ b/hw/misc/trace-events
@@ -466,3 +466,4 @@ pcal6408a_reg_read(uint8_t reg, uint8_t val) "PCAL6408A reg 0x%02x -> 0x%02x"
 pcal6408a_reg_write(uint8_t reg, uint8_t val) "PCAL6408A reg 0x%02x <- 0x%02x"
 pf53_reg_read(uint8_t reg, uint8_t val) "PF53 reg 0x%02x -> 0x%02x"
 pf53_reg_write(uint8_t reg, uint8_t val) "PF53 reg 0x%02x <- 0x%02x"
+imx95_dpu_cmdseq_status(uint32_t value) "CMDSEQ_STATUS -> 0x%08x"
-- 
2.34.1