[PATCH v2 02/12] drm/i915/mchbar: Provide intel_mchbar_read*() abstraction

[Ville Syrjala <ville.syrjala@linux.intel.com>]

From: Ville Syrjälä <ville.syrjala@linux.intel.com>

MCHBAR registers are a bit special in that:
- we access them through the mirror
- the mirror is read only on HSW+
- the mirror requires the actual MCHBAR to be enabled in device 0:0.0
- the mirror is gone on MTL+

So I'd prefer to treat MCHBAR registers as a bit special in
the code as well, and do all accesses to them via dedicated
functions. Prodive such functions in the form of
intel_mchbar_read*().

v2: Put the function arguments on one line
    No intel_uncore_read64() on xe, use intel_uncore_read64_2x32()
    Name the new function intel_mchbar_read64_2x32() as well

Reviewed-by: Jani Nikula <jani.nikula@intel.com> #v1
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
---
 drivers/gpu/drm/i915/Makefile               |  1 +
 drivers/gpu/drm/i915/display/intel_mchbar.c | 30 +++++++++++++++++++++
 drivers/gpu/drm/i915/display/intel_mchbar.h | 19 +++++++++++++
 drivers/gpu/drm/xe/Makefile                 |  1 +
 4 files changed, 51 insertions(+)
 create mode 100644 drivers/gpu/drm/i915/display/intel_mchbar.c
 create mode 100644 drivers/gpu/drm/i915/display/intel_mchbar.h

diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
index b677720a1c2d..0e48305df8b2 100644
--- a/drivers/gpu/drm/i915/Makefile
+++ b/drivers/gpu/drm/i915/Makefile
@@ -295,6 +295,7 @@ i915-y += \
 	display/intel_link_bw.o \
 	display/intel_load_detect.o \
 	display/intel_lpe_audio.o \
+	display/intel_mchbar.o \
 	display/intel_modeset_lock.o \
 	display/intel_modeset_setup.o \
 	display/intel_modeset_verify.o \
diff --git a/drivers/gpu/drm/i915/display/intel_mchbar.c b/drivers/gpu/drm/i915/display/intel_mchbar.c
new file mode 100644
index 000000000000..2636fe60ef37
--- /dev/null
+++ b/drivers/gpu/drm/i915/display/intel_mchbar.c
@@ -0,0 +1,30 @@
+// SPDX-License-Identifier: MIT
+/*
+ * Copyright © 2026 Intel Corporation
+ */
+
+#include "intel_display_core.h"
+#include "intel_mchbar.h"
+#include "intel_uncore.h"
+
+u16 intel_mchbar_read16(struct intel_display *display, i915_reg_t reg)
+{
+	struct intel_uncore *uncore = to_intel_uncore(display->drm);
+
+	return intel_uncore_read16(uncore, reg);
+}
+
+u32 intel_mchbar_read(struct intel_display *display, i915_reg_t reg)
+{
+	struct intel_uncore *uncore = to_intel_uncore(display->drm);
+
+	return intel_uncore_read(uncore, reg);
+}
+
+u64 intel_mchbar_read64_2x32(struct intel_display *display, i915_reg_t reg)
+{
+	struct intel_uncore *uncore = to_intel_uncore(display->drm);
+	i915_reg_t upper_reg = _MMIO(i915_mmio_reg_offset(reg) + 4);
+
+	return intel_uncore_read64_2x32(uncore, reg, upper_reg);
+}
diff --git a/drivers/gpu/drm/i915/display/intel_mchbar.h b/drivers/gpu/drm/i915/display/intel_mchbar.h
new file mode 100644
index 000000000000..002a4454e8ed
--- /dev/null
+++ b/drivers/gpu/drm/i915/display/intel_mchbar.h
@@ -0,0 +1,19 @@
+/* SPDX-License-Identifier: MIT */
+/*
+ * Copyright © 2026 Intel Corporation
+ */
+
+#ifndef __INTEL_MCHBAR_H__
+#define __INTEL_MCHBAR_H__
+
+#include <linux/types.h>
+
+#include "i915_reg_defs.h"
+
+struct intel_display;
+
+u16 intel_mchbar_read16(struct intel_display *display, i915_reg_t reg);
+u32 intel_mchbar_read(struct intel_display *display, i915_reg_t reg);
+u64 intel_mchbar_read64_2x32(struct intel_display *display, i915_reg_t reg);
+
+#endif /* __INTEL_MCHBAR_H__ */
diff --git a/drivers/gpu/drm/xe/Makefile b/drivers/gpu/drm/xe/Makefile
index 9dacb0579a7d..a86c553dc996 100644
--- a/drivers/gpu/drm/xe/Makefile
+++ b/drivers/gpu/drm/xe/Makefile
@@ -306,6 +306,7 @@ xe-$(CONFIG_DRM_XE_DISPLAY) += \
 	i915-display/intel_link_bw.o \
 	i915-display/intel_lspcon.o \
 	i915-display/intel_lt_phy.o \
+	i915-display/intel_mchbar.o \
 	i915-display/intel_modeset_lock.o \
 	i915-display/intel_modeset_setup.o \
 	i915-display/intel_modeset_verify.o \
-- 
2.52.0