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

[Jani Nikula <jani.nikula@linux.intel.com>]

On Wed, 25 Mar 2026, Ville Syrjala <ville.syrjala@linux.intel.com> wrote:
> 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*().
>
> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/Makefile               |  1 +
>  drivers/gpu/drm/i915/display/intel_mchbar.c | 32 +++++++++++++++++++++
>  drivers/gpu/drm/i915/display/intel_mchbar.h | 22 ++++++++++++++
>  drivers/gpu/drm/xe/Makefile                 |  1 +
>  4 files changed, 56 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..950a36d586c3
> --- /dev/null
> +++ b/drivers/gpu/drm/i915/display/intel_mchbar.c
> @@ -0,0 +1,32 @@
> +// 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)

Throughout the patch, surely these would fit on one line?

Regardless,

Reviewed-by: Jani Nikula <jani.nikula@intel.com>


> +{
> +	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(struct intel_display *display,
> +			i915_reg_t reg)
> +{
> +	struct intel_uncore *uncore = to_intel_uncore(display->drm);
> +
> +	return intel_uncore_read64(uncore, 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..ced5023c4522
> --- /dev/null
> +++ b/drivers/gpu/drm/i915/display/intel_mchbar.h
> @@ -0,0 +1,22 @@
> +/* 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(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 49de1c22a469..6ae190316b95 100644
> --- a/drivers/gpu/drm/xe/Makefile
> +++ b/drivers/gpu/drm/xe/Makefile
> @@ -304,6 +304,7 @@ 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 \

-- 
Jani Nikula, Intel