Re: [PATCH 5/9] drm/i915/display: Add bits for link_n_exended for DISPLAY >= 14

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

On Mon, 21 Jul 2025, Ankit Nautiyal <ankit.k.nautiyal@intel.com> wrote:
> LINK_N register has bits 31:24 for extended link N value used for
> HDMI2.1 and for an alternate mode of operation of DP TG DDA
> (Bspec:50488).
>
> Add support for these extra bits.
>
> For displays with version 14 or higher, the `PIPE_LINK_N1_EXTENDED_MASK`
> (bits 31:24) is used to handle the extended link N bits.
> For older platforms, the `DATA_LINK_M_N_MASK` (bits 23:0) is used to
> handle the standard link N bits. This distinction ensures clarity and
> maintains the semantics for platforms that support the extended bits.
> In subsequent changes the logic is updated to conditionally apply the
> extended link N bits.
>
> v2: Drop extra link_n_ext member. (Jani)
> v3: Avoid link_n_ext in set_m_n helper. (Jani)
> v4: Rebase, and update commit message.
>
> Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
> ---
>  drivers/gpu/drm/i915/display/intel_display.c   | 18 ++++++++++++++++--
>  .../gpu/drm/i915/display/intel_display_regs.h  |  2 ++
>  2 files changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/display/intel_display.c b/drivers/gpu/drm/i915/display/intel_display.c
> index f32a4956c926..5232478613aa 100644
> --- a/drivers/gpu/drm/i915/display/intel_display.c
> +++ b/drivers/gpu/drm/i915/display/intel_display.c
> @@ -2578,14 +2578,22 @@ void intel_set_m_n(struct intel_display *display,
>  		   i915_reg_t data_m_reg, i915_reg_t data_n_reg,
>  		   i915_reg_t link_m_reg, i915_reg_t link_n_reg)
>  {
> +	u32 link_n = m_n->link_n;
> +
>  	intel_de_write(display, data_m_reg, TU_SIZE(m_n->tu) | m_n->data_m);
>  	intel_de_write(display, data_n_reg, m_n->data_n);
>  	intel_de_write(display, link_m_reg, m_n->link_m);
> +
> +	if (DISPLAY_VER(display) >= 14)
> +		link_n &= ~PIPE_LINK_N1_EXTENDED_MASK;
> +	else
> +		link_n &= DATA_LINK_M_N_MASK;
> +
>  	/*
>  	 * On BDW+ writing LINK_N arms the double buffered update
>  	 * of all the M/N registers, so it must be written last.
>  	 */
> -	intel_de_write(display, link_n_reg, m_n->link_n);
> +	intel_de_write(display, link_n_reg, link_n);
>  }
>  
>  bool intel_cpu_transcoder_has_m2_n2(struct intel_display *display,
> @@ -3321,7 +3329,13 @@ void intel_get_m_n(struct intel_display *display,
>  		   i915_reg_t link_m_reg, i915_reg_t link_n_reg)
>  {
>  	m_n->link_m = intel_de_read(display, link_m_reg) & DATA_LINK_M_N_MASK;
> -	m_n->link_n = intel_de_read(display, link_n_reg) & DATA_LINK_M_N_MASK;
> +	m_n->link_n = intel_de_read(display, link_n_reg);
> +
> +	if (DISPLAY_VER(display) >= 14)
> +		m_n->link_n &= ~PIPE_LINK_N1_EXTENDED_MASK;
> +	else
> +		m_n->link_n &= DATA_LINK_M_N_MASK;
> +
>  	m_n->data_m = intel_de_read(display, data_m_reg) & DATA_LINK_M_N_MASK;
>  	m_n->data_n = intel_de_read(display, data_n_reg) & DATA_LINK_M_N_MASK;
>  	m_n->tu = REG_FIELD_GET(TU_SIZE_MASK, intel_de_read(display, data_m_reg)) + 1;
> diff --git a/drivers/gpu/drm/i915/display/intel_display_regs.h b/drivers/gpu/drm/i915/display/intel_display_regs.h
> index 7bd09d981cd2..9248561aec5f 100644
> --- a/drivers/gpu/drm/i915/display/intel_display_regs.h
> +++ b/drivers/gpu/drm/i915/display/intel_display_regs.h
> @@ -1027,6 +1027,8 @@
>  
>  #define _PIPEA_LINK_N1		0x60044
>  #define _PIPEB_LINK_N1		0x61044
> +#define  PIPE_LINK_N1_EXTENDED_MASK	REG_GENMASK(31, 24)
> +#define  PIPE_LINK_N1_EXTENDED(val)	REG_FIELD_PREP(PIPE_LINK_N1_EXTENDED_MASK, (val))

Please read the comment near the top of i915_reg.h, and send patches to
amend the comment if there's something unclear.

BR,
Jani.

>  #define PIPE_LINK_N1(dev_priv, tran) _MMIO_TRANS2(dev_priv, tran, _PIPEA_LINK_N1)
>  
>  #define _PIPEA_LINK_M2		0x60048

-- 
Jani Nikula, Intel