Re: [PATCH 1/4] Add bits for link_n_exended for DISPLAY >= 14

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

On Fri, Mar 21, 2025 at 08:31:22PM +0200, Ville Syrjälä wrote:
> On Fri, Mar 21, 2025 at 04:56:47PM +0530, Ankit Nautiyal 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 ++++++++++++++++--
> >  drivers/gpu/drm/i915/i915_reg.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 3afb85fe8536..8fb0df388571 100644
> > --- a/drivers/gpu/drm/i915/display/intel_display.c
> > +++ b/drivers/gpu/drm/i915/display/intel_display.c
> > @@ -2583,14 +2583,22 @@ 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;
> 
> There should never be anything in those bits or we've screwed up
> somewhere.
> 
> The actual w/a for the M/N > 10.0 looks like a pile of annoying
> hacks in the hardware. I wonder if we could just live without it
> to avoid complicating the code?
> 
> As for the 10.0 limit, I suspect there's nothing platform specific
> about it and it's always been there. It's just not possible to hit
> it with 8b/10b encoding.

Hmm. Maybe it would be possible to hit it with DSC+bigjoiner on ICL+.

Without some kind of joiner it's definitely not possible due to
cdclk limiting the max dotclock well below 1.62 GHz.

> The correct place to handle this would seem
> to be the link rate calculation, ie. just bump up the link rate until
> the limit is no longer an issue (or error out if we can't increase the
> link rate sufficiently).
> 
> > +
> >  	/*
> >  	 * 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,
> > @@ -3279,7 +3287,13 @@ 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/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> > index c5064eebe063..a2054aced4f8 100644
> > --- a/drivers/gpu/drm/i915/i915_reg.h
> > +++ b/drivers/gpu/drm/i915/i915_reg.h
> > @@ -1869,6 +1869,8 @@
> >  
> >  #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))
> >  #define PIPE_LINK_N1(dev_priv, tran) _MMIO_TRANS2(dev_priv, tran, _PIPEA_LINK_N1)
> >  
> >  #define _PIPEA_LINK_M2		0x60048
> > -- 
> > 2.45.2
> 
> -- 
> Ville Syrjälä
> Intel

-- 
Ville Syrjälä
Intel