Re: [PATCH] drm/i915: Reduce Data Link N value for 1 lane DP->hdmi converters

From: Jani Nikula <jani.nikula@intel.com>
To: clinton.a.taylor@intel.com, Intel-gfx@lists.freedesktop.org
Subject: Re: [PATCH] drm/i915: Reduce Data Link N value for 1 lane DP->hdmi converters
Date: Thu, 23 Mar 2017 14:30:50 +0200	[thread overview]
Message-ID: <871sto89ad.fsf@intel.com> (raw)
In-Reply-To: <1490225256-11667-1-git-send-email-clinton.a.taylor@intel.com>

On Thu, 23 Mar 2017, clinton.a.taylor@intel.com wrote:
> From: Clint Taylor <clinton.a.taylor@intel.com>
>
> Several major vendor USB-C->HDMI converters fail to recover a 5.4 GHz 1 lane
> signal if the Data Link N is greater than 0x80000.
> Patch detects when 1 lane 5.4 GHz signal is being used and makes the maximum
> value 20 bit instead of the maximum specification supported 24 bit value.
>
> Cc: Jani Nikula <jani.nikula@intel.com>
> Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
>

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93578

> Signed-off-by: Clint Taylor <clinton.a.taylor@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_reg.h      |    2 ++
>  drivers/gpu/drm/i915/intel_display.c |   15 +++++++++++----
>  2 files changed, 13 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
> index 04c8f69..838d8d5 100644
> --- a/drivers/gpu/drm/i915/i915_reg.h
> +++ b/drivers/gpu/drm/i915/i915_reg.h
> @@ -4869,6 +4869,8 @@ enum {
>  
>  #define  DATA_LINK_M_N_MASK	(0xffffff)
>  #define  DATA_LINK_N_MAX	(0x800000)
> +/* Maximum N value useable on some DP->HDMI converters */
> +#define  DATA_LINK_REDUCED_N_MAX (0x80000)
>  
>  #define _PIPEA_DATA_N_G4X	0x70054
>  #define _PIPEB_DATA_N_G4X	0x71054
> diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
> index 010e5dd..6e1fdd2 100644
> --- a/drivers/gpu/drm/i915/intel_display.c
> +++ b/drivers/gpu/drm/i915/intel_display.c
> @@ -6315,9 +6315,10 @@ static int intel_crtc_compute_config(struct intel_crtc *crtc,
>  }
>  
>  static void compute_m_n(unsigned int m, unsigned int n,
> -			uint32_t *ret_m, uint32_t *ret_n)
> +			uint32_t *ret_m, uint32_t *ret_n,
> +			uint32_t max_link_n)
>  {
> -	*ret_n = min_t(unsigned int, roundup_pow_of_two(n), DATA_LINK_N_MAX);
> +	*ret_n = min_t(unsigned int, roundup_pow_of_two(n), max_link_n);

If there's evidence suggesting "certain other operating systems" always
use a max (or fixed value) of 0x80000, perhaps we should just follow
suit? Simpler and less magical.

>  	*ret_m = div_u64((uint64_t) m * *ret_n, n);
>  	intel_reduce_m_n_ratio(ret_m, ret_n);
>  }
> @@ -6327,14 +6328,20 @@ static void compute_m_n(unsigned int m, unsigned int n,
>  		       int pixel_clock, int link_clock,
>  		       struct intel_link_m_n *m_n)
>  {
> +	uint32_t max_link_n = DATA_LINK_N_MAX;
>  	m_n->tu = 64;
>  
> +	if ((nlanes==1) && (link_clock >= 540000))

Is the problem really dependent on these conditions? You can get the
same problematic N value with nlanes == 2 && link_clock == 270000 too.

BR,
Jani.

> +		max_link_n = DATA_LINK_REDUCED_N_MAX;
> +
>  	compute_m_n(bits_per_pixel * pixel_clock,
>  		    link_clock * nlanes * 8,
> -		    &m_n->gmch_m, &m_n->gmch_n);
> +		    &m_n->gmch_m, &m_n->gmch_n,
> +		    max_link_n);
>  
>  	compute_m_n(pixel_clock, link_clock,
> -		    &m_n->link_m, &m_n->link_n);
> +		    &m_n->link_m, &m_n->link_n,
> +		    max_link_n);
>  }
>  
>  static inline bool intel_panel_use_ssc(struct drm_i915_private *dev_priv)

-- 
Jani Nikula, Intel Open Source Technology Center
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