Re: [PATCH 1/3] drm/i915: Check pixel rate for DP to VGA dongle

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

On Tue, May 03, 2016 at 02:46:36PM +0300, Mika Kahola wrote:
> Prep work to improve DP branch device handling.
> 
> Filter out a mode that exceeds the max pixel rate setting
> for DP to VGA dongle. This is defined in DPCD register 0x81
> if detailed cap info i.e. info field is 4 bytes long and
> it is available for DP downstream port.
> 
> The register defines the pixel rate divided by 8 in MP/s.
> 
> Signed-off-by: Mika Kahola <mika.kahola@intel.com>
> ---
>  drivers/gpu/drm/i915/intel_dp.c  | 34 ++++++++++++++++++++++++++++++++++
>  drivers/gpu/drm/i915/intel_drv.h |  9 +++++++++
>  2 files changed, 43 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> index 3633002..74a04ce 100644
> --- a/drivers/gpu/drm/i915/intel_dp.c
> +++ b/drivers/gpu/drm/i915/intel_dp.c
> @@ -201,6 +201,13 @@ intel_dp_mode_valid(struct drm_connector *connector,
>  	int max_rate, mode_rate, max_lanes, max_link_clock;
>  	int max_dotclk = to_i915(connector->dev)->max_dotclk_freq;
>  
> +	/* DP to VGA dongle may define max pixel rate in DPCD */
> +	if (intel_dp->dfp.present &&
> +	    intel_dp->dfp.detailed_cap_info &&
> +	    (intel_dp->dfp.type & DP_DS_PORT_TYPE_VGA) &&
> +	    (intel_dp->dfp.dot_clk > 0))
> +		max_dotclk = min(max_dotclk, intel_dp->dfp.dot_clk);

What's dfp?

Looks like most of this stuff is not really needed. Just storing a max
dotclock per downstream port would seem to suffice.

> +
>  	if (is_edp(intel_dp) && fixed_mode) {
>  		if (mode->hdisplay > fixed_mode->hdisplay)
>  			return MODE_PANEL;
> @@ -4566,6 +4573,28 @@ static const struct drm_encoder_funcs intel_dp_enc_funcs = {
>  	.destroy = intel_dp_encoder_destroy,
>  };
>  
> +static void intel_dp_get_dfp(struct intel_dp *intel_dp)
> +{
> +	uint8_t dfp_info[4];
> +
> +	intel_dp->dfp.detailed_cap_info = intel_dp->dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DETAILED_CAP_INFO_AVAILABLE;
> +
> +	if (drm_dp_dpcd_read(&intel_dp->aux, DP_DOWNSTREAM_PORT_0, dfp_info, 4) < 0) {
> +		intel_dp->dfp.present = false;
> +		intel_dp->dfp.detailed_cap_info = false;
> +		return; /* aux transfer failed */
> +	}
> +
> +	intel_dp->dfp.type = dfp_info[0] & DP_DS_PORT_TYPE_MASK;
> +
> +	if (intel_dp->dfp.detailed_cap_info) {
> +		if (intel_dp->dfp.type & DP_DS_PORT_TYPE_VGA) {
> +			intel_dp->dfp.dot_clk = dfp_info[1] * 8 * 1000;
> +			DRM_DEBUG_KMS("max pixel rate for VGA is %d kHz\n", intel_dp->dfp.dot_clk);
> +		}

I would suggest putting this sort of stuff into the dp helper. I once
started to hatch something to deal with these downstream port limits,
but never finished it. I pushed my WIP stuff (mostly ideas how to parse
these port caps) to 

git://github.com/vsyrjala/linux.git dp_downstream_ports

maybe you can to see if there's anything useful for you there.

> +	}
> +}
> +
>  enum irqreturn
>  intel_dp_hpd_pulse(struct intel_digital_port *intel_dig_port, bool long_hpd)
>  {
> @@ -4599,6 +4628,11 @@ intel_dp_hpd_pulse(struct intel_digital_port *intel_dig_port, bool long_hpd)
>  	power_domain = intel_display_port_aux_power_domain(intel_encoder);
>  	intel_display_power_get(dev_priv, power_domain);
>  
> +	intel_dp->dfp.present = intel_dp->dpcd[DP_DOWNSTREAMPORT_PRESENT] & 0x1;
> +
> +	if (intel_dp->dfp.present)
> +		intel_dp_get_dfp(intel_dp);
> +
>  	if (long_hpd) {
>  		/* indicate that we need to restart link training */
>  		intel_dp->train_set_valid = false;
> diff --git a/drivers/gpu/drm/i915/intel_drv.h b/drivers/gpu/drm/i915/intel_drv.h
> index 21dee3f..9798a59 100644
> --- a/drivers/gpu/drm/i915/intel_drv.h
> +++ b/drivers/gpu/drm/i915/intel_drv.h
> @@ -794,6 +794,13 @@ enum link_m_n_set {
>  	M2_N2
>  };
>  
> +struct intel_dp_dfp {
> +	bool present;
> +	int type;
> +	bool detailed_cap_info;
> +	int dot_clk; /* pixel rate for VGA dongles */
> +};
> +
>  struct intel_dp {
>  	i915_reg_t output_reg;
>  	i915_reg_t aux_ch_ctl_reg;
> @@ -861,6 +868,8 @@ struct intel_dp {
>  
>  	bool train_set_valid;
>  
> +	struct intel_dp_dfp dfp;
> +
>  	/* Displayport compliance testing */
>  	unsigned long compliance_test_type;
>  	unsigned long compliance_test_data;
> -- 
> 1.9.1
> 
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-- 
Ville Syrjälä
Intel OTC
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