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drm/i915/dp: Fix the math in intel_dp_link_required

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The previous code was confused about units, which is pretty reasonable
given that the units themselves are confusing.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>
This commit is contained in:
Adam Jackson 2011-10-14 12:43:49 -04:00 committed by Keith Packard
parent f52c619a59
commit cd9dde44f4
1 changed files with 21 additions and 5 deletions
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26
drivers/gpu/drm/i915/intel_dp.c
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@ -184,9 +184,25 @@ intel_dp_link_clock(uint8_t link_bw)
return 162000;
}
/* I think this is a fiction */
/*
* The units on the numbers in the next two are... bizarre. Examples will
* make it clearer; this one parallels an example in the eDP spec.
*
* intel_dp_max_data_rate for one lane of 2.7GHz evaluates as:
*
* 270000 * 1 * 8 / 10 == 216000
*
* The actual data capacity of that configuration is 2.16Gbit/s, so the
* units are decakilobits. ->clock in a drm_display_mode is in kilohertz -
* or equivalently, kilopixels per second - so for 1680x1050R it'd be
* 119000. At 18bpp that's 2142000 kilobits per second.
*
* Thus the strange-looking division by 10 in intel_dp_link_required, to
* get the result in decakilobits instead of kilobits.
*/
static int
intel_dp_link_required(struct drm_device *dev, struct intel_dp *intel_dp, int pixel_clock)
intel_dp_link_required(struct intel_dp *intel_dp, int pixel_clock)
{
struct drm_crtc *crtc = intel_dp->base.base.crtc;
struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
@ -195,7 +211,7 @@ intel_dp_link_required(struct drm_device *dev, struct intel_dp *intel_dp, int pi
if (intel_crtc)
bpp = intel_crtc->bpp;
return (pixel_clock * bpp + 7) / 8;
return (pixel_clock * bpp + 9) / 10;
}
static int
@ -223,7 +239,7 @@ intel_dp_mode_valid(struct drm_connector *connector,
/* only refuse the mode on non eDP since we have seen some weird eDP panels
which are outside spec tolerances but somehow work by magic */
if (!is_edp(intel_dp) &&
(intel_dp_link_required(connector->dev, intel_dp, mode->clock)
(intel_dp_link_required(intel_dp, mode->clock)
> intel_dp_max_data_rate(max_link_clock, max_lanes)))
return MODE_CLOCK_HIGH;
@ -670,7 +686,7 @@ intel_dp_mode_fixup(struct drm_encoder *encoder, struct drm_display_mode *mode,
for (clock = 0; clock <= max_clock; clock++) {
int link_avail = intel_dp_max_data_rate(intel_dp_link_clock(bws[clock]), lane_count);
if (intel_dp_link_required(encoder->dev, intel_dp, mode->clock)
if (intel_dp_link_required(intel_dp, mode->clock)
<= link_avail) {
intel_dp->link_bw = bws[clock];
intel_dp->lane_count = lane_count;