[PATCH] drm/i915/dp: reduce link M/N parameters

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

Several major vendor USB-C->HDMI converters, in particular the DA200,
fail to recover a 5.4 GHz 1 lane signal if the link N is greater than
0x80000.

The link M and N depend on the pixel clock and link clock ratio. With
current code link N exceeds 0x80000 only when link clock >= 540000
kHz. Except for the eDP intermediate link clocks, at least the four
least significant bits are always zero. Just one bit shift right would
be enough to bring even the DP 1.4 810000 kHz link clock under 0x80000
link N. The pixel clock for modes that require a link clock >= 540000
kHz would also have several least significant bits zero. Unless the user
provides a mode with an odd pixel clock value, we can reduce the numbers
to reach the goal, with no loss in precision.

The DP spec even mentions sources making choices that "allow for static
and relatively small Mvid and Nvid values", thus reducing the link M/N
regardless of the sink in question seems justified.

Everything here is based on the work and information gathered by Clint
Taylor <clinton.a.taylor@intel.com>. This is just an iteration to reduce
the parameters regardless of lane count, link rate, or sink.

Reference: http://patchwork.freedesktop.org/patch/msgid/1490225256-11667-1-git-send-email-clinton.a.taylor@intel.com
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93578
Tested-by: Mads <mads@ab3.no>
Tested-by: PJ <foobar@pjmodos.net>
Tested-by: François Guerraz <kubrick@fgv6.net>
Tested-by: Lev Popov <leo@nabam.net>
Tested-by: Igor Krivenko <igor.s.krivenko@gmail.com>
Cc: Clint Taylor <clinton.a.taylor@intel.com>
Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

---

This is cc: stable material, but due to the slight risk of regressions
(there's always the risk, however small, when you change parameters that
affect all sinks) I'd prefer letting this simmer for a while, and asking
for an explicit stable backport afterwards.
---
 drivers/gpu/drm/i915/intel_display.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 9a28a8917dc1..55bb6cf2a2d3 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -6337,6 +6337,15 @@ intel_reduce_m_n_ratio(uint32_t *num, uint32_t *den)
 static void compute_m_n(unsigned int m, unsigned int n,
 			uint32_t *ret_m, uint32_t *ret_n)
 {
+	/*
+	 * Reduce M/N as much as possible without loss in precision. Several DP
+	 * dongles in particular seem to be fussy about too large M/N values.
+	 */
+	while ((m & 1) == 0 && (n & 1) == 0) {
+		m >>= 1;
+		n >>= 1;
+	}
+
 	*ret_n = min_t(unsigned int, roundup_pow_of_two(n), DATA_LINK_N_MAX);
 	*ret_m = div_u64((uint64_t) m * *ret_n, n);
 	intel_reduce_m_n_ratio(ret_m, ret_n);
-- 
2.1.4

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