linux-kernel - [PATCH] drm/i915: rework the error handling in *_dpll

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Date:   Fri,  4 Mar 2022 13:03:55 -0800
From:   trix@...hat.com
To:     jani.nikula@...ux.intel.com, joonas.lahtinen@...ux.intel.com,
        rodrigo.vivi@...el.com, tvrtko.ursulin@...ux.intel.com,
        airlied@...ux.ie, daniel@...ll.ch, nathan@...nel.org,
        ndesaulniers@...gle.com, ville.syrjala@...ux.intel.com,
        matthew.d.roper@...el.com, lucas.demarchi@...el.com,
        airlied@...hat.com, imre.deak@...el.com
Cc:     intel-gfx@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev,
        Tom Rix <trix@...hat.com>
Subject: [PATCH] drm/i915: rework the error handling in *_dpll_params

From: Tom Rix <trix@...hat.com>

Clang static analysis reports this issue
intel_dpll.c:472:31: warning: The left operand of '-'
  is a garbage value [core.UndefinedBinaryOperatorResult]
  this_err = abs(clock.dot - target);
                 ~~~~~~~~~ ^

In a loop clock.dot is set on successful call to
i9xx_calc_dpll_params().  If the call fails, the later
*is_valid() will use the previous loop's clock.dot.

The *_dpll_params functions return an arithmetic statement
with the clock.dot as the variable.  Change the error handler
to set clock.dot to 0 and jump to the return statement.

Fixes: dccbea3b0704 ("drm/i915: calculate the port clock rate along with other PLL params")
Signed-off-by: Tom Rix <trix@...hat.com>
---
 drivers/gpu/drm/i915/display/intel_dpll.c | 32 ++++++++++++++---------
 1 file changed, 20 insertions(+), 12 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dpll.c b/drivers/gpu/drm/i915/display/intel_dpll.c
index 0ae37fdbf2a5b..ba7cada704288 100644
--- a/drivers/gpu/drm/i915/display/intel_dpll.c
+++ b/drivers/gpu/drm/i915/display/intel_dpll.c
@@ -309,11 +309,13 @@ int pnv_calc_dpll_params(int refclk, struct dpll *clock)
 {
 	clock->m = clock->m2 + 2;
 	clock->p = clock->p1 * clock->p2;
-	if (WARN_ON(clock->n == 0 || clock->p == 0))
-		return 0;
+	if (WARN_ON(clock->n == 0 || clock->p == 0)) {
+		clock->dot = 0;
+		goto end;
+	}
 	clock->vco = DIV_ROUND_CLOSEST(refclk * clock->m, clock->n);
 	clock->dot = DIV_ROUND_CLOSEST(clock->vco, clock->p);
-
+end:
 	return clock->dot;
 }
 
@@ -326,11 +328,13 @@ int i9xx_calc_dpll_params(int refclk, struct dpll *clock)
 {
 	clock->m = i9xx_dpll_compute_m(clock);
 	clock->p = clock->p1 * clock->p2;
-	if (WARN_ON(clock->n + 2 == 0 || clock->p == 0))
-		return 0;
+	if (WARN_ON(clock->n + 2 == 0 || clock->p == 0)) {
+		clock->dot = 0;
+		goto end;
+	}
 	clock->vco = DIV_ROUND_CLOSEST(refclk * clock->m, clock->n + 2);
 	clock->dot = DIV_ROUND_CLOSEST(clock->vco, clock->p);
-
+end:
 	return clock->dot;
 }
 
@@ -338,11 +342,13 @@ int vlv_calc_dpll_params(int refclk, struct dpll *clock)
 {
 	clock->m = clock->m1 * clock->m2;
 	clock->p = clock->p1 * clock->p2;
-	if (WARN_ON(clock->n == 0 || clock->p == 0))
-		return 0;
+	if (WARN_ON(clock->n == 0 || clock->p == 0)) {
+		clock->dot = 0;
+		goto end;
+	}
 	clock->vco = DIV_ROUND_CLOSEST(refclk * clock->m, clock->n);
 	clock->dot = DIV_ROUND_CLOSEST(clock->vco, clock->p);
-
+end:
 	return clock->dot / 5;
 }
 
@@ -350,12 +356,14 @@ int chv_calc_dpll_params(int refclk, struct dpll *clock)
 {
 	clock->m = clock->m1 * clock->m2;
 	clock->p = clock->p1 * clock->p2;
-	if (WARN_ON(clock->n == 0 || clock->p == 0))
-		return 0;
+	if (WARN_ON(clock->n == 0 || clock->p == 0)) {
+		clock->dot = 0;
+		goto end;
+	}
 	clock->vco = DIV_ROUND_CLOSEST_ULL(mul_u32_u32(refclk, clock->m),
 					   clock->n << 22);
 	clock->dot = DIV_ROUND_CLOSEST(clock->vco, clock->p);
-
+end:
 	return clock->dot / 5;
 }
 
-- 
2.26.3