[PATCH] phy: stm32: work around constant-value overflow assertion

[Arnd Bergmann <arnd@kernel.org>]

From: Arnd Bergmann <arnd@arndb.de>

FIELD_PREP() checks that a constant fits into the available bitfield,
but if one of the two lookup tables in stm32_impedance_tune() does
not find a matching entry, the index is out of range, which gcc
correctly complains about:

In file included from <command-line>:
In function 'stm32_impedance_tune',
    inlined from 'stm32_combophy_pll_init' at drivers/phy/st/phy-stm32-combophy.c:247:9:
include/linux/compiler_types.h:517:38: error: call to '__compiletime_assert_447' declared with attribute error: FIELD_PREP: value too large for the field
  517 |  _compiletime_assert(condition, msg, __compiletime_assert_, __COUNTER__)
      |                                      ^
include/linux/bitfield.h:68:3: note: in expansion of macro 'BUILD_BUG_ON_MSG'
   68 |   BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ?  \
  115 |   __BF_FIELD_CHECK(_mask, 0ULL, _val, "FIELD_PREP: "); \
      |   ^~~~~~~~~~~~~~~~
drivers/phy/st/phy-stm32-combophy.c:162:8: note: in expansion of macro 'FIELD_PREP'
  162 |        FIELD_PREP(STM32MP25_PCIEPRG_IMPCTRL_VSWING, vswing_of));
      |        ^~~~~~~~~~

Rework this so the field value gets set inside of the loop and otherwise
set to zero.

Fixes: 47e1bb6b4ba0 ("phy: stm32: Add support for STM32MP25 COMBOPHY.")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/phy/st/phy-stm32-combophy.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/drivers/phy/st/phy-stm32-combophy.c b/drivers/phy/st/phy-stm32-combophy.c
index 765bb34fe358..49e9fa90a681 100644
--- a/drivers/phy/st/phy-stm32-combophy.c
+++ b/drivers/phy/st/phy-stm32-combophy.c
@@ -122,6 +122,7 @@ static int stm32_impedance_tune(struct stm32_combophy *combophy)
 	u32 max_vswing = imp_lookup[imp_size - 1].vswing[vswing_size - 1];
 	u32 min_vswing = imp_lookup[0].vswing[0];
 	u32 val;
+	u32 regval;
 
 	if (!of_property_read_u32(combophy->dev->of_node, "st,output-micro-ohms", &val)) {
 		if (val < min_imp || val > max_imp) {
@@ -129,16 +130,20 @@ static int stm32_impedance_tune(struct stm32_combophy *combophy)
 			return -EINVAL;
 		}
 
-		for (imp_of = 0; imp_of < ARRAY_SIZE(imp_lookup); imp_of++)
-			if (imp_lookup[imp_of].microohm <= val)
+		regval = 0;
+		for (imp_of = 0; imp_of < ARRAY_SIZE(imp_lookup); imp_of++) {
+			if (imp_lookup[imp_of].microohm <= val) {
+				regval = FIELD_PREP(STM32MP25_PCIEPRG_IMPCTRL_OHM, imp_of);
 				break;
+			}
+		}
 
 		dev_dbg(combophy->dev, "Set %u micro-ohms output impedance\n",
 			imp_lookup[imp_of].microohm);
 
 		regmap_update_bits(combophy->regmap, SYSCFG_PCIEPRGCR,
 				   STM32MP25_PCIEPRG_IMPCTRL_OHM,
-				   FIELD_PREP(STM32MP25_PCIEPRG_IMPCTRL_OHM, imp_of));
+				   regval);
 	} else {
 		regmap_read(combophy->regmap, SYSCFG_PCIEPRGCR, &val);
 		imp_of = FIELD_GET(STM32MP25_PCIEPRG_IMPCTRL_OHM, val);
@@ -150,16 +155,20 @@ static int stm32_impedance_tune(struct stm32_combophy *combophy)
 			return -EINVAL;
 		}
 
-		for (vswing_of = 0; vswing_of < ARRAY_SIZE(imp_lookup[imp_of].vswing); vswing_of++)
-			if (imp_lookup[imp_of].vswing[vswing_of] >= val)
+		regval = 0;
+		for (vswing_of = 0; vswing_of < ARRAY_SIZE(imp_lookup[imp_of].vswing); vswing_of++) {
+			if (imp_lookup[imp_of].vswing[vswing_of] >= val) {
+				regval = FIELD_PREP(STM32MP25_PCIEPRG_IMPCTRL_VSWING, vswing_of);
 				break;
+			}
+		}
 
 		dev_dbg(combophy->dev, "Set %u microvolt swing\n",
 			 imp_lookup[imp_of].vswing[vswing_of]);
 
 		regmap_update_bits(combophy->regmap, SYSCFG_PCIEPRGCR,
 				   STM32MP25_PCIEPRG_IMPCTRL_VSWING,
-				   FIELD_PREP(STM32MP25_PCIEPRG_IMPCTRL_VSWING, vswing_of));
+				   regval);
 	}
 
 	return 0;
-- 
2.39.5