[PATCH v6] i2c: designware: Enable SAR in slave mode for Multiple SAR controllers

[Umesh Kumar <ukumar@marvell.com>]

DesignWare controllers synthesised with the Multiple SAR feature (e.g.
Marvell CN20K) gate the primary slave address behind IC_ENABLE.SAR_EN
(bit 19). Per the Synopsys DW_apb_i2c DataBook v2.03a (Multiple SAR
feature), when this bit is set the core acknowledges the address matching
IC_SAR; otherwise the address is NACKed. The slave enable path writes only
IC_ENABLE.ENABLE (bit 0), so on such controllers the target never ACKs
its address and a remote master cannot reach it.

Set SAR_EN together with ENABLE when entering slave mode. The bit is
reserved (and ignored) on single-SAR controllers, so existing users are
unaffected.

Signed-off-by: Umesh Kumar <ukumar@marvell.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
Changes in v6:
 - Added Andy's Reviewed-by (missed in v5).

Changes in v5:
 - Fixed missing spaces in commit message.

Changes in v4:
 - Added DataBook version (v2.03a); removed duplicate comment from core.h.

Changes in v3:
 - Referenced the Synopsys DW_apb_i2c DataBook (Andy Shevchenko).

Changes in v2:
 - Dropped RFC tag; added Mika's Acked-by.

v1 (RFC): https://lore.kernel.org/linux-i2c/20260804041842.3205356-1-ukumar@marvell.com/

 drivers/i2c/busses/i2c-designware-common.c | 10 +++++++++-
 drivers/i2c/busses/i2c-designware-core.h   |  1 +
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/drivers/i2c/busses/i2c-designware-common.c b/drivers/i2c/busses/i2c-designware-common.c
index 4dc57fd56170..880b9a74b3ff 100644
--- a/drivers/i2c/busses/i2c-designware-common.c
+++ b/drivers/i2c/busses/i2c-designware-common.c
@@ -374,7 +374,15 @@ static void i2c_dw_configure_mode(struct dw_i2c_dev *dev, int mode)
 		regmap_write(dev->map, DW_IC_CON, dev->slave_cfg);
 		regmap_write(dev->map, DW_IC_SAR, dev->slave->addr);
 		regmap_write(dev->map, DW_IC_INTR_MASK, DW_IC_INTR_SLAVE_MASK);
-		__i2c_dw_enable(dev);
+		/*
+		 * Per the Synopsys DW_apb_i2c DataBook v2.03a (Multiple SAR
+		 * feature), controllers synthesised with multiple SARs only
+		 * acknowledge traffic to IC_SAR when IC_ENABLE.SAR_EN is set;
+		 * a plain enable is not enough. The bit is reserved (ignored) on
+		 * single-SAR controllers.
+		 */
+		regmap_write(dev->map, DW_IC_ENABLE,
+			     DW_IC_ENABLE_ENABLE | DW_IC_ENABLE_SAR_EN);
 		break;
 	default:
 		WARN(1, "Invalid mode %d\n", mode);
diff --git a/drivers/i2c/busses/i2c-designware-core.h b/drivers/i2c/busses/i2c-designware-core.h
index 9d8d104cc391..1390c8862a68 100644
--- a/drivers/i2c/busses/i2c-designware-core.h
+++ b/drivers/i2c/busses/i2c-designware-core.h
@@ -125,6 +125,7 @@
 
 #define DW_IC_ENABLE_ENABLE			BIT(0)
 #define DW_IC_ENABLE_ABORT			BIT(1)
+#define DW_IC_ENABLE_SAR_EN			BIT(19)
 
 #define DW_IC_STATUS_ACTIVITY			BIT(0)
 #define DW_IC_STATUS_TFE			BIT(2)
-- 
2.43.0