[PATCH v1 1/3] misc: amd-sbi: use sized DMA-safe I3C tx/rx buffers

[Akshay Gupta <Akshay.Gupta@amd.com>]

From: Prathima Lk <Prathima.Lk@amd.com>

This change replaces the open-coded tx[2] array and single-byte scalar rx
field in struct sbtsi_i3c_priv with named, sized buffers (SBTSI_TX_LEN and
SBTSI_RX_LEN) and makes rx a proper array. The motivation is an upcoming
patch adding the SB-TSI Trace Buffer Acquire Interface, which shares the
same I3C target core and needs larger buffers. Preparing the sizes now
lets that follow-up patch grow the buffers without reworking the struct or
register access code.
Each buffer stays aligned to ARCH_DMA_MINALIGN for
DMA safety on noncoherent platforms. Affected areas are tsi-core.c and
tsi-core.h.
Tested by building the driver and verifying I3C register reads
and writes still function correctly on the target hardware.

Signed-off-by: Prathima Lk <Prathima.Lk@amd.com>
Signed-off-by: Akshay Gupta <Akshay.Gupta@amd.com>
---
 drivers/misc/amd-sbi/tsi-core.c | 4 ++--
 drivers/misc/amd-sbi/tsi-core.h | 8 ++++++--
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/misc/amd-sbi/tsi-core.c b/drivers/misc/amd-sbi/tsi-core.c
index 5c178702c67a..1920e4dc7b67 100644
--- a/drivers/misc/amd-sbi/tsi-core.c
+++ b/drivers/misc/amd-sbi/tsi-core.c
@@ -60,13 +60,13 @@ static int sbtsi_i3c_read(struct sbtsi_data *data, u8 reg, u8 *val)
 	/* Read the data byte into a separate buffer (DMA_FROM_DEVICE). */
 	xfers[1].rnw = true;
 	xfers[1].len = 1;
-	xfers[1].data.in = &priv->rx;
+	xfers[1].data.in = priv->rx;
 
 	ret = i3c_device_do_xfers(data->i3cdev, xfers, 2, I3C_SDR);
 	if (ret)
 		return ret;
 
-	*val = priv->rx;
+	*val = priv->rx[0];
 	return ret;
 }
 
diff --git a/drivers/misc/amd-sbi/tsi-core.h b/drivers/misc/amd-sbi/tsi-core.h
index 4cf55c46230e..8efe7e220f4a 100644
--- a/drivers/misc/amd-sbi/tsi-core.h
+++ b/drivers/misc/amd-sbi/tsi-core.h
@@ -11,6 +11,10 @@
 #include <linux/cache.h>
 #include <linux/misc/tsi.h>
 
+/* SB-TSI register access transaction sizes in bytes. */
+#define SBTSI_TX_LEN	2	/* [0] register address, [1] value */
+#define SBTSI_RX_LEN	1	/* single data byte */
+
 /**
  * struct sbtsi_i3c_priv - per-device state for I3C SBTSI (includes DMA-safe buffers)
  * @data: public device state exposed via dev_set_drvdata()
@@ -19,8 +23,8 @@
  */
 struct sbtsi_i3c_priv {
 	struct sbtsi_data data;
-	u8 tx[2];
-	u8 rx __aligned(ARCH_DMA_MINALIGN);
+	u8 tx[SBTSI_TX_LEN] __aligned(ARCH_DMA_MINALIGN);
+	u8 rx[SBTSI_RX_LEN] __aligned(ARCH_DMA_MINALIGN);
 };
 
 int create_misc_tsi_device(struct sbtsi_data *data, struct device *dev);
-- 
2.34.1