i2c: xlr: fix extra read/write at end of rx transfer

The BYTECNT register holds the transfer size minus one.  Setting it to
the correct value removes the need for a dummy read/write at the end of
each transfer.  As zero-length transfers are not supported, do not
advertise I2C_FUNC_SMBUS_QUICK.

In other words, this patch makes the driver transfer the number of bytes
requested unless this is zero, which is not supported by the hardware
and is thus refused.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

1 files changed, 27 insertions, 23 deletions

diff --git a/drivers/i2c/busses/i2c-xlr.c b/drivers/i2c/busses/i2c-xlr.c
index 10fb916fa8c7..2cfaba7c1f92 100644
--- a/drivers/i2c/busses/i2c-xlr.c
+++ b/drivers/i2c/busses/i2c-xlr.c
@@ -89,38 +89,43 @@ static int xlr_i2c_tx(struct xlr_i2c_private *priv,  u16 len,
         unsigned long timeout, stoptime, checktime;
         u32 i2c_status;
         int pos, timedout;
-        u8 offset, byte;
+        u8 offset;
+        u32 xfer;
+
+        if (!len)
+                return -EOPNOTSUPP;
 
         offset = buf[0];
         xlr_i2c_wreg(priv->iobase, XLR_I2C_ADDR, offset);
         xlr_i2c_wreg(priv->iobase, XLR_I2C_DEVADDR, addr);
         xlr_i2c_wreg(priv->iobase, XLR_I2C_CFG,
                         XLR_I2C_CFG_ADDR | priv->cfg->cfg_extra);
-        xlr_i2c_wreg(priv->iobase, XLR_I2C_BYTECNT, len - 1);
 
         timeout = msecs_to_jiffies(XLR_I2C_TIMEOUT);
         stoptime = jiffies + timeout;
         timedout = 0;
-        pos = 1;
-retry:
+
         if (len == 1) {
-                xlr_i2c_wreg(priv->iobase, XLR_I2C_STARTXFR,
-                                XLR_I2C_STARTXFR_ND);
+                xlr_i2c_wreg(priv->iobase, XLR_I2C_BYTECNT, len - 1);
+                xfer = XLR_I2C_STARTXFR_ND;
+                pos = 1;
         } else {
-                xlr_i2c_wreg(priv->iobase, XLR_I2C_DATAOUT, buf[pos]);
-                xlr_i2c_wreg(priv->iobase, XLR_I2C_STARTXFR,
-                                XLR_I2C_STARTXFR_WR);
+                xlr_i2c_wreg(priv->iobase, XLR_I2C_BYTECNT, len - 2);
+                xlr_i2c_wreg(priv->iobase, XLR_I2C_DATAOUT, buf[1]);
+                xfer = XLR_I2C_STARTXFR_WR;
+                pos = 2;
         }
 
+retry:
+        /* retry can only happen on the first byte */
+        xlr_i2c_wreg(priv->iobase, XLR_I2C_STARTXFR, xfer);
+
         while (!timedout) {
                 checktime = jiffies;
                 i2c_status = xlr_i2c_rdreg(priv->iobase, XLR_I2C_STATUS);
 
-                if (i2c_status & XLR_I2C_SDOEMPTY) {
-                        pos++;
-                        /* need to do a empty dataout after the last byte */
-                        byte = (pos < len) ? buf[pos] : 0;
-                        xlr_i2c_wreg(priv->iobase, XLR_I2C_DATAOUT, byte);
+                if ((i2c_status & XLR_I2C_SDOEMPTY) && pos < len) {
+                        xlr_i2c_wreg(priv->iobase, XLR_I2C_DATAOUT, buf[pos++]);
 
                         /* reset timeout on successful xmit */
                         stoptime = jiffies + timeout;
@@ -149,11 +154,13 @@ static int xlr_i2c_rx(struct xlr_i2c_private *priv, u16 len, u8 *buf, u16 addr)
         u32 i2c_status;
         unsigned long timeout, stoptime, checktime;
         int nbytes, timedout;
-        u8 byte;
+
+        if (!len)
+                return -EOPNOTSUPP;
 
         xlr_i2c_wreg(priv->iobase, XLR_I2C_CFG,
                         XLR_I2C_CFG_NOADDR | priv->cfg->cfg_extra);
-        xlr_i2c_wreg(priv->iobase, XLR_I2C_BYTECNT, len);
+        xlr_i2c_wreg(priv->iobase, XLR_I2C_BYTECNT, len - 1);
         xlr_i2c_wreg(priv->iobase, XLR_I2C_DEVADDR, addr);
 
         timeout = msecs_to_jiffies(XLR_I2C_TIMEOUT);
@@ -167,14 +174,11 @@ retry:
                 checktime = jiffies;
                 i2c_status = xlr_i2c_rdreg(priv->iobase, XLR_I2C_STATUS);
                 if (i2c_status & XLR_I2C_RXRDY) {
-                        if (nbytes > len)
+                        if (nbytes >= len)
                                 return -EIO;    /* should not happen */
 
-                        /* we need to do a dummy datain when nbytes == len */
-                        byte = xlr_i2c_rdreg(priv->iobase, XLR_I2C_DATAIN);
-                        if (nbytes < len)
-                                buf[nbytes] = byte;
-                        nbytes++;
+                        buf[nbytes++] =
+                                xlr_i2c_rdreg(priv->iobase, XLR_I2C_DATAIN);
 
                         /* reset timeout on successful read */
                         stoptime = jiffies + timeout;
@@ -228,7 +232,7 @@ static int xlr_i2c_xfer(struct i2c_adapter *adap,
 static u32 xlr_func(struct i2c_adapter *adap)
 {
         /* Emulate SMBUS over I2C */
-        return I2C_FUNC_SMBUS_EMUL | I2C_FUNC_I2C;
+        return (I2C_FUNC_SMBUS_EMUL & ~I2C_FUNC_SMBUS_QUICK) | I2C_FUNC_I2C;
 }
 
 static struct i2c_algorithm xlr_i2c_algo = {