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 <m...@mansr.com>
---
Changes:
- refuse zero-length transfers and stop advertising I2C_FUNC_SMBUS_QUICK
---
 drivers/i2c/busses/i2c-xlr.c | 50 ++++++++++++++++++++++++--------------------
 1 file changed, 27 insertions(+), 23 deletions(-)

diff --git a/drivers/i2c/busses/i2c-xlr.c b/drivers/i2c/busses/i2c-xlr.c
index 10fb916..2cfaba7 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 = {
-- 
2.6.3

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