Re: [PATCH v2 7/7] i2c: omap: implement handling for 'transferred' bytes

[Paul Walmsley]

Hi

On Thu, 25 Oct 2012, Felipe Balbi wrote:

 this is important in cases where client driver
 wants to know how many bytes were actually
 transferred.
 
 There is one trick here: if transfer is completed,
 meaning I2C_CNT reaches zero, then ARDY will be
 asserted to let SW know that it can program a
 new transfer.
 
 When ARDY is asserted, I2C_CNT is reset to the
 original value (msg-len), which means that
 for a successful message, msg-transferred = msg-len
 and we don't need to spend time with a register
 read.
 
 In case of NACK condition, however, I2C_CNT will
 remain with the end value which is the amount of
 data transferred until NACK condition found on the
 bus inclusive. In this situation, msg-transferred
 needs to be initialized with:
 
 msg-len - read(I2C_CNT) - 1;
 
 This patch implements exactly that handling.
 
 Signed-off-by: Felipe Balbi ba...@ti.com
 ---
  drivers/i2c/busses/i2c-omap.c | 12 
  1 file changed, 12 insertions(+)
 
 diff --git a/drivers/i2c/busses/i2c-omap.c b/drivers/i2c/busses/i2c-omap.c
 index 699fa12..d268e92 100644
 --- a/drivers/i2c/busses/i2c-omap.c
 +++ b/drivers/i2c/busses/i2c-omap.c
 @@ -588,6 +588,9 @@ static int omap_i2c_xfer_msg(struct i2c_adapter *adap,
   goto out;
   }
  
 + msg-transferred = msg-len;
 + wmb();

If this barrier is being used to ensure that the compiler doesn't move 
this store past the point at which the I2C interrupt handler can be 
entered, then please consider the same comments as on the other 
patch that adds a barrier:

http://marc.info/?l=linux-omapm=135129247524136w=2

http://marc.info/?l=linux-omapm=135135357305794w=2


- Paul
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[PATCH v2 7/7] i2c: omap: implement handling for 'transferred' bytes

[Felipe Balbi]

this is important in cases where client driver
wants to know how many bytes were actually
transferred.

There is one trick here: if transfer is completed,
meaning I2C_CNT reaches zero, then ARDY will be
asserted to let SW know that it can program a
new transfer.

When ARDY is asserted, I2C_CNT is reset to the
original value (msg-len), which means that
for a successful message, msg-transferred = msg-len
and we don't need to spend time with a register
read.

In case of NACK condition, however, I2C_CNT will
remain with the end value which is the amount of
data transferred until NACK condition found on the
bus inclusive. In this situation, msg-transferred
needs to be initialized with:

msg-len - read(I2C_CNT) - 1;

This patch implements exactly that handling.

Signed-off-by: Felipe Balbi ba...@ti.com
---
 drivers/i2c/busses/i2c-omap.c | 12 
 1 file changed, 12 insertions(+)

diff --git a/drivers/i2c/busses/i2c-omap.c b/drivers/i2c/busses/i2c-omap.c
index 699fa12..d268e92 100644
--- a/drivers/i2c/busses/i2c-omap.c
+++ b/drivers/i2c/busses/i2c-omap.c
@@ -588,6 +588,9 @@ static int omap_i2c_xfer_msg(struct i2c_adapter *adap,
goto out;
}
 
+   msg-transferred = msg-len;
+   wmb();
+
/* We have an error */
if (dev-cmd_err  (OMAP_I2C_STAT_AL | OMAP_I2C_STAT_ROVR |
OMAP_I2C_STAT_XUDF)) {
@@ -597,6 +600,15 @@ static int omap_i2c_xfer_msg(struct i2c_adapter *adap,
}
 
if (dev-cmd_err  OMAP_I2C_STAT_NACK) {
+   /* In case of a NACK, we need to check how many bytes we
+* actually transferred, so we can tell our client driver about
+* it.
+*
+* Let's check it here and overwrite msg-transferred.
+*/
+   w = omap_i2c_read_reg(dev, OMAP_I2C_CNT_REG);
+   msg-transferred = msg-len - w - 1;
+
if (msg-flags  I2C_M_IGNORE_NAK)
return 0;
 
-- 
1.8.0

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