w1: ds2408: reset on output_write retry with readback [Linux 5.2]

w1: ds2408: reset on output_write retry with readback [Linux 5.2]

This Linux kernel change "w1: ds2408: reset on output_write retry with readback" is included in the Linux 5.2 release. This change is authored by Jean-Francois Dagenais <jeff.dagenais [at] gmail.com> on Thu Mar 28 12:41:11 2019 -0400. The commit for this change in Linux stable tree is 49695ac (patch).

w1: ds2408: reset on output_write retry with readback

When we have success in 'Channel Access Write' but reading back latch
states fails, a write is retried without doing a proper slave reset.
This leads to protocol errors as the slave treats the next 'Channel
Access Write' as the continuation of previous command.

This commit is fixing this by making sure if the retry loop re-runs, a
reset is performed, whatever the failure (CONFIRM_BYTE or the read
back).

The loop was quite due for a cleanup and this change mandated it. By
isolating the CONFIG_W1_SLAVE_DS2408_READBACK case into it's own
function, we vastly reduce the visual and branching(runtime and
compile-time) noise.

Reported-by: Mariusz Bialonczyk <manio@skyboo.net>
Tested-by: Mariusz Bialonczyk <manio@skyboo.net>
Signed-off-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

There are 76 lines of Linux source code added/deleted in this change. Code changes to Linux kernel are as follows.

 drivers/w1/slaves/w1_ds2408.c | 76 ++++++++++++++++++++++---------------------
 1 file changed, 39 insertions(+), 37 deletions(-)

diff --git a/drivers/w1/slaves/w1_ds2408.c b/drivers/w1/slaves/w1_ds2408.c
index b535d5e..92e8f07 100644
--- a/drivers/w1/slaves/w1_ds2408.c
+++ b/drivers/w1/slaves/w1_ds2408.c
@@ -138,14 +138,37 @@ static ssize_t status_control_read(struct file *filp, struct kobject *kobj,
        W1_F29_REG_CONTROL_AND_STATUS, buf);
 }

+#ifdef fCONFIG_W1_SLAVE_DS2408_READBACK
+static bool optional_read_back_valid(struct w1_slave *sl, u8 expected)
+{
+   u8 w1_buf[3];
+
+   if (w1_reset_resume_command(sl->master))
+       return false;
+
+   w1_buf[0] = W1_F29_FUNC_READ_PIO_REGS;
+   w1_buf[1] = W1_F29_REG_OUTPUT_LATCH_STATE;
+   w1_buf[2] = 0;
+
+   w1_write_block(sl->master, w1_buf, 3);
+
+   return (w1_read_8(sl->master) == expected);
+}
+#else
+static bool optional_read_back_valid(struct w1_slave *sl, u8 expected)
+{
+   return true;
+}
+#endif
+
 static ssize_t output_write(struct file *filp, struct kobject *kobj,
                struct bin_attribute *bin_attr, char *buf,
                loff_t off, size_t count)
 {
    struct w1_slave *sl = kobj_to_w1_slave(kobj);
    u8 w1_buf[3];
-   u8 readBack;
    unsigned int retries = W1_F29_RETRIES;
+   ssize_t bytes_written = -EIO;

    if (count != 1 || off != 0)
        return -EFAULT;
@@ -155,54 +178,33 @@ static ssize_t output_write(struct file *filp, struct kobject *kobj,
    dev_dbg(&sl->dev, "mutex locked");

    if (w1_reset_select_slave(sl))
-       goto error;
+       goto out;

-   while (retries--) {
+   do {
        w1_buf[0] = W1_F29_FUNC_CHANN_ACCESS_WRITE;
        w1_buf[1] = *buf;
        w1_buf[2] = ~(*buf);
-       w1_write_block(sl->master, w1_buf, 3);

-       readBack = w1_read_8(sl->master);
+       w1_write_block(sl->master, w1_buf, 3);

-       if (readBack != W1_F29_SUCCESS_CONFIRM_BYTE) {
-           if (w1_reset_resume_command(sl->master))
-               goto error;
-           /* try again, the slave is ready for a command */
-           continue;
+       if (w1_read_8(sl->master) == W1_F29_SUCCESS_CONFIRM_BYTE &&
+           optional_read_back_valid(sl, *buf)) {
+           bytes_written = 1;
+           goto out;
        }

-#ifdef CONFIG_W1_SLAVE_DS2408_READBACK
-       /* here the master could read another byte which
-          would be the PIO reg (the actual pin logic state)
-          since in this driver we don't know which pins are
-          in and outs, there's no value to read the state and
-          compare. with (*buf) so end this command abruptly: */
        if (w1_reset_resume_command(sl->master))
-           goto error;
+           goto out; /* unrecoverable error */
+       /* try again, the slave is ready for a command */
+   } while (--retries);

-       /* go read back the output latches */
-       /* (the direct effect of the write above) */
-       w1_buf[0] = W1_F29_FUNC_READ_PIO_REGS;
-       w1_buf[1] = W1_F29_REG_OUTPUT_LATCH_STATE;
-       w1_buf[2] = 0;
-       w1_write_block(sl->master, w1_buf, 3);
-       /* read the result of the READ_PIO_REGS command */
-       if (w1_read_8(sl->master) == *buf)
-#endif
-       {
-           /* success! */
-           mutex_unlock(&sl->master->bus_mutex);
-           dev_dbg(&sl->dev,
-               "mutex unlocked, retries:%d", retries);
-           return 1;
-       }
-   }
-error:
+out:
    mutex_unlock(&sl->master->bus_mutex);
-   dev_dbg(&sl->dev, "mutex unlocked in error, retries:%d", retries);

-   return -EIO;
+   dev_dbg(&sl->dev, "%s, mutex unlocked retries:%d\n",
+       (bytes_written > 0) ? "succeeded" : "error", retries);
+
+   return bytes_written;
 }

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