Re: [PATCH v2] i2c-mpc: generate START condition after STOP caused by read i2c_msg

[Grant Likely <grant.likely@secretlab.ca>]

Hi Esben and Ben,

Sorry for the lateness in reviewing this.  FWIW, here are my comments.

g.

On Mon, May 18, 2009 at 11:22 PM, Esben Haabendal
<eha@doredevelopment.dk> wrote:
> This fixes MAL (arbitration lost) bug caused by illegal use of
> RSTA (repeated START) after STOP condition generated after last byte
> of reads. With this patch, it is possible to do an i2c_transfer() with
> additional i2c_msg's following the I2C_M_RD messages.
>
> It still needs to be resolved if it is possible to fix this issue
> by removing the STOP condition after reads in a robust way.
>
> Signed-off-by: Esben Haabendal <eha@doredevelopment.dk>
> ---
>  drivers/i2c/busses/i2c-mpc.c |    9 +++++++--
>  1 files changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/i2c/busses/i2c-mpc.c b/drivers/i2c/busses/i2c-mpc.c
> index 4af5c09..0199f9a 100644
> --- a/drivers/i2c/busses/i2c-mpc.c
> +++ b/drivers/i2c/busses/i2c-mpc.c
> @@ -456,17 +456,22 @@ static int mpc_xfer(struct i2c_adapter *adap, struct
> i2c_msg *msgs, int num)
>        }
>
>        for (i = 0; ret >= 0 && i < num; i++) {
> +               int restart = i;
>                pmsg = &msgs[i];
>                dev_dbg(i2c->dev,
>                        "Doing %s %d bytes to 0x%02x - %d of %d messages\n",
>                        pmsg->flags & I2C_M_RD ? "read" : "write",
>                        pmsg->len, pmsg->addr, i + 1, num);
> +               if (i > 0 && ((pmsg - 1)->flags & I2C_M_RD))
> +                       restart = 0;
>                if (pmsg->flags & I2C_M_RD)
>                        ret =
> -                           mpc_read(i2c, pmsg->addr, pmsg->buf, pmsg->len,
> i);
> +                           mpc_read(i2c, pmsg->addr, pmsg->buf, pmsg->len,
> +                                    restart);
>                else
>                        ret =
> -                           mpc_write(i2c, pmsg->addr, pmsg->buf, pmsg->len,
> i);
> +                           mpc_write(i2c, pmsg->addr, pmsg->buf, pmsg->len,
> +                                     restart);
>        }

Hmmm, seems to be a rather convoluted code path.  Surely this could be
handled in a clearer way.  The whole (pmsg - 1) bit raises my eyebrows
(I'd rather see msgs[i-1]).  At the very least this deserves an
comment describing what it is doing.  The following might be better
for the next person who has to read this code:

+      int restart = 0;
       for (i = 0; ret >= 0 && i < num; i++) {
               pmsg = &msgs[i];
               dev_dbg(i2c->dev,
                       "Doing %s %d bytes to 0x%02x - %d of %d messages\n",
                       pmsg->flags & I2C_M_RD ? "read" : "write",
                       pmsg->len, pmsg->addr, i + 1, num);
               if (pmsg->flags & I2C_M_RD)
                       ret =
-                           mpc_read(i2c, pmsg->addr, pmsg->buf, pmsg->len,
i);
+                           mpc_read(i2c, pmsg->addr, pmsg->buf, pmsg->len,
+                                    restart);
               else
                       ret =
-                           mpc_write(i2c, pmsg->addr, pmsg->buf, pmsg->len,
i);
+                           mpc_write(i2c, pmsg->addr, pmsg->buf, pmsg->len,
+                                     restart);
+               /* Only set the restart flag if this was not an
I2C_M_RD transaction
+                * because it causes an illegal use of
+                * RSTA (repeated START) after STOP condition
generated after last byte
+                * of reads  */
+               restart = (pmsg->flags & I2C_M_RD == 0);
       }

g.

-- 
Grant Likely, B.Sc., P.Eng.
Secret Lab Technologies Ltd.