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

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

On Tue, Jun 2, 2009 at 5:12 PM, Grant Likely <grant.likely@secretlab.ca> wrote:
> 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);
>       }

BTW, since you're touching these lines anyway, please clean up the
whitespace usage.  Since you have to break the line anyway, it no
longer makes sense for the "ret = " to be on a separate line anymore.

g.

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