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

[Joakim Tjernlund]

>
> Wolfram Sang wrote:
> > On Thu, May 14, 2009 at 10:10:03AM +0200, Esben Haabendal wrote:
> >
> >> This fixes MAL (arbitration lost) bug caused by illegal use of
> >> RSTA (repeated START) after STOP condition generated after last byte
> >> of reads.
> >>
> >>
> >
> > Could you elaborate a bit, please? Like an example when the original bug
> > occured (so I could reproduce easily) and how this patch actually solves
> > the problem.
> >
> Sure. I used the following code to initialize a CS4265 chip:
> (simplified here for the example)
>
> static int
> i2c_init_chip(void)
> {
>     int err=0;
>     char write_dac_control_1[] = { 0x03, chip->regs.dac_control_1 };
>     char seek_chip_id[] = { 0x01 };
>     char write_power_control[] = { 0x02, chip->regs.power_control };
>     struct i2c_msg cs4265_init_cmds[] = {
>         { chip->i2c_addr, 0, 2, write_dac_control_1 },
>         { chip->i2c_addr, 0, 1, seek_chip_id },
>         { chip->i2c_addr, I2C_M_RD, 1, &regs.chip_id },
>         { chip->i2c_addr, 0, 2, write_power_control },
>     };
>
>     err = i2c_transfer(i2c, cs4265_init_cmds,
>                sizeof(cs4265_init_cmds)/sizeof(*cs4265_init_cmds));
>     if (err < 0) {
>         printk(KERN_ERR "i2c_transfer failed: %d\n", err);
>         return err;
>     }
>
>     return 0;
> }
>
> I have added some additional debug output and traces something like this:
>
> [   36.114297] writeccr 80 [MEN]
> [   36.114324] mpc_xfer: 2 bytes to 4f:W - 1 of 5 messages
> [   36.120176] next 1 bytes to 4f:W
> [   36.124159] mpc_write addr=4f len=2
> [   36.128044] writeccr 80 [MEN]
> [   36.128062] writeccr f0 [MEN MIEN MSTA MTX] -> START
> [   36.128219] I2C: SR=a2
> [   36.131528] I2C: SR=a6
> [   36.134406] I2C: SR=a2
>
> [   36.137165] mpc_xfer: 1 bytes to 4f:W - 2 of 5 messages
> [   36.142802] next 1 bytes to 4f:R
> [   36.146699] mpc_write addr=4f len=1
> [   36.150583] writeccr f4 [MEN MIEN MSTA MTX RSTA] -> repeated START
> [   36.150758] I2C: SR=a2
> [   36.153851] I2C: SR=a6
>
> [   36.156236] mpc_xfer: 1 bytes to 4f:R - 3 of 5 messages
> [   36.162081] next 2 bytes to 4f:W
> [   36.166062] mpc_read addr=4f len=1
> [   36.169858] writeccr f4 [MEN MIEN MSTA MTX RSTA] -> repeated START
> [   36.170031] I2C: SR=a6
> [   36.172993] writeccr e8 [MEN MIEN MSTA TXAK] -> receive & no ACK
> [   36.173143] I2C: SR=a7
> [   36.175511] writeccr c8 [MEN MIEN TXAK] -> STOP
>
> [   36.175529] mpc_xfer: 2 bytes to 4f:W - 4 of 5 messages
> [   36.181804] next 2 bytes to 4f:W
> [   36.185788] mpc_write addr=4f len=2
> [   36.189672] writeccr f4 [MEN MIEN MSTA MTX RSTA] -> repeated START
> [   36.189704] I2C: SR=b7
> [   36.192072] I2C: MAL
> [   36.195075] i2c_wait(address) error: -5
> [   36.199311] writeccr 80
>
> The problem is that after the STOP condition, the following i2c_msg will be
> attempted with a repeated START, which according to specification will
> cause a MAL, which also happens.  My MPC8360ERM reads:
>
> "Attempting a repeated START at the wrong time (or if the bus is owned
> by another master), results in loss of arbitration."
>
> Which I know read as it that we must own the I2C bus when using RSTA.

I agree with the theory, but isn't the problem here that STOP is performed in
the middle of this transaction?
Remove the STOP and RSTA will work I think.

 Jocke