Linux, I2C_RDWR, stop conditions

[Joakim Tjernlund]

> Replying to myself:
>
> Le dim 29/06/2003 à 14:24, Albert ARIBAUD a écrit :
> >
> > I am experiencing an annoying I2C problem, using ELDK 2.0.2 and its 2.4
> > kernel, while trying to read an I2C 24C128 eeprom. Its read sequence is
> > composed as follows :
> > - start condition, slave write of two bytes : high then low part of adr.
> > - start condition, slave read of up to 64 bytes.
> > - stop condition.
> > (notice no stop condition in between: this is a double start).
> >
> > Using either the r360 adapter or the rpx adapter, I tried a cyclic read
> > with a while-loop at shell level calling a C program doing the eeprom
> > read. This works between a couple and several hundred reads, then stops
> > with te following condition : the data read part fails with a short read
> > of 1, and from then on i2c transfers find a low SDA.
> >
> > This looked like chip errata CPM6.I2C (single-byte buffer after failed
> > transaction) and CPM7.I2C (I2C receiver locks holding SDA low), however:
> > - I use an MPC855T mask rev D.4 which claims all I2C errata fixed;
> > - the previous write shows no sign of having failed (so no CPM6.I2C)
> > - I have PDIV=00 and FLT=1, so it would rule out CPM7.I2C.
> >
> > This not an I2C/SPI relocation case either, since SCC1 is used as an
> > UART and SCC2 does not exist in the 855T.
> >
> > However, I have checked i2c drivers in the following code:
> > - Linux kernel 2.4.4 provided with ELDK 2.0.2;
> > - linux-2.4 CVS from Denx;
> > - i2c package v2.7.0 from lm_sensors.
> > All versions share the following characteristic in the the I2C_RDWR
> > ioctl implementation: i2c-algo-8xx simply concatenates i2c reads and
> > writes (thus ending each individual transfer with a stop condition),
> > while i2c-algo-pcf and i2c-algo-bit do not send stop conditions until
> > after the last transfer.
> >
> > Am I right in thinking that the correct I2C_RDWD behaviour is that of
> > pcf and bit-banging, and that the 8xx implementation is wrong in this
> > respect?
> >
> > And would it explain the behaviour I see with the 24C128 eeprom?
> >
> > Thanks in advance,
> >
> > Albert.
> I have implemented a more correctly-behaving, albeit badly-styled,
> solution by heavily patching i2c-algo-8xx.c and i2c-rpx.c. Miracle : the
> eeprom that I could not write to suddenly works like a charm, and reads
> seem not to break anymore. Hurray!
>
> Well, to be honest, I have a small quirk left (wouldn't be fun if it all
> worked OK, would it?): when doing an i2c read, I sometimes get TXE set
> in I2CER along with the expected RXB. However, the data was read
> correctly, as far as I can tell. I'll run a check (zero-fill the receive
> buffer before starting the I2C exchange.
>
> Broadly, my solution prepares as many Tx and Rx buffers as required to
> realize the whole exchange in a single run. This effectively makes the
> I2C_RDWR ioctl work as expected.
>
> I expect few people were using I2C_RDWR with double-start sequences,
> since they would have failed :), but anyone simply interested in i2c on
> the MPC8xx may have a look at the code and try it; I would like to know
> if this approach is a good candidate for a patch submission, both to
> lm_sensors and the linux ppc kernel, so a little regression testing
> would only be fair. :)
>
> Note, however, that my solution is currently a bit lavish with
> allocating Rx and Tx BDs; before considering a patch, I will change it
> so that it creates Rx/TxBD templates in kmalloc()ed memory, and 'hot
> swap' them in real time into the 2 Rx and 2 Tx BDs currently allocated
> in i2c-rpx.c.
>
> All the best,
>

Hi Albert

A few months ago I fixed a few bugs in the i2c-algo-8xx.c and submitted these
to linuxppc_2_4_devel. I too had problems similar to yours. I suggest
you check if your version matches the one in linuxppc_2_4_devel.

 Jocke


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