Replying to myself: Le dim 29/06/2003 ? 14:24, Albert ARIBAUD a ?crit : > > 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, Albert, simply happy to be able to write to an i2c eeprom. :) ** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/
