Dear Lucas Stach, > Am Mittwoch, den 03.07.2013, 06:37 +0200 schrieb Marek Vasut: > > Hi, > > > > > Dear Lucas Stach, > > > > > > > Am Montag, den 01.07.2013, 18:14 -0300 schrieb Fabio Estevam: > > > > > From: Fabio Estevam <fabio.estevam@xxxxxxxxxxxxx> > > > > > > > > > > Recently we have been seing some reports about PIO mode not working > > > > > properly. > > > > > > > > > > - http://www.spinics.net/lists/linux-i2c/msg11985.html > > > > > - http://marc.info/?l=linux-i2c&m=137235593101385&w=2 > > > > > - https://lkml.org/lkml/2013/6/24/430 > > > > > > > > > > Let's use DMA mode even for small transfers. > > > > > > > > > > Without this patch, i2c reads the incorrect sgtl5000 version on a > > > > > mx28evk when touchscreen is enabled: > > > > > > > > > > [ 5.856270] sgtl5000 0-000a: Device with ID register 0 is not a > > > > > sgtl5000 [ 9.877307] sgtl5000 0-000a: ASoC: failed to probe > > > > > CODEC -19 [ 9.883528] mxs-sgtl5000 sound.12: ASoC: failed to > > > > > instantiate card -19 [ 9.892955] mxs-sgtl5000 sound.12: > > > > > snd_soc_register_card failed (-19) > > > > > > > > > > Cc: <stable@xxxxxxxxxxxxxxx> > > > > > Signed-off-by: Fabio Estevam <fabio.estevam@xxxxxxxxxxxxx> > > > > > --- > > > > > Applies against 3.10 > > > > > > > > > > Changes since v1: > > > > > - Keep the PIO code > > > > > > > > > > drivers/i2c/busses/i2c-mxs.c | 2 +- > > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > > > diff --git a/drivers/i2c/busses/i2c-mxs.c > > > > > b/drivers/i2c/busses/i2c-mxs.c index 2039f23..6d8094d 100644 > > > > > --- a/drivers/i2c/busses/i2c-mxs.c > > > > > +++ b/drivers/i2c/busses/i2c-mxs.c > > > > > @@ -494,7 +494,7 @@ static int mxs_i2c_xfer_msg(struct i2c_adapter > > > > > *adap, struct i2c_msg *msg, > > > > > > > > > > * based on this empirical measurement and a lot of previous > > > > > frobbing. */ > > > > > > > > > > i2c->cmd_err = 0; > > > > > > > > > > - if (msg->len < 8) { > > > > > + if (0) { /* disable PIO mode until a proper fix is made */ > > > > > > > > > > ret = mxs_i2c_pio_setup_xfer(adap, msg, flags); > > > > > if (ret) > > > > > > > > > > mxs_i2c_reset(i2c); > > > > > > > > I still plan to take another look at the PIO mode, but higher > > > > priority things keep popping up in front of me. So this patch is: > > > > Acked-by: Lucas Stach <l.stach@xxxxxxxxxxxxxx> > > > > > > Ok, update. So far I arrived at a point where I can avoid the PIO > > > trouble. > > > > > > If I only do transfers shorter or equal in length to 3 bytes via PIO, > > > it works as expected. If the transfer is longer, the LA shows very > > > long transfer happening actually. Therefore, the pumping of data in > > > loop to/from PIO via the DATA register doesn't work. > > > > > > I will update you more later, once I figure out something else. Now I > > > need some sleep. > > > > I'm attaching a patch. Alex, please give it a go and see if it fixes your > > issue. It is _VERY_ ugly. > > > > The basic idea behind the the patch is that, as (attempted to be) > > explained above, subsequent writes to DATA register in PIO mode cause > > constant generation of clock on the bus and therefore a very long > > transfer of zero data. This confuses the I2C peripherals of course. > > > > The patch implements clock stretching for PIO writes (maybe we need this > > for reads too) by making the controller blast out only 4 (or less) bytes > > of data in each write into the DATA register. To prevent interruption of > > the transfer between writes into the DATA register, the SCK is held low > > using the RETAIN_CLOCK bit. > > > > But (!) here comes the caveat. The PIO was introduced to speed up small > > transfers. Introducing clock stretching into PIO mode operation might > > completely remove this advantage. This has to be measured again. > > > > I will continue once I sleep a little. Pardon my {language, code}, it's > > too early in the morning already. > > Ok, so if this patch works, we may just want to restrict PIO mode to 4 > or 3 Byte transfers But then we will end up with pretty useless PIO implementation, since most of the register IO is around 4-bytes long ;-) > where we don't need to use this ugly clock > stretching. This way we may gain the PIO speedup for single register > reads/writes, which is a pretty common operation, but fall back to the > more reliable DMA mode for bigger transfers. I will need to check some more and especially do some profiling and timing, otherwise further discussion here is moot . Best regards, Marek Vasut -- To unsubscribe from this list: send the line "unsubscribe linux-i2c" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
