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 <[email protected]> > > > > > > > > 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: <[email protected]> > > > > Signed-off-by: Fabio Estevam <[email protected]> > > > > --- > > > > 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 <[email protected]> > > > > 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, 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. Regards, Lucas -- Pengutronix e.K. | Lucas Stach | Industrial Linux Solutions | http://www.pengutronix.de/ | Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-5076 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 | -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
