On Wednesday 11 December 2013 13:00:33 Tim Kryger wrote: > On Wed, Dec 11, 2013 at 6:00 AM, Heikki Krogerus > > <heikki.krogerus@xxxxxxxxxxxxxxx> wrote: > > Hi, > > > > On Tue, Dec 10, 2013 at 10:28:04PM +0000, James Hogan wrote: > >> From: James Hogan <james.hogan@xxxxxxxxxx> > >> > >> Commit c49436b657d0 (serial: 8250_dw: Improve unwritable LCR workaround) > >> caused a regression. It added a check that the LCR was written properly > >> to detect and workaround the busy quirk, but the behaviour of bit 5 > >> (UART_LCR_SPAR) differs between IP versions 3.00a and 3.14c per the > >> docs. On older versions this caused the check to fail and it would > >> repeatedly force idle and rewrite the LCR register, causing delays and > >> preventing any input from serial being received. > >> > >> This is fixed by masking out UART_LCR_SPAR before making the comparison. > >> > >> Signed-off-by: James Hogan <james.hogan@xxxxxxxxxx> > >> Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> > >> Cc: Jiri Slaby <jslaby@xxxxxxx> > >> Cc: Tim Kryger <tim.kryger@xxxxxxxxxx> > >> Cc: Ezequiel Garcia <ezequiel.garcia@xxxxxxxxxxxxxxxxxx> > >> Cc: Matt Porter <matt.porter@xxxxxxxxxx> > >> Cc: Markus Mayer <markus.mayer@xxxxxxxxxx> > >> Cc: Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx> > >> Tested-by: Tim Kryger <tim.kryger@xxxxxxxxxx> > >> Tested-by: Ezequiel Garcia <ezequiel.garcia@xxxxxxxxxxxxxxxxxx> > > > > I tested this with HW without the busy functionality > > (UART_16550_COMPATIBLE == yes). No problems here. FWIW: > > > > Tested-by: Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx> > > Greg, can you consider taking this patch as a regression fix for 3.13? > > Just in case you want a little more background beyond what is in the > commit message... > > Statically declared DW UARTs will execute autoconfig_16550a during > probe which attempts to set the SPAR bit of LCR. Some old versions of > the DW IP don't implement the stick parity feature and for these > versions the SPAR bit always reads back as zero. Since these writes > aren't fully accepted by the hardware, they trigger the workaround > which interprets the partially rejected write to mean the UART was > busy. A number of attempts are made to idle the UART and re-write the > LCR but eventually the workaround gives up and prints a warning. DT > declared DW UARTs don't run autoconfig and don't see this issue as far > as I know. Hi Tim Thanks for the extra description. FYI, I was instantiating DW UART from DT when I saw the problem. Cheers James
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