Re: [PATCH] serial: 8250_dw: Fix LCR workaround regression

[James Hogan]

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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