Re: [PATCH v2] serial: 8250_dw: Improve unwritable LCR workaround

[Heikki Krogerus]

Hi,

On Tue, Oct 01, 2013 at 10:18:08AM -0700, Tim Kryger wrote:
> When configured with UART_16550_COMPATIBLE=NO or in versions prior to
> the introduction of this option, the Designware UART will ignore writes
> to the LCR if the UART is busy.  The current workaround saves a copy of
> the last written LCR and re-writes it in the ISR for a special interrupt
> that is raised when a write was ignored.
> 
> Unfortunately, interrupts are typically disabled prior to performing a
> sequence of register writes that include the LCR so the point at which
> the retry occurs is too late.  An example is serial8250_do_set_termios()
> where an ignored LCR write results in the baud divisor not being set and
> instead a garbage character is sent out the transmitter.
> 
> Furthermore, since serial_port_out() offers no way to indicate failure,
> a serious effort must be made to ensure that the LCR is actually updated
> before returning back to the caller.  This is difficult, however, as a
> UART that was busy during the first attempt is likely to still be busy
> when a subsequent attempt is made unless some extra action is taken.
> 
> This updated workaround reads back the LCR after each write to confirm
> that the new value was accepted by the hardware.  Should the hardware
> ignore a write, the TX/RX FIFOs are cleared and the receive buffer read
> before attempting to rewrite the LCR out of the hope that doing so will
> force the UART into an idle state.  While this may seem unnecessarily
> aggressive, writes to the LCR are used to change the baud rate, parity,
> stop bit, or data length so the data that may be lost is likely not
> important.  Admittedly, this is far from ideal but it seems to be the
> best that can be done given the hardware limitations.
> 
> Lastly, the revised workaround doesn't touch the LCR in the ISR, so it
> avoids the possibility of a "serial8250: too much work for irq" lock up.
> This problem is rare in real situations but can be reproduced easily by
> wiring up two UARTs and running the following commands.
> 
>   # stty -F /dev/ttyS1 echo
>   # stty -F /dev/ttyS2 echo
>   # cat /dev/ttyS1 &
>   [1] 375
>   # echo asdf > /dev/ttyS1
>   asdf
> 
>   [   27.700000] serial8250: too much work for irq96
>   [   27.700000] serial8250: too much work for irq96
>   [   27.710000] serial8250: too much work for irq96
>   [   27.710000] serial8250: too much work for irq96
>   [   27.720000] serial8250: too much work for irq96
>   [   27.720000] serial8250: too much work for irq96
>   [   27.730000] serial8250: too much work for irq96
>   [   27.730000] serial8250: too much work for irq96
>   [   27.740000] serial8250: too much work for irq96
> 
> Signed-off-by: Tim Kryger <tim.kryger@xxxxxxxxxx>
> Reviewed-by: Matt Porter <matt.porter@xxxxxxxxxx>
> Reviewed-by: Markus Mayer <markus.mayer@xxxxxxxxxx>
> ---
> 
> Changes in v2:
>   - Rebased on tty-next
>   - Updated commit messsage to mention UART_16550_COMPATIBLE
>   - Removed potentially unnecessary read of LSR and MSR
>   - Only attempt workaround when LCR write is ignored

I'm OK with this.

Reviewed-by: Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx>


Br,

-- 
heikki
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