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Subject: Re: [PATCH] tty: serial: msm_serial: Fix XON/XOFF
To:     Stephen Boyd <sboyd@kernel.org>, agross@kernel.org,
        david.brown@linaro.org, gregkh@linuxfoundation.org
Cc:     jslaby@suse.com, keescook@chromium.org, anton@enomsg.org,
        ccross@android.com, tony.luck@intel.com,
        linux-arm-msm@vger.kernel.org, linux-serial@vger.kernel.org,
        linux-kernel@vger.kernel.org, khasim.mohammed@linaro.org,
        agsumit@qti.qualcomm.com
References: <20190520103435.30850-1-jorge.ramirez-ortiz@linaro.org>
 <20190520145110.7BDAE21721@mail.kernel.org>
From:   Jorge Ramirez <jorge.ramirez-ortiz@linaro.org>
Message-ID: <254704a2-ee20-30cd-8362-6e1bd23ec090@linaro.org>
Date:   Mon, 20 May 2019 16:56:59 +0200
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On 5/20/19 16:51, Stephen Boyd wrote:
> Quoting Jorge Ramirez-Ortiz (2019-05-20 03:34:35)
>> When the tty layer requests the uart to throttle, the current code
>> executing in msm_serial will trigger "Bad mode in Error Handler" and
>> generate an invalid stack frame in pstore before rebooting (that is if
>> pstore is indeed configured: otherwise the user shall just notice a
>> reboot with no further information dumped to the console).
>>
>> This patch replaces the PIO byte accessor with the word accessor
>> already used in PIO mode.
> 
> Because the hardware only accepts word based accessors and fails
> otherwise? I can believe that.
> 
> I wonder if the earlier UART hardware this driver used to support (i.e.
> pre-DM) would accept byte access to the registers. It's possible, but we
> don't really care because those boards aren't supported.

ok.

also the PIO path uses iowrite32_rep to write a number of bytes (from 1
to 4) so I think it is also appropriate to use it for XON/XOFF.

> 
>>
>> Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
>> ---
> 
> Reviewed-by: Stephen Boyd <swboyd@chromium.org>
> 
>>  drivers/tty/serial/msm_serial.c | 5 ++++-
>>  1 file changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/tty/serial/msm_serial.c b/drivers/tty/serial/msm_serial.c
>> index 109096033bb1..23833ad952ba 100644
>> --- a/drivers/tty/serial/msm_serial.c
>> +++ b/drivers/tty/serial/msm_serial.c
>> @@ -869,10 +870,12 @@ static void msm_handle_tx(struct uart_port *port)
>>                 else
>>                         tf = port->membase + UART_TF;
>>  
>> +               buf[0] = port->x_char;
>> +
>>                 if (msm_port->is_uartdm)
>>                         msm_reset_dm_count(port, 1);
>>  
>> -               iowrite8_rep(tf, &port->x_char, 1);
>> +               iowrite32_rep(tf, buf, 1);
> 
> I suppose it's OK to write some extra zeroes here?
> 
> 

yeah, semantically confusing msm_reset_dm_count is what really matters:
it tells the hardware to only take n bytes (in this case only one) so
the others will be ignored