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Subject: Re: [PATCH] tty: serial: msm_serial: Fix XON/XOFF
To:     Bjorn Andersson <bjorn.andersson@linaro.org>
Cc:     Stephen Boyd <sboyd@kernel.org>, agross@kernel.org,
        david.brown@linaro.org, gregkh@linuxfoundation.org,
        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>
 <254704a2-ee20-30cd-8362-6e1bd23ec090@linaro.org>
 <f0c89b84-7c3d-596d-06e1-cb5172e62970@linaro.org>
 <20190520151101.GN2085@tuxbook-pro> <20190520151206.GO2085@tuxbook-pro>
From:   Jorge Ramirez <jorge.ramirez-ortiz@linaro.org>
Message-ID: <13116b66-451b-bdf9-d631-abb8ada66e01@linaro.org>
Date:   Mon, 20 May 2019 17:20:48 +0200
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On 5/20/19 17:12, Bjorn Andersson wrote:
> On Mon 20 May 08:11 PDT 2019, Bjorn Andersson wrote:
> 
>> On Mon 20 May 07:58 PDT 2019, Jorge Ramirez wrote:
>>
>>> On 5/20/19 16:56, Jorge Ramirez wrote:
>>>> 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
>>>
>>> um after I said this, maybe iowrite32_rep should only be applied to
>>> uartdm ... what do you think?
>>>
>>
>> If I read the history correctly this write was a writel() up until
>> 68252424a7c7 ("tty: serial: msm: Support big-endian CPUs").
>>
>> So I think you should just change this back to a iowrite32_rep() and add
>> a Fixes tag.
>>
> 
> I mean...

ok. cool

> 
> Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
> 
> Regards,
> Bjorn
>