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Subject: Re: [PATCH] serial: qcom_geni_serial: Fix recent kdb hang
To:     Douglas Anderson <dianders@chromium.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     kgdb-bugreport@lists.sourceforge.net,
        Mukesh Savaliya <msavaliy@codeaurora.org>,
        Andy Gross <agross@kernel.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Evan Green <evgreen@chromium.org>,
        Jiri Slaby <jirislaby@kernel.org>,
        linux-arm-msm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-serial@vger.kernel.org
References: <20200806221904.1.I4455ff86f0ef5281c2a0cd0a4712db614548a5ca@changeid>
From:   Akash Asthana <akashast@codeaurora.org>
Message-ID: <adaef6bf-7887-feea-fedf-d3bc5566bb9d@codeaurora.org>
Date:   Mon, 10 Aug 2020 18:01:54 +0530
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Hi Doug,

On 8/7/2020 10:49 AM, Douglas Anderson wrote:
> The commit e42d6c3ec0c7 ("serial: qcom_geni_serial: Make kgdb work
> even if UART isn't console") worked pretty well and I've been doing a
> lot of debugging with it.  However, recently I typed "dmesg" in kdb
> and then held the space key down to scroll through the pagination.  My
> device hung.  This was repeatable and I found that it was introduced
> with the aforementioned commit.
>
> It turns out that there are some strange boundary cases in geni where
> in some weird situations it will signal RX_LAST but then will put 0 in
> RX_LAST_BYTE.  This means that the entire last FIFO entry is valid.

IMO that means we received a word in RX_FIFO and it is the last word 
hence RX_LAST bit is set.

RX_LAST_BYTE is 0 means none of the bytes are valid in the last word.

In such scenario we should just read RX_FIFO buffer (to empty it), 
discard the word and return NO_POLL_CHAR. Something like below.

---------------------------------------------------------------------------------------------------------------------------------------------------------

                 else
                         private_data->poll_cached_bytes_cnt = 4;

                 private_data->poll_cached_bytes =
                         readl(uport->membase + SE_GENI_RX_FIFOn);
         }

+        if (!private_data->poll_cached_bytes_cnt)
+              return NO_POLL_CHAR;
         private_data->poll_cached_bytes_cnt--;
         ret = private_data->poll_cached_bytes & 0xff;
-------------------------------------------------------------------------------------------------------------------------------------------------------------

Please let me know whether above code helps.

I am not sure about what all scenario can leads to this behavior from 
hardware, I will try to get an answer from hardware team.

Any error bit was set for SE_GENI_S_IRQ_STATUS & SE_GENI_M_IRQ_STATUS 
registers?


I guess the hang was seen because *poll_cached_bytes_cnt* is unsigned 
int and it's value was 0, when it's decremented by 1 it's value become 
'4294967295' (very large) and dummy RX (0x00) would happen that

many times before reading any actual RX transfers/bytes.

Regards,

Akash


> This weird corner case is handled in qcom_geni_serial_handle_rx()
> where you can see that we only honor RX_LAST_BYTE if RX_LAST is set
> _and_ RX_LAST_BYTE is non-zero.  If either of these is not true we use
> BYTES_PER_FIFO_WORD (4) for the size of the last FIFO word.
>
> Let's fix kgdb.  While at it, also use the proper #define for 4.
>
> Fixes: e42d6c3ec0c7 ("serial: qcom_geni_serial: Make kgdb work even if UART isn't console")
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
>
>   drivers/tty/serial/qcom_geni_serial.c | 9 +++++++--
>   1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/tty/serial/qcom_geni_serial.c b/drivers/tty/serial/qcom_geni_serial.c
> index 07b7b6b05b8b..e27077656939 100644
> --- a/drivers/tty/serial/qcom_geni_serial.c
> +++ b/drivers/tty/serial/qcom_geni_serial.c
> @@ -361,11 +361,16 @@ static int qcom_geni_serial_get_char(struct uart_port *uport)
>   			return NO_POLL_CHAR;
>   
>   		if (word_cnt == 1 && (status & RX_LAST))
> +			/*
> +			 * NOTE: If RX_LAST_BYTE_VALID is 0 it needs to be
> +			 * treated as if it was BYTES_PER_FIFO_WORD.
> +			 */
>   			private_data->poll_cached_bytes_cnt =
>   				(status & RX_LAST_BYTE_VALID_MSK) >>
>   				RX_LAST_BYTE_VALID_SHFT;
> -		else
> -			private_data->poll_cached_bytes_cnt = 4;
> +
> +		if (private_data->poll_cached_bytes_cnt == 0)
> +			private_data->poll_cached_bytes_cnt = BYTES_PER_FIFO_WORD;
>   
>   		private_data->poll_cached_bytes =
>   			readl(uport->membase + SE_GENI_RX_FIFOn);

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