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Message-ID: <ZlmLe4d10TrnoyjQ@smile.fi.intel.com>
Date: Fri, 31 May 2024 11:34:03 +0300
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Douglas Anderson <dianders@...omium.org>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jirislaby@...nel.org>, linux-arm-msm@...r.kernel.org,
John Ogness <john.ogness@...utronix.de>,
Uwe Kleine-König <u.kleine-koenig@...gutronix.de>,
Tony Lindgren <tony@...mide.com>,
Stephen Boyd <swboyd@...omium.org>, linux-serial@...r.kernel.org,
Yicong Yang <yangyicong@...ilicon.com>,
Johan Hovold <johan+linaro@...nel.org>,
linux-kernel@...r.kernel.org,
Bjorn Andersson <andersson@...nel.org>,
Konrad Dybcio <konrad.dybcio@...aro.org>,
Thomas Gleixner <tglx@...utronix.de>,
Vijaya Krishna Nivarthi <quic_vnivarth@...cinc.com>
Subject: Re: [PATCH v2 2/7] serial: qcom-geni: Fix the timeout in
qcom_geni_serial_poll_bit()
On Thu, May 30, 2024 at 03:45:54PM -0700, Douglas Anderson wrote:
> The qcom_geni_serial_poll_bit() is supposed to be able to be used to
> poll a bit that's will become set when a TX transfer finishes. Because
> of this it tries to set its timeout based on how long the UART will
> take to shift out all of the queued bytes. There are two problems
> here:
> 1. There appears to be a hidden extra word on the firmware side which
> is the word that the firmware has already taken out of the FIFO and
> is currently shifting out. We need to account for this.
> 2. The timeout calculation was assuming that it would only need 8 bits
> on the wire to shift out 1 byte. This isn't true. Typically 10 bits
> are used (8 data bits, 1 start and 1 stop bit), but as much as 13
> bits could be used (14 if we allowed 9 bits per byte, which we
> don't).
>
> The too-short timeout was seen causing problems in a future patch
> which more properly waited for bytes to transfer out of the UART
> before cancelling.
..
> + /*
> + * Add 1 to tx_fifo_depth to account for the hidden register
> + * on the firmware side that can hold a word.
> + */
> + max_queued_bytes =
> + DIV_ROUND_UP((port->tx_fifo_depth + 1) * port->tx_fifo_width,
> + BITS_PER_BYTE);
BITS_TO_BYTES()
..
> - timeout_us = ((fifo_bits * USEC_PER_SEC) / baud) + 500;
> + timeout_us = ((max_queued_bits * USEC_PER_SEC) / baud) + 500;
Too many parentheses. (The outer ones can be dropped.
--
With Best Regards,
Andy Shevchenko
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