linux-kernel - Re: [PATCH v2 2/7] serial: qcom-geni: Fix the timeout in qcom_geni_serial_poll

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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