linux-kernel - Re: [PATCH v2 2/3] spi: pxa2xx: Prepare for edge-triggered interrupts

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Date:   Tue, 17 Jan 2017 08:54:56 +0100
From:   Robert Jarzmik <robert.jarzmik@...e.fr>
To:     Jan Kiszka <jan.kiszka@...mens.com>
Cc:     Mark Brown <broonie@...nel.org>, linux-spi@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Daniel Mack <daniel@...que.org>,
        Haojian Zhuang <haojian.zhuang@...il.com>,
        linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>,
        Sascha Weisenberger <sascha.weisenberger@...mens.com>
Subject: Re: [PATCH v2 2/3] spi: pxa2xx: Prepare for edge-triggered interrupts

Jan Kiszka <jan.kiszka@...mens.com> writes:

> When using the a device with edge-triggered interrupts, such as MSIs,
> the interrupt handler has to ensure that there is a point in time during
> its execution where all interrupts sources are silent so that a new
> event can trigger a new interrupt again.
>
> This is achieved here by looping over SSSR evaluation. We need to take
> into account that SSCR1 may be changed by the transfer handler, thus we
> need to redo the mask calculation, at least regarding the volatile
> interrupt enable bit (TIE).
>
> Signed-off-by: Jan Kiszka <jan.kiszka@...mens.com>
Hi Jan,

> +	while (1) {
This bit worries me a bit, as this can be either :
 - hogging the SoC's CPU, endlessly running
 - or even worse, blocking the CPU for ever

The question behind is, should this be done in a top-half, or moved to a irq
thread ?

> +		/* Ignore possible writes if we don't need to write */
> +		if (!(sccr1_reg & SSCR1_TIE))
> +			mask &= ~SSSR_TFS;
>  
> -	if (!drv_data->master->cur_msg) {
> -		handle_bad_msg(drv_data);
> -		/* Never fail */
> -		return IRQ_HANDLED;
> -	}
> +		if (!(status & mask))
> +			return ret;
> +
> +		if (!drv_data->master->cur_msg) {
> +			handle_bad_msg(drv_data);
> +			/* Never fail */
> +			return IRQ_HANDLED;
> +		}
> +
> +		ret |= drv_data->transfer_handler(drv_data);
Mmm that looks weird to me, oring a irqreturn.

Imagine that on first iteration the handler returns IRQ_NONE, and on second
IRQ_HANDLED. This makes ret IRQ_HANDLED. Yet after the first iteration the
handler should have exited, especially if the interrupt is shared with another
driver.

Another thing which is along what Andy already said : it would be better
practice to have this loop in the form :
do {
...
} while (exit_condition_not_met);

Just for maintainability, it's better, and it concentrates the test on the
"exit_condition_not_met" in one place, which will enable us to review better the
algorithm.

Cheers.

-- 
Robert