linux-kernel - Re: [PATCH] spi: fsl-espi: Only process interrupts for expected events

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Date:   Wed, 23 Sep 2020 22:27:33 +0200
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Chris Packham <chris.packham@...iedtelesis.co.nz>,
        broonie@...nel.org, npiggin@...il.com
Cc:     linux-spi@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH] spi: fsl-espi: Only process interrupts for expected
 events

On 04.09.2020 02:28, Chris Packham wrote:
> The SPIE register contains counts for the TX FIFO so any time the irq
> handler was invoked we would attempt to process the RX/TX fifos. Use the
> SPIM value to mask the events so that we only process interrupts that
> were expected.
> 
> This was a latent issue exposed by commit 3282a3da25bd ("powerpc/64:
> Implement soft interrupt replay in C").
> 
> Signed-off-by: Chris Packham <chris.packham@...iedtelesis.co.nz>
> Cc: stable@...r.kernel.org
> ---
> 
> Notes:
>     I've tested this on a T2080RDB and a custom board using the T2081 SoC. With
>     this change I don't see any spurious instances of the "Transfer done but
>     SPIE_DON isn't set!" or "Transfer done but rx/tx fifo's aren't empty!" messages
>     and the updates to spi flash are successful.
>     
>     I think this should go into the stable trees that contain 3282a3da25bd but I
>     haven't added a Fixes: tag because I think 3282a3da25bd exposed the issue as
>     opposed to causing it.
> 
>  drivers/spi/spi-fsl-espi.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/spi/spi-fsl-espi.c b/drivers/spi/spi-fsl-espi.c
> index 7e7c92cafdbb..cb120b68c0e2 100644
> --- a/drivers/spi/spi-fsl-espi.c
> +++ b/drivers/spi/spi-fsl-espi.c
> @@ -574,13 +574,14 @@ static void fsl_espi_cpu_irq(struct fsl_espi *espi, u32 events)
>  static irqreturn_t fsl_espi_irq(s32 irq, void *context_data)
>  {
>  	struct fsl_espi *espi = context_data;
> -	u32 events;
> +	u32 events, mask;
>  
>  	spin_lock(&espi->lock);
>  
>  	/* Get interrupt events(tx/rx) */
>  	events = fsl_espi_read_reg(espi, ESPI_SPIE);
> -	if (!events) {
> +	mask = fsl_espi_read_reg(espi, ESPI_SPIM);
> +	if (!(events & mask)) {
>  		spin_unlock(&espi->lock);
>  		return IRQ_NONE;

Sorry, I was on vacation and therefore couldn't comment earlier.
I'm fine with the change, just one thing could be improved IMO.
If we skip an unneeded interrupt now, then returning IRQ_NONE
causes reporting this interrupt as spurious. This isn't too nice
as spurious interrupts typically are seen as a problem indicator.
Therefore returning IRQ_HANDLED should be more appropriate.
This would just require a comment in the code explaining why we
do this, and why it can happen that we receive interrupts
we're not interested in.

>  	}
>