linux-kernel - Re: [PATCH] spi: bcm2835: Enable shared interrupt support

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Date:   Fri, 29 May 2020 19:53:50 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>,
        Mark Brown <broonie@...nel.org>, Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        bcm-kernel-feedback-list@...adcom.com,
        Martin Sperl <kernel@...tin.sperl.org>,
        linux-spi@...r.kernel.org, linux-rpi-kernel@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] spi: bcm2835: Enable shared interrupt support

On Fri, May 29, 2020 at 10:46:01AM -0700, Florian Fainelli wrote:
> On 5/29/20 10:43 AM, Lukas Wunner wrote:
> > On Thu, May 28, 2020 at 08:58:04PM +0200, Nicolas Saenz Julienne wrote:
> >> --- a/drivers/spi/spi-bcm2835.c
> >> +++ b/drivers/spi/spi-bcm2835.c
> >> @@ -379,6 +379,10 @@ static irqreturn_t bcm2835_spi_interrupt(int irq, void *dev_id)
> >>  	if (bs->tx_len && cs & BCM2835_SPI_CS_DONE)
> >>  		bcm2835_wr_fifo_blind(bs, BCM2835_SPI_FIFO_SIZE);
> >>  
> >> +	/* check if we got interrupt enabled */
> >> +	if (!(bcm2835_rd(bs, BCM2835_SPI_CS) & BCM2835_SPI_CS_INTR))
> >> +		return IRQ_NONE;
> >> +
> >>  	/* Read as many bytes as possible from FIFO */
> >>  	bcm2835_rd_fifo(bs);
> >>  	/* Write as many bytes as possible to FIFO */
[...]
> > Finally, it would be nice if the check would be optimized away when
> > compiling for pre-RasPi4 products, maybe something like:
> > 
> > +	if (IS_ENABLED(CONFIG_ARM_LPAE) && !(cs & BCM2835_SPI_CS_INTR))
> > +		return IRQ_NONE;
> 
> Rather than keying this off ARM_LPAE or any other option, this should be
> keyed off a compatible string, that way we can even conditionally pass
> IRQF_SHARED to the interrupt handler if we care so much about performance.

But a compatible string can't be checked at compile time, can it?

The point is that at the least the Foundation compiles and ships a separate
kernel for each of the three platforms BCM2835, BCM2837, BCM2711.  It's
unnecessary to check whether an interrupt was actually raised if we *know*
in advance that it's not shared (as is the case with kernels compiled for
BCM2835 and BCM2837).

Thanks,

Lukas