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

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Message-ID: <36dd65bb-18a9-9697-b9b6-76eaf8cbe45c@gmail.com>
Date:   Fri, 29 May 2020 10:46:01 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Lukas Wunner <lukas@...ner.de>,
        Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     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 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 */
> 
> This definitely looks wrong.  The check whether the interrupt is enabled
> should be moved *before* the conditional calls to bcm2835_rd_fifo_blind()
> and bcm2835_wr_fifo_blind(), i.e. to the top of the function.
> 
> Otherwise if an interrupt is raised by another SPI controller,
> this function may perform read/write accesses to the FIFO and
> interfere with an ongoing transfer in DMA or poll mode.
> 
> Also, instead of performing an MMIO read, just use the "cs" variable
> which was assigned at the top of the function.
> 
> The code comment should explain that the check is necessary because the
> interrupt may be shared with other controllers on the BCM2711.
> 
> 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.
-- 
Florian