linux-kernel - Re: [PATCH] spi: rockchip: Disable local irq when pio write out of interrupt service

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Message-ID: <YqxpTqvaQb4cohTh@sirena.org.uk>
Date:   Fri, 17 Jun 2022 12:45:18 +0100
From:   Mark Brown <broonie@...nel.org>
To:     Jon Lin <jon.lin@...k-chips.com>
Cc:     heiko@...ech.de, linux-spi@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-rockchip@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] spi: rockchip: Disable local irq when pio write out of
 interrupt service

On Fri, Jun 17, 2022 at 02:24:10PM +0800, Jon Lin wrote:
> On 2022/6/13 20:37, Mark Brown wrote:
> > On Mon, Jun 13, 2022 at 05:27:44PM +0800, Jon Lin wrote:

> > > +	spin_unlock_irqrestore(&rs->lock, flags);

> > So this is effectively just disabling interrupts during PIO, there's no
> > other users of the lock which is rather heavyweight.  What's the actual
> > issue here?  We should also have something saying what's going on in the
> > code since right now the lock just looks redundant.

> For lock: In order to avoid special situations, such as when the CPU
> frequency drops to close to the IO rate, the water line interrupt is
> triggered during FIFO filling (triggered by other CPUs), resulting in
> critical resources still not being protected in place. For local IRQ

So essentially we're so slow in filling the FIFO when starting a
transfer that the interrupt triggers in the middle of the initial FIFO
fill?  Something that tricky *really* needs a comment adding.

Ideally we'd just leave the interrupt masked until the FIFO is filled
though, looking at the driver I see that there is an interrupt mask
register which seems to have some level of masking available and I do
note that in rockchip_spi_prepare_irq() we unmask interrupts before we
start filling the FIFO rather than afterwards.  Would reversing the
unmask order there address the issue more cleanly?

> disable: Turning off the local interrupt is mainly to prevent the CPU
> schedule from being interrupted when filling FIFO.

If it were just this then there's preempt_disable(), but what's the
problem with being preempted during the FIFO fill?

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