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

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Message-ID: <41af9d13-fff5-a416-87cc-4f69e77d6409@rock-chips.com>
Date:   Fri, 17 Jun 2022 20:46:24 +0800
From:   Jon Lin <jon.lin@...k-chips.com>
To:     Mark Brown <broonie@...nel.org>
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 2022/6/17 19:45, Mark Brown wrote:
> 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?

This idea is workable, and it's more efficient than previous code, So I 
send a new commit:
https://patchwork.kernel.org/project/spi-devel-general/patch/20220617124251.5051-1-jon.lin@rock-chips.com/
> 
>> 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?

I think