linux-kernel - Re: [PATCH v4 3/3] gpio: ws16c48: Migrate to the regmap API

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Date:   Tue, 7 Mar 2023 21:51:26 -0500
From:   William Breathitt Gray <william.gray@...aro.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     linus.walleij@...aro.org, brgl@...ev.pl,
        linux-gpio@...r.kernel.org, linux-kernel@...r.kernel.org,
        broonie@...nel.org, techsupport@...systems.com,
        Paul Demetrotion <pdemetrotion@...systems.com>
Subject: Re: [PATCH v4 3/3] gpio: ws16c48: Migrate to the regmap API

On Mon, Mar 06, 2023 at 04:20:03PM +0200, Andy Shevchenko wrote:
> On Mon, Mar 06, 2023 at 07:59:53AM -0500, William Breathitt Gray wrote:
> 
> ...
> 
> > -	raw_spinlock_t lock;
> > +	spinlock_t lock;
> 
> This is a regression.
> That said, do we need a support of raw spin locks in the regmap IRQ?

So this code has a similar need as the gpio-pcie-idio-24 patch: guard
registers between handle_mask_sync() and set_type_config(); however, now
we also need to protect registers in regmap_irq_thread(). We can't use a
mutex here because regmap_irq_thread() is executed in an interrupt
context so we cannot sleep.

This might be a mistake in my understanding: I chose spinlock_t here
because I believed it to map out to a raw_spinlock_t anyway underneath,
whereas on RT kernels it would map out to whatever the equivalent is. I
suspect this is not actually the case. Would using raw_spinlock_t
explicitly be the correct way to go for this particular case?

> > +	u8 irq_mask[WS16C48_NUM_IRQS / WS16C48_NGPIO_PER_REG];
> 
> Can this be a bitmap? Or is it too over engineered with it?

I also considered a bitmap at first, but I believe it adds an
unnecessary abstraction in this particular case: irq_mask is just a
buffer to hold the previous mask_buf state to check if it's changed when
ws16c48_handle_mask_sync() is called. Since all we do with it is save
the mask_buf directly, using the bitmap API seems like overkill.

William Breathitt Gray

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