linux-kernel - Re: [PATCH v4 5/5] mfd: tqmx86: add I2C IRQ support

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Message-ID: <20241016131832.GC1152434@google.com>
Date: Wed, 16 Oct 2024 14:18:32 +0100
From: Lee Jones <lee@...nel.org>
To: Matthias Schiffer <matthias.schiffer@...tq-group.com>
Cc: linux@...tq-group.com, linux-kernel@...r.kernel.org,
	Gregor Herburger <gregor.herburger@...group.com>
Subject: Re: [PATCH v4 5/5] mfd: tqmx86: add I2C IRQ support

On Wed, 16 Oct 2024, Matthias Schiffer wrote:

> On Wed, 2024-10-16 at 14:07 +0100, Lee Jones wrote:
> > 
> > On Wed, 16 Oct 2024, Matthias Schiffer wrote:
> > 
> > > On Tue, 2024-10-15 at 12:38 +0100, Lee Jones wrote:
> > > > 
> > > > On Mon, 07 Oct 2024, Matthias Schiffer wrote:
> > > > 
> > > > > From: Gregor Herburger <gregor.herburger@...group.com>
> > > > > 
> > > > > The i2c-ocores controller can run in interrupt mode on tqmx86 modules.
> > > > > Add module parameter to allow configuring the IRQ number, similar to the
> > > > > handling of the GPIO IRQ.
> > > > > 
> > > > > Signed-off-by: Gregor Herburger <gregor.herburger@...group.com>
> > > > > Signed-off-by: Matthias Schiffer <matthias.schiffer@...tq-group.com>
> > > > > ---
> > > > > 
> > > > > v2: improve module parameter description (was patch 4/4)
> > > > > v3: replace IRQ 0 resource with an empty placeholder to simplify error handling
> > > > > v4: no changes
> > > > > 
> > > > >  drivers/mfd/tqmx86.c | 20 +++++++++++++++++++-
> > > > >  1 file changed, 19 insertions(+), 1 deletion(-)
> > > > > 
> > > > > diff --git a/drivers/mfd/tqmx86.c b/drivers/mfd/tqmx86.c
> > > > > index e444fcd2a3e9d..057d035b71d33 100644
> > > > > --- a/drivers/mfd/tqmx86.c
> > > > > +++ b/drivers/mfd/tqmx86.c
> > > > > @@ -50,6 +50,7 @@
> > > > >  #define TQMX86_REG_IO_EXT_INT_9			2
> > > > >  #define TQMX86_REG_IO_EXT_INT_12		3
> > > > >  #define TQMX86_REG_IO_EXT_INT_MASK		0x3
> > > > > +#define TQMX86_REG_IO_EXT_INT_I2C_SHIFT		0
> > > > >  #define TQMX86_REG_IO_EXT_INT_GPIO_SHIFT	4
> > > > >  #define TQMX86_REG_SAUC		0x17
> > > > >  
> > > > > @@ -60,7 +61,16 @@ static uint gpio_irq;
> > > > >  module_param(gpio_irq, uint, 0);
> > > > >  MODULE_PARM_DESC(gpio_irq, "GPIO IRQ number (valid parameters: 7, 9, 12)");
> > > > >  
> > > > > -static const struct resource tqmx_i2c_soft_resources[] = {
> > > > > +static uint i2c_irq;
> > > > > +module_param(i2c_irq, uint, 0);
> > > > > +MODULE_PARM_DESC(i2c_irq, "I2C IRQ number (valid parameters: 7, 9, 12)");
> > > > > +
> > > > > +static struct resource tqmx_i2c_soft_resources[] = {
> > > > > +	/*
> > > > > +	 * Placeholder for IRQ resource - must come first to be filled in by the
> > > > > +	 * probe function.
> > > > > +	 */
> > > > > +	{},
> > > > 
> > > > Having a NULLed entry in the first slot doesn't sit well with me at all.
> > > > 
> > > > In order for us to avoid wasting memory, it would be better to place the
> > > > entry at the end of the array with a blank entry:
> > > > 
> > > >   DEFINE_RES_IRQ(0),
> > > > 
> > > > Later comes the matching code which updates the 0 value to something sane.
> > > > 
> > > > Before you call to the add the devices, check to see if the value has
> > > > changed.  If it hasn't, deprecate num_resources, effectively masking the
> > > > last entry in the array.  Then when platform_device_add_resources()
> > > > comes to duplicate the array, it will only copy the relevant entries.
> > > 
> > > I chose my current solution because it resulted in more simple and maintainable code:
> > > 
> > > - No dynamic array access, the IRQ resource is always written to index 0
> > 
> > Which is fragile (even with the comment).  If you're going to directly
> > index array elements, please do so with a unique identifier rather than
> > relying on it happening to reside in the first.
> > 
> > > - No surprises regarding num_resources, it is always equal to ARRAY_SIZE(resources)
> > 
> > No surprises, but sometimes it's incorrect.
> > 
> > > It also allowed to make all mfd_cell const; to make num_resources modifyable, either the const would
> > > need to be removed, or each mfd_cell would need to be copied to a stack variable in the probe
> > > function.
> > > 
> > > In my opinion, these benefits outweigh the 2*64 bytes saved for the additional resources allocated
> > > by platform_device_add_resources() - and 128 bytes doesn't seem significant at all, in particular
> > > considering that this driver is used on x86_64 only.
> > 
> > But doesn't outweigh my disliking for placing a NULL element at the
> > start of the array.  If you _must_ do something like this, please place
> > it at the end of the array.
> > 
> 
> Ok, will move the placeholder to the end, and access it using a defined index instead of 0. I would
> still prefer to keep num_resources constant instead of adjusting it. Does this sound acceptable to
> you?

It would allow me to sleep at night, yes. :)

-- 
Lee Jones [李琼斯]