Subject: Re: [Patch V6 2/6] irqchip: xilinx: Clean up irqdomain argument and
 read/write
To: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>,
        Thomas Gleixner <tglx@linutronix.de>
References: <1477916207-25157-1-git-send-email-Zubair.Kakakhel@imgtec.com>
 <1477916207-25157-3-git-send-email-Zubair.Kakakhel@imgtec.com>
 <alpine.DEB.2.20.1610311335380.5011@nanos>
 <fea1e962-1af2-d666-2b0d-55a8e230430c@imgtec.com>
Cc: monstr@monstr.eu, jason@lakedaemon.net, linux-kernel@vger.kernel.org,
        michal.simek@xilinx.com, linuxppc-dev@lists.ozlabs.org,
        mpe@ellerman.id.au
From: Marc Zyngier <marc.zyngier@arm.com>
Organization: ARM Ltd
Message-ID: <30e73e32-3144-3222-c758-dd8affd9b276@arm.com>
Date: Wed, 9 Nov 2016 15:53:04 +0000
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On 01/11/16 11:05, Zubair Lutfullah Kakakhel wrote:
> Hi,
> 
> Thanks for the review.
> 
> On 10/31/2016 07:51 PM, Thomas Gleixner wrote:
>> On Mon, 31 Oct 2016, Zubair Lutfullah Kakakhel wrote:
>>> The drivers read/write function handling is a bit quirky.
>>
>> Can you please explain in more detail what's quirky and why it should be
>> done differently,
>>
>>> And the irqmask is passed directly to the handler.
>>
>> I can't make any sense out of that sentence.  Which handler? If you talk
>> about the write function, then I don't see a change. So what are you
>> talking about?
> 
> Thanks. I'll add more detail in v7 if this patch survives.
> 
>>
>>> Add a new irqchip struct to pass to the handler and
>>> cleanup read/write handling.
>>
>> I still don't see what it cleans up. You move the write function pointer
>> into a data structure, which is exposed by another pointer. So you create
>> two levels of indirection in the hotpath. The function prototype is still
>> the same. So all this does is making things slower unless I'm missing
>> something.
> 
> I wrote this patch/cleanup based on a review of driver by Marc when I moved the
> driver from arch/microblaze to drivers/irqchip
> 
> "Marc Zyngier
> 
> ...
> 
>  >  arch/microblaze/kernel/intc.c   | 196 ----------------------------------------
>  >  drivers/irqchip/irq-axi-intc.c  | 196 ++++++++++++++++++++++++++++++++++++++++
> 
> ...
> 
>  > +	/* Yeah, okay, casting the intr_mask to a void* is butt-ugly, but I'm
>  > +	 * lazy and Michal can clean it up to something nicer when he tests
>  > +	 * and commits this patch.  ~~gcl */
>  > +	root_domain = irq_domain_add_linear(intc, nr_irq, &xintc_irq_domain_ops,
>  > +							(void *)intr_mask);
> 
> Since you're now reworking this driver, how about addressing this
> ugliness? You could store the intr_mask together with intc_baseaddr,
> and the read/write functions in a global structure, and pass a
> pointer to it? That would make the code a bit nicer...
> "
> 
> https://patchwork.kernel.org/patch/9287933/
> 
>>
>>> -static unsigned int (*read_fn)(void __iomem *);
>>> -static void (*write_fn)(u32, void __iomem *);
>>> +struct xintc_irq_chip {
>>> +	void __iomem *base;
>>> +	struct	irq_domain *domain;
>>> +	struct	irq_chip chip;
>>
>> The tabs between struct and the structure name are bogus.
>>
>>> +	u32	intr_mask;
>>> +	unsigned int (*read)(void __iomem *iomem);
>>> +	void (*write)(u32 data, void __iomem *iomem);
>>
>> Please structure that like a table:
>>
>> 	void			__iomem *base;
>> 	struct irq_domain	*domain;
>> 	struct irq_chip		chip;
>> 	u32			intr_mask;
>> 	unsigned int		(*read)(void __iomem *iomem);
>> 	void			(*write)(u32 data, void __iomem *iomem);
>>
>> Can you see how that makes parsing the struct simpler, because the data
>> types are clearly to identify?
> 
> That does make it look much better.
> 
>>
>>> +static struct xintc_irq_chip *xintc_irqc;
>>>
>>>  static void intc_write32(u32 val, void __iomem *addr)
>>>  {
>>> @@ -54,6 +60,18 @@ static unsigned int intc_read32_be(void __iomem *addr)
>>>  	return ioread32be(addr);
>>>  }
>>>
>>> +static inline unsigned int xintc_read(struct xintc_irq_chip *xintc_irqc,
>>> +					     int reg)
>>> +{
>>> +	return xintc_irqc->read(xintc_irqc->base + reg);
>>> +}
>>> +
>>> +static inline void xintc_write(struct xintc_irq_chip *xintc_irqc,
>>> +				     int reg, u32 data)
>>> +{
>>> +	xintc_irqc->write(data, xintc_irqc->base + reg);
>>> +}
>>> +
>>>  static void intc_enable_or_unmask(struct irq_data *d)
>>>  {
>>>  	unsigned long mask = 1 << d->hwirq;
>>> @@ -65,21 +83,21 @@ static void intc_enable_or_unmask(struct irq_data *d)
>>>  	 * acks the irq before calling the interrupt handler
>>>  	 */
>>>  	if (irqd_is_level_type(d))
>>> -		write_fn(mask, intc_baseaddr + IAR);
>>> +		xintc_write(xintc_irqc, IAR, mask);
>>
>> So this whole thing makes only sense, when you want to support multiple
>> instances of that chip and then you need to store the xintc_irqc pointer as
>> irqchip data and retrieve it from there. Unless you do that, this "cleanup"
>> is just churn for nothing with the effect of making things less efficient.
>>
> 
> Indeed the driver doesn't support multiple instances of the Xilinx Interrupt controller.
> I don't have a use-case or the hardware for that.
> 
> So what would be the recommended course of action?

If you really don't want/need to support multi-instance, then this is
indeed overkill. You're then better off having a simple static key that
deals with the endianess of the peripheral, and have a global structure
that contains the relevant data:

static DEFINE_STATIC_KEY_FALSE(xintc_is_be);

static void xintc_write(int reg, u32 data)
{
	if (static_branch_unlikely(&xintc_is_be))
		iowrite32be(data, xint_irqc.base + reg);
	else
		iowrite32(data, xint_irqc.base + reg);
}

and something similar for the write accessor.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...