Date:   Fri, 1 Sep 2017 18:25:01 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Stafford Horne <shorne@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Openrisc <openrisc@...ts.librecores.org>,
        Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jason Cooper <jason@...edaemon.net>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Jonas Bonn <jonas@...thpole.se>, devicetree@...r.kernel.org
Subject: Re: [PATCH 05/13] irqchip: add initial support for ompic

On 01/09/17 02:24, Stafford Horne wrote:
> On Thu, Aug 31, 2017 at 10:28:01AM +0100, Marc Zyngier wrote:
>> On 30/08/17 22:58, Stafford Horne wrote:
>>> From: Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>
>>>
>>> IPI driver for OpenRISC Multicore programmable interrupt controller as
>>> described in the Multicore support section of the OpenRISC 1.2
>>> proposed architecture specification:
>>>
>>>   https://github.com/stffrdhrn/doc/raw/arch-1.2-proposal/openrisc-arch-1.2-rev0.pdf
>>>
>>> Each OpenRISC core contains a full interrupt controller which is used in
>>> the SMP architecture for interrupt balancing.  This IPI device is the
>>> only external device required for enabling SMP on OpenRISC.
>>>
>>> Pending ops are stored in a memory bit mask which can allow multiple
>>> pending operations to be set and serviced at a time. This is mostly
>>> borrowed from the alpha IPI implementation.
>>>
>>> Signed-off-by: Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>
>>> [shorne@...il.com: converted ops to bitmask, wrote commit message]
>>> Signed-off-by: Stafford Horne <shorne@...il.com>
>>> ---
>>>  .../bindings/interrupt-controller/ompic.txt        |  22 ++++
>>>  arch/openrisc/Kconfig                              |   1 +
>>>  drivers/irqchip/Kconfig                            |   4 +
>>>  drivers/irqchip/Makefile                           |   1 +
>>>  drivers/irqchip/irq-ompic.c                        | 117 +++++++++++++++++++++
>>>  5 files changed, 145 insertions(+)
>>>  create mode 100644 Documentation/devicetree/bindings/interrupt-controller/ompic.txt
>>>  create mode 100644 drivers/irqchip/irq-ompic.c
>>>
>>> diff --git a/Documentation/devicetree/bindings/interrupt-controller/ompic.txt b/Documentation/devicetree/bindings/interrupt-controller/ompic.txt
>>> new file mode 100644
>>> index 000000000000..4176ecc3366d
>>> --- /dev/null
>>> +++ b/Documentation/devicetree/bindings/interrupt-controller/ompic.txt
>>> @@ -0,0 +1,22 @@
>>> +OpenRISC Multicore Programmable Interrupt Controller
>>> +
>>> +Required properties:
>>> +
>>> +- compatible : This should be "ompic"
>>> +- reg : Specifies base physical address and size of the register space. The
>>> +  size can be arbitrary based on the number of cores the controller has
>>> +  been configured to handle, typically 8 bytes per core.
>>> +- interrupt-controller : Identifies the node as an interrupt controller
>>> +- #interrupt-cells : Specifies the number of cells needed to encode an
>>> +  interrupt source. The value shall be 1.
>>> +- interrupts : Specifies the interrupt line to which the ompic is wired.
>>> +
>>> +Example:
>>> +
>>> +ompic: ompic {
>>> +	compatible = "ompic";
>>> +	reg = <0x98000000 16>;
>>> +	#interrupt-cells = <1>;
>>> +	interrupt-controller;
>>> +	interrupts = <1>;
>>> +};
>>> diff --git a/arch/openrisc/Kconfig b/arch/openrisc/Kconfig
>>> index 214c837ce597..dd7e55e7e42d 100644
>>> --- a/arch/openrisc/Kconfig
>>> +++ b/arch/openrisc/Kconfig
>>> @@ -30,6 +30,7 @@ config OPENRISC
>>>  	select NO_BOOTMEM
>>>  	select ARCH_USE_QUEUED_SPINLOCKS
>>>  	select ARCH_USE_QUEUED_RWLOCKS
>>> +	select OMPIC if SMP
>>>  
>>>  config CPU_BIG_ENDIAN
>>>  	def_bool y
>>> diff --git a/drivers/irqchip/Kconfig b/drivers/irqchip/Kconfig
>>> index f1fd5f44d1d4..3fa60e6667a7 100644
>>> --- a/drivers/irqchip/Kconfig
>>> +++ b/drivers/irqchip/Kconfig
>>> @@ -145,6 +145,10 @@ config CLPS711X_IRQCHIP
>>>  	select SPARSE_IRQ
>>>  	default y
>>>  
>>> +config OMPIC
>>> +	bool
>>> +	select IRQ_DOMAIN
>>
>> Why do you need to select IRQ_DOMAIN? This driver doesn't seem to use any...
> 
> Right, I will look to remove that.
> 
>>> +
>>>  config OR1K_PIC
>>>  	bool
>>>  	select IRQ_DOMAIN
>>> diff --git a/drivers/irqchip/Makefile b/drivers/irqchip/Makefile
>>> index e88d856cc09c..123047d7a20d 100644
>>> --- a/drivers/irqchip/Makefile
>>> +++ b/drivers/irqchip/Makefile
>>> @@ -17,6 +17,7 @@ obj-$(CONFIG_DW_APB_ICTL)		+= irq-dw-apb-ictl.o
>>>  obj-$(CONFIG_METAG)			+= irq-metag-ext.o
>>>  obj-$(CONFIG_METAG_PERFCOUNTER_IRQS)	+= irq-metag.o
>>>  obj-$(CONFIG_CLPS711X_IRQCHIP)		+= irq-clps711x.o
>>> +obj-$(CONFIG_OMPIC)			+= irq-ompic.o
>>>  obj-$(CONFIG_OR1K_PIC)			+= irq-or1k-pic.o
>>>  obj-$(CONFIG_ORION_IRQCHIP)		+= irq-orion.o
>>>  obj-$(CONFIG_OMAP_IRQCHIP)		+= irq-omap-intc.o
>>> diff --git a/drivers/irqchip/irq-ompic.c b/drivers/irqchip/irq-ompic.c
>>> new file mode 100644
>>> index 000000000000..438819f8a5a7
>>> --- /dev/null
>>> +++ b/drivers/irqchip/irq-ompic.c
>>> @@ -0,0 +1,117 @@
>>> +/*
>>> + * Open Multi-Processor Interrupt Controller driver
>>> + *
>>> + * Copyright (C) 2014 Stefan Kristiansson <stefan.kristiansson@...nalahti.fi>
>>> + *
>>> + * This file is licensed under the terms of the GNU General Public License
>>> + * version 2.  This program is licensed "as is" without any warranty of any
>>> + * kind, whether express or implied.
>>> + */
>>> +
>>> +#include <linux/io.h>
>>> +#include <linux/interrupt.h>
>>> +#include <linux/smp.h>
>>> +#include <linux/of.h>
>>> +#include <linux/of_irq.h>
>>> +#include <linux/of_address.h>
>>> +#include <linux/irqchip/chained_irq.h>
>>
>> Don't think you need this.
>>
>>> +#include <linux/delay.h>
>>
>> Nor this.
> 
> OK on both.
> 
>>> +
>>> +#include <linux/irqchip.h>
>>> +
>>> +#define OMPIC_IPI_BASE			0x0
>>> +#define OMPIC_IPI_CTRL(cpu)		(OMPIC_IPI_BASE + 0x0 + (cpu)*8)
>>> +#define OMPIC_IPI_STAT(cpu)		(OMPIC_IPI_BASE + 0x4 + (cpu)*8)
>>
>> In the DT binding you say that "size can be arbitrary based on the
>> number of cores the controller has been configured to handle, typically
>> 8 bytes per core". Here, this is 8 bytes, always, which is not exactly
>> the same. What is the architectural value, if any? If there is none,
>> then the per-core size should either come from DT or some other mean
>> (register?).
> 
> I mean the address space is 8 bytes x number-of-cores.  Thats what I meant
> by arbitrary, I guess its better for me to be explicit. There is no
> register that we can check to confirm the configuration of ompic.  But I
> guess we can check the CPU NUMCORES register and compare it to the DT
> address space to do a sanity check.

Thanks for the explanation. So the code is OK, but the DT should be more
rigorous in saying that there is 8 bytes per CPU. And yes to the check
if that can be done at this stage.

> 
>>> +
>>> +#define OMPIC_IPI_CTRL_IRQ_ACK		(1 << 31)
>>> +#define OMPIC_IPI_CTRL_IRQ_GEN		(1 << 30)
>>> +#define OMPIC_IPI_CTRL_DST(cpu)		(((cpu) & 0x3fff) << 16)
>>> +
>>> +#define OMPIC_IPI_STAT_IRQ_PENDING	(1 << 30)
>>> +
>>> +#define OMPIC_IPI_DATA(x)		((x) & 0xffff)
>>> +
>>> +static struct {
>>> +	unsigned long ops;
>>> +} ipi_data[NR_CPUS];
>>
>> In general, a per cpu data structure is better expressed as a percpu
>> data structure (yes, I'm in a funny mood this morning). Otherwise,
>> you're going to thrash more than just the receiver and the sender, but
>> also all the other CPUs that have their data in the same cache line.
> 
> Right, that makes sense, I am not sure why that was done this way. I think
> I borrowed from alpha which has the extra __cacheline_aligned annotations.
> I forgot to come back and fix this.  Ill do as you suggest, thank you.
> 
> Excerpt from alpha:
> 
> static struct {
>         unsigned long bits ____cacheline_aligned;
> } ipi_data[NR_CPUS] __cacheline_aligned;

Yup, __cacheline_aligned is key here, as it will have the same effect as
the percpu stuff from that point of view.

>>> +
>>> +static void __iomem *ompic_base;
>>> +
>>> +static inline u32 ompic_readreg(void __iomem *base, loff_t offset)
>>> +{
>>> +	return ioread32be(base + offset);
>>> +}
>>> +
>>> +static void ompic_writereg(void __iomem *base, loff_t offset, u32 data)
>>> +{
>>> +	iowrite32be(data, base + offset);
>>> +}
>>> +
>>> +#ifdef CONFIG_SMP
>>
>> This code is only selected when CONFIG_SMP=y.
> 
> Yes, that is right, as below:
> 
>   set_smp_cross_call(ompic_raise_softirq);
> 
> The set_smp_cross_call() function from smp.c is only defined for smp.  Do
> you think thats wrong or needed extra comments?  This is similar to other
> chips in irqchip/ for archs which use set_smp_cross_call().

Other irqchips can often be compiled for both SMP and !SMP, hence the
need of a #ifdef. In your case, this driver is only compiled when SMP is
selected, so you're pretty much guaranteed that this symbol is available.

> 
>>> +void ompic_raise_softirq(const struct cpumask *mask, unsigned int irq)
>>> +{
>>
>> What is "irq" here? How is it guaranteed to fit in an unsigned long?
> 
> Here irq is the `enum ipi_msg_type` which is guaranteed to fit in unsigned
> long.  Porbably its better to rename as msg or ipi_msg?

You should certainly make sure your "enum ipi_msg_type" is capped at the
number of bits of unsigned long. And yes to ipi_msg, which is a better
name than irq. Also, you could change the types of ompic_raise_softirq
and set_smp_cross_call, so that you use the enum instead of an int here.

> 
>>> +	unsigned int dst_cpu;
>>> +	unsigned int src_cpu = smp_processor_id();
>>> +
>>> +	for_each_cpu(dst_cpu, mask) {
>>> +		set_bit(irq, &ipi_data[dst_cpu].ops);
>>> +
>>> +		ompic_writereg(ompic_base, OMPIC_IPI_CTRL(src_cpu),
>>> +			       OMPIC_IPI_CTRL_IRQ_GEN |
>>> +			       OMPIC_IPI_CTRL_DST(dst_cpu) |
>>> +			       OMPIC_IPI_DATA(1));
>>
>> What guarantees that the action of set_bit can be observed by the target
>> CPU? set-bit gives you atomicity, but no barrier.
> 
> The bit will not be read by target CPUs until after the ompic_writereg()
> which will trigger the target CPU interrupt to be raised.  OpenRISC stores
> are ordered.

Are they really ordered irrespective of the memory type (one is
cacheable RAM, the other is a device...)?

And assuming they are (which I find a bit odd), that doesn't necessarily
guarantee that the interrupt will only be delivered once the effect of
set_bit can be seen on the target CPU...

> This will work on OpenRISC, but should I be thinking of other architectures
> as well for all drivers?  Or do you think this will be an issue on
> OpenRISC?

I definitely think this could be an issue with OpenRISC, but only
someone familiar with the OpenRISC architecture can say whether I'm
right or wrong. I'm just guessing at the moment.

[...]

> Thank you for the feedback, I will clean this up and resubmit with the
> comments on the other thread.
> 
> In terms of commit path, do you think its ok for this to go in via the
> OpenRISC arch path?

Sure, that's fine by me.

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

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