Re: [RFC PATCH 4/4] ARM: Xilinx: Adding Xilinx board support

[Arnd Bergmann <arnd@arndb.de>]

On Monday 02 May 2011 07:08:00 Grant Likely wrote:
> The 1st board support is minimal to get a system up and running
> on the Xilinx platform.
> 
> This platform reuses the clock implementation from plat-versatile, and
> it depends entirely on CONFIG_OF support.  There is only one board
> support file which obtains all device information from a device tree
> dtb file which is passed to the kernel at boot time.

Very cool stuff!

> +
> +	amba {
> +		compatible = "simple-bus";
> +		#address-cells = <1>;
> +		#size-cells = <1>;
> +		ranges;

Shouldn't we have a way to identify amba buses by the compatible property
as well, rather than only declaring it simple-bus?

We might be able to do some more automatic probing in the amba
code in cases where the architected registers are actually filled
with meaningful data there.

Is the empty ranges property technically correct? Excuse my poor
knowledge of amba, but I would have assumed that only a range
of addresses is actually put on the bus, while others (e.g. RAM)
are not visible on AMBA.

> +static struct of_device_id zynq_of_bus_ids[] __initdata = {
> +	{ .compatible = "simple-bus", },
> +	{}
> +};
> +
> +static struct of_device_id gic_match[] = {
> +	{ .compatible = "arm,gic", },
> +	{}
> +};
> +
> +/**
> + * xilinx_init_machine() - System specific initialization, intended to be
> + *			   called from board specific initialization.
> + */
> +void __init xilinx_init_machine(void)
> +{
> +	struct device_node *node = of_find_matching_node(NULL, gic_match);
> +
> +	if (node)
> +		of_irq_domain_add_simple(node, 0, NR_IRQS);
> +
> +#ifdef CONFIG_CACHE_L2X0
> +	/*
> +	 * 64KB way size, 8-way associativity, parity disabled
> +	 */
> +	l2x0_init(PL310_L2CC_BASE, 0x02060000, 0xF0F0FFFF);
> +#endif
> +
> +/**
> + * xilinx_irq_init() - Interrupt controller initialization for the GIC.
> + */
> +void __init xilinx_irq_init(void)
> +{
> +	gic_init(0, 29, SCU_GIC_DIST_BASE, SCU_GIC_CPU_BASE);
> +}

I think we can do better than this, there is still more hardcoded stuff
in here than I think should be. I understand that you tried to minimize
the size of the patch set for obvious reasons, but none of this
seems too board specific to put into common code in one way or another.

Of course we can not fix all of this at once, but maybe we can have
an explanation for each hardcoded setting on why it is still needed
and what would have to be done to make it go away.

> +/* The minimum devices needed to be mapped before the VM system is up and
> + * running include the GIC, UART and Timer Counter.
> + */
> +
> +static struct map_desc io_desc[] __initdata = {
> +	{
> +		.virtual	= TTC0_VIRT,
> +		.pfn		= __phys_to_pfn(TTC0_PHYS),
> +		.length		= SZ_4K,
> +		.type		= MT_DEVICE,
> +	}, {
> +		.virtual	= SCU_PERIPH_VIRT,
> +		.pfn		= __phys_to_pfn(SCU_PERIPH_PHYS),
> +		.length		= SZ_8K,
> +		.type		= MT_DEVICE,
> +	}, {
> +		.virtual	= PL310_L2CC_VIRT,
> +		.pfn		= __phys_to_pfn(PL310_L2CC_PHYS),
> +		.length		= SZ_4K,
> +		.type		= MT_DEVICE,
> +	},

I especially dislike the idea of having to set up iotables, these
seem completely counterintuitive when we probe the devices from the
device tree.

AFAICT, all of init_irq, time_init and init_machine are called way after
mm_init, so you should have ioremap available by the time you need to
access these virtual memory ranges.

I can understand why you'd want to special-case PCI I/O space windows
and early serial port access, but I think we should handle them differently
and give them fixed machine independent virtual addresses.

> --- /dev/null
> +++ b/arch/arm/mach-zynq/include/mach/io.h
> --- /dev/null
> +++ b/arch/arm/mach-zynq/include/mach/irqs.h
> --- /dev/null
> +++ b/arch/arm/mach-zynq/include/mach/memory.h
> --- /dev/null
> +++ b/arch/arm/mach-zynq/include/mach/system.h
> --- /dev/null
> +++ b/arch/arm/mach-zynq/include/mach/timex.h
> --- /dev/null
> +++ b/arch/arm/mach-zynq/include/mach/vmalloc.h

These are all either completely generic versions that could be reused
on a number of other machines, or they could be turned into such files.

For the global inlude/asm-generic files, we have just added a mechanism
to share that kind of file across multiple architectures using some
Makefile magic. Should we perhaps do something similar here to avoid
having to create more of the same files?

> --- /dev/null
> +++ b/arch/arm/mach-zynq/include/mach/uart.h
> --- /dev/null
> +++ b/arch/arm/mach-zynq/include/mach/uncompress.h

For these, it obviously won't work.

> diff --git a/arch/arm/mach-zynq/timer.c b/arch/arm/mach-zynq/timer.c
> new file mode 100644
> index 0000000..c2c96cc
> --- /dev/null
> +++ b/arch/arm/mach-zynq/timer.c

In the light of the recent discussions of drivers moving out of
platforms into common Linux code, do we expect this to also
happen to timers any time soon?

	Arnd