[RFC PATCH] ARM: mvebu: Let the device-tree determine smp_ops

Thomas Petazzoni thomas.petazzoni at free-electrons.com
Thu Nov 6 06:58:23 PST 2014

Dear Chris Packham,

On Thu,  6 Nov 2014 17:49:56 +1300, Chris Packham wrote:
> The machine specific SMP operations can be configured either via
> setup_arch or via arm_dt_init_cpu_maps. For the ARMADA_370_XP_DT devices
> both of these are called and setup_arch wins because it is called last.
> This means that it is not possible to substitute a different set of SMP
> operations via the device-tree.
> All of the ARMADA_370_XP_DT compatible devices are either single core or
> declare an enable-method in the dts (via one of armada-xp-mv78230.dtsi,
> armada-xp-mv78260.dtsi or armada-xp-mv78460.dtsi). Remove the smp
> assignment from board-v7.c so that the SMP operations set via the
> device-tree aren't discarded.
> Signed-off-by: Chris Packham <chris.packham at alliedtelesis.co.nz>
> ---
> Hi,
> (This is the first patch I've sent to the arm-lkml so beware of my newbie-ness)
> I'm working on a new board that uses a integrated CPU that according to the
> vendor is "based on the ARMADA-XP", more on that to come in the future
> hopefully. For the most part things just work if I use a .dts based on the
> mv78260.
> One difference I've encountered is in the way the secondary CPU is initialised,
> which requires me to supply a slightly different set of machine specific SMP
> operations so I can bring up the 2nd core. It looks like I should be able to
> supply a different /cpus/enable-method in the .dts and once I define the
> corresponding set of operations it should be picked up. 
> Which leads me to what I think could be a bug (or just a lack of clear
> specification). The smp ops are set by both arm_dt_init_cpu_maps (from the
> enable-method) and setup_arch (from mdesc). The latter always wins because it
> is called last and doesn't check to see if smp_ops has already been set.
> This is my attempt at fixing the problem by not setting mdesc.smp for the
> ARMADA_370_XP_DT machines thus preventing setup_arch from overriding what has
> been setup by arm_dt_init_cpu_maps.
> Thanks,
> Chris
>  arch/arm/mach-mvebu/board-v7.c | 1 -
>  1 file changed, 1 deletion(-)
> diff --git a/arch/arm/mach-mvebu/board-v7.c b/arch/arm/mach-mvebu/board-v7.c
> index 6478626..894974b 100644
> --- a/arch/arm/mach-mvebu/board-v7.c
> +++ b/arch/arm/mach-mvebu/board-v7.c
> @@ -206,7 +206,6 @@ static const char * const armada_370_xp_dt_compat[] = {
>  DT_MACHINE_START(ARMADA_370_XP_DT, "Marvell Armada 370/XP (Device Tree)")
>  	.l2c_aux_val	= 0,
>  	.l2c_aux_mask	= ~0,
> -	.smp		= smp_ops(armada_xp_smp_ops),
>  	.init_machine	= mvebu_dt_init,
>  	.init_irq       = mvebu_init_irq,
>  	.restart	= mvebu_restart,

There is a very good reason to keep this .smp initialization. The
Device Tree for Armada XP used to *not* have the enable-method
property, since the SMP enable-method binding did not exist at the
time we introduced the Armada XP SMP support. Therefore, if we want to
keep backward compatibility with the old Armada XP DTs and continue to
have SMP support working with those, we need to keep this .smp
initialization essentially forever.

Yes, backward compatibility sometimes sucks :-/

Best regards,

Thomas Petazzoni, CTO, Free Electrons
Embedded Linux, Kernel and Android engineering

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