[U-Boot] [PATCH 5/7] riscv: add support for multi-hart systems

[Anup Patel]

> -----Original Message-----
> From: Lukas Auer [mailto:lukas.auer at aisec.fraunhofer.de]
> Sent: Tuesday, February 12, 2019 3:44 AM
> To: u-boot at lists.denx.de
> Cc: Atish Patra <Atish.Patra at wdc.com>; Anup Patel
> <Anup.Patel at wdc.com>; Bin Meng <bmeng.cn at gmail.com>; Andreas
> Schwab <schwab at suse.de>; Palmer Dabbelt <palmer at sifive.com>;
> Alexander Graf <agraf at suse.de>; Lukas Auer
> <lukas.auer at aisec.fraunhofer.de>; Anup Patel <anup at brainfault.org>; Rick
> Chen <rick at andestech.com>; Baruch Siach <baruch at tkos.co.il>; Stefan
> Roese <sr at denx.de>
> Subject: [PATCH 5/7] riscv: add support for multi-hart systems
> 
> On RISC-V, all harts boot independently. To be able to run on a multi-hart
> system, U-Boot must be extended with the functionality to manage all harts
> in the system. A new config option, CONFIG_MAIN_HART, is used to select
> the hart U-Boot runs on. All other harts are halted.
> U-Boot can delegate functions to them using smp_call_function().
> 
> Every hart has a valid pointer to the global data structure and a 8KiB stack by
> default. The stack size is set with CONFIG_STACK_SIZE_SHIFT.
> 
> Signed-off-by: Lukas Auer <lukas.auer at aisec.fraunhofer.de>
> ---
> 
>  arch/riscv/Kconfig           |  12 +++++
>  arch/riscv/cpu/start.S       | 102 ++++++++++++++++++++++++++++++++++-
>  arch/riscv/include/asm/csr.h |   1 +
>  3 files changed, 114 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig index
> 3a51339c4d..af8d0f8d67 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -140,4 +140,16 @@ config SBI_IPI
>  	default y if RISCV_SMODE
>  	depends on SMP
> 
> +config MAIN_HART
> +	int "Main hart in system"
> +	default 0
> +	help
> +	  Some SoCs include harts of various sizes, some of which might not
> +	  be suitable for running U-Boot. CONFIG_MAIN_HART is used to
> select
> +	  the hart U-Boot runs on.

This config option can be avoided altogether if we have
lottery based system to select "Main HART" in start.S.

With the MAIN_HART config option in-place, every system
will have to pick a "Main HART". What if the "Main HART"
itself does not come online due to HW failure.

Regards,
Anup