[U-Boot] [PATCH v2 03/11] riscv: generic: Ensure that U-Boot runs within 4GB for 64bit systems

[Bin Meng]

On Fri, Jan 18, 2019 at 7:19 PM Anup Patel <Anup.Patel at wdc.com> wrote:
>
> On 64bit systems, the DRAM top can be easily beyond 4GB and U-Boot
> DMA mapping APIs will generate DMA addresses beyond 4GB. This
> breaks DMA programming in 32bit DMA capable devices (such as
> Cadence MACB ethernet). For example, If DRAM is more then 2GB
> on QEMU sifive_u machine then Cadence MACB ethernet stops working
> for U-Boot because it is a 32bit DMA capable device.
>
> To handle 32bit DMA capable devices on 64bit systems, we provide
> custom implementation of board_get_usable_ram_top() which ensures
> that usable ram top is not more then 4GB. This in-turn ensures
> that U-Boot always runs within 4GB hence DMA addresses generated
> by DMA mapping APIs will be within 4GB too.
>
> Signed-off-by: Atish Patra <atish.patra at wdc.com>
> Signed-off-by: Anup Patel <anup.patel at wdc.com>
> Reviewed-by: Alexander Graf <agraf at suse.de>
> ---
>  arch/riscv/cpu/generic/dram.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
>

Reviewed-by: Bin Meng <bmeng.cn at gmail.com>

But see one comment below:

> diff --git a/arch/riscv/cpu/generic/dram.c b/arch/riscv/cpu/generic/dram.c
> index 84d87d2a7f..5725d3c7ae 100644
> --- a/arch/riscv/cpu/generic/dram.c
> +++ b/arch/riscv/cpu/generic/dram.c
> @@ -5,6 +5,9 @@
>
>  #include <common.h>
>  #include <fdtdec.h>
> +#include <linux/sizes.h>
> +
> +DECLARE_GLOBAL_DATA_PTR;
>
>  int dram_init(void)
>  {
> @@ -15,3 +18,20 @@ int dram_init_banksize(void)
>  {
>         return fdtdec_setup_memory_banksize();
>  }
> +
> +ulong board_get_usable_ram_top(ulong total_size)
> +{
> +#ifdef CONFIG_64BIT
> +       /*
> +        * Ensure that we run from first 4GB so that all
> +        * addresses used by U-Boot are 32bit addresses.
> +        *
> +        * This in-turn ensures that 32bit DMA capabale
> +        * devices work fine because DMA mapping APIs will
> +        * provide 32bit DMA addresses only.
> +        */
> +       if (gd->ram_top > SZ_4G)
> +               return SZ_4G;
> +#endif
> +       return gd->ram_top;
> +}

I was wondering whether we should change this for 32-bit too, given
it's a valid configuration to have more than 4GB memory in a 32-bit
system.

Regards,
Bin