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

[Alexander Graf]

On 01/17/2019 11:38 AM, Anup Patel 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: Anup Patel <anup.patel at wdc.com>
> Signed-off-by: Atish Patra <atish.patra at wdc.com>

You could probably write that with MIN() more easily, but this way works 
fine too.

Reviewed-by: Alexander Graf <agraf at suse.de>

Alex