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

[Anup Patel]

> -----Original Message-----
> From: Auer, Lukas [mailto:lukas.auer at aisec.fraunhofer.de]
> Sent: Monday, January 21, 2019 1:35 AM
> To: sjg at chromium.org; bmeng.cn at gmail.com; rick at andestech.com; Anup
> Patel <Anup.Patel at wdc.com>; joe.hershberger at ni.com;
> yamada.masahiro at socionext.com
> Cc: paul.walmsley at sifive.com; palmer at sifive.com; hch at infradead.org; u-
> boot at lists.denx.de; agraf at suse.de; Atish Patra <Atish.Patra at wdc.com>
> Subject: Re: [PATCH v2 03/11] riscv: generic: Ensure that U-Boot runs within
> 4GB for 64bit systems
> 
> On Fri, 2019-01-18 at 11:18 +0000, 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: 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: Lukas Auer <lukas.auer at aisec.fraunhofer.de>
> 
> With one nit 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
> 
> nit: capable

Sure, will update.

> 
> > +	 * 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;
> > +}

Regards,
Anup