> -----Original Message----- > From: Bin Meng [mailto:[email protected]] > Sent: Tuesday, January 22, 2019 7:37 AM > To: Anup Patel <[email protected]> > Cc: Rick Chen <[email protected]>; Joe Hershberger > <[email protected]>; Lukas Auer <[email protected]>; > Masahiro Yamada <[email protected]>; Simon Glass > <[email protected]>; Alexander Graf <[email protected]>; Palmer Dabbelt > <[email protected]>; Paul Walmsley <[email protected]>; Atish > Patra <[email protected]>; Christoph Hellwig <[email protected]>; U- > Boot Mailing List <[email protected]> > Subject: Re: [PATCH v2 03/11] riscv: generic: Ensure that U-Boot runs within > 4GB for 64bit systems > > On Fri, Jan 18, 2019 at 7:19 PM Anup Patel <[email protected]> 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 <[email protected]> > > Signed-off-by: Anup Patel <[email protected]> > > Reviewed-by: Alexander Graf <[email protected]> > > --- > > arch/riscv/cpu/generic/dram.c | 20 ++++++++++++++++++++ > > 1 file changed, 20 insertions(+) > > > > Reviewed-by: Bin Meng <[email protected]> > > 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.
We had considered this but "gd->ram_top" is "unsigned long" so on 32bit system it will be 32bit only. In other words, value of "gd->ram_top" on 32bit system can never exceed 4GB. Regards, Anup _______________________________________________ U-Boot mailing list [email protected] https://lists.denx.de/listinfo/u-boot
