On Wed, Nov 21, 2018 at 9:58 PM Palmer Dabbelt palmer@sifive.com wrote:
On Tue, 20 Nov 2018 19:41:10 PST (-0800), anup@brainfault.org wrote:
This patch adds kconfig option RISCV_SMODE to run u-boot in S-mode. When this opition is enabled we use s<xyz> CSRs instead of m<xyz> CSRs.
It is important to note that there is no equivalent S-mode CSR for misa and mhartid CSRs so we expect M-mode runtime firmware (BBL or equivalent) to emulate misa and mhartid CSR read.
In-future, we will have more patches to avoid accessing misa and mhartid CSRs from S-mode.
Ya, I don't like this. Our current boot protocol puts mhartid in a1 upon entering the supervisor, and allows the detection of misa via a device tree pointer provided in a0.
As long as everyone agrees this isn't what we're actually looking for then I'm fine with the patch, I just don't want to end up requiring this "M mode emulates some CSRs for S mode" interface. Since we don't have an actual platform spec we've got to be careful to avoid a bunch of defacto interfaces that we'll need to support later.
Yes, it's a temporary thingy to emulate misa and mhartid in BBL.
Eventually, U-Boot will be fixed to avoid direct access of misa and mhartid.
Regards, Anup