From mboxrd@z Thu Jan 1 00:00:00 1970 From: Leo Liang Date: Thu, 17 Sep 2020 19:15:49 +0800 Subject: [PATCH v2 7/7] riscv: Add some comments to start.S In-Reply-To: <20200914142303.21307-8-seanga2@gmail.com> References: <20200914142303.21307-1-seanga2@gmail.com> <20200914142303.21307-8-seanga2@gmail.com> Message-ID: <20200917111545.GA685@andestech.com> List-Id: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: u-boot@lists.denx.de On Mon, Sep 14, 2020 at 10:23:03AM -0400, Sean Anderson wrote: > This adds comments regarding the ordering and purpose of certain > instructions as I understand them. > > Signed-off-by: Sean Anderson > Reviewed-by: Bin Meng > Reviewed-by: Rick Chen > --- > > Changes in v2: > - Clarify comments regarding tp > > arch/riscv/cpu/start.S | 19 +++++++++++++++++-- > 1 file changed, 17 insertions(+), 2 deletions(-) > > diff --git a/arch/riscv/cpu/start.S b/arch/riscv/cpu/start.S > index a16af79fbe..cb1347559c 100644 > --- a/arch/riscv/cpu/start.S > +++ b/arch/riscv/cpu/start.S > @@ -43,7 +43,10 @@ _start: > csrr a0, CSR_MHARTID > #endif > > - /* save hart id and dtb pointer */ > + /* > + * Save hart id and dtb pointer. The thread pointer register is not > + * modified by C code. It is used by secondary_hart_loop. > + */ > mv tp, a0 > mv s1, a1 > > @@ -54,10 +57,18 @@ _start: > */ > mv gp, zero > > + /* > + * Set the trap handler. This must happen after initializing gp because > + * the handler may use it. > + */ > la t0, trap_entry > csrw MODE_PREFIX(tvec), t0 > > - /* mask all interrupts */ > + /* > + * Mask all interrupts. Interrupts are disabled globally (in m/sstatus) > + * for U-Boot, but we will need to read m/sip to determine if we get an > + * IPI > + */ > csrw MODE_PREFIX(ie), zero > > #if CONFIG_IS_ENABLED(SMP) > @@ -410,6 +421,10 @@ secondary_hart_relocate: > mv gp, a2 > #endif > > +/* > + * Interrupts are disabled globally, but they can still be read from m/sip. The > + * wfi function will wake us up if we get an IPI, even if we do not trap. > + */ > secondary_hart_loop: > wfi > Reviewed-by: Leo Liang