Re: [PATCH] MIPS: Make check condition for SDBBP consistent with EJTAG spec

[Tiezhu Yang]

On 02/09/2021 10:36 AM, Jiaxun Yang wrote:
> On Tue, Feb 9, 2021, at 12:32 AM, Jiaxun Yang wrote:
> > On Mon, Feb 8, 2021, at 9:14 PM, Tiezhu Yang wrote:
> > > According to MIPS EJTAG Specification [1], a Debug Breakpoint
> > > exception occurs when an SDBBP instruction is executed, the
> > > CP0_DEBUG bit DBp indicates that a Debug Breakpoint exception
> > > occurred, just check bit DBp for SDBBP is more accurate.
> > >
> > > [1] http://www.t-es-t.hu/download/mips/md00047f.pdf
> > >
> > > Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
> > > ---
> > >   arch/mips/kernel/genex.S | 4 ++--
> > >   1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/mips/kernel/genex.S b/arch/mips/kernel/genex.S
> > > index bcce32a..6336826 100644
> > > --- a/arch/mips/kernel/genex.S
> > > +++ b/arch/mips/kernel/genex.S
> > > @@ -349,8 +349,8 @@ NESTED(ejtag_debug_handler, PT_SIZE, sp)
> > >   	MTC0	k0, CP0_DESAVE
> > >   	mfc0	k0, CP0_DEBUG
> > >   
> > > -	sll	k0, k0, 30	# Check for SDBBP.
> > > -	bgez	k0, ejtag_return
> > > +	andi	k0, k0, 0x2	# Check for SDBBP.
> > > +	beqz	k0, ejtag_return
> > You'd better define a marco for it to prevent further confusion.

OK,  thanks, I will do it in v2.

> > Btw I'm curious about how do kernel receive EJTAG exception?
> > In my understanding there are only two possible EJTAG exception vectors,
> > 0xbfc00480 and DSEG one. Both of them are reachable by kernel.
> ^ not
>
> > How do this piece of code work?

We can see some useful explanations from the following comment,
the firmware needs to make sure jump to except_vec_ejtag_debug.

arch/mips/kernel/genex.S
/*
 * EJTAG debug exception handler.
 * The EJTAG debug exception entry point is 0xbfc00480, which
 * normally is in the boot PROM, so the boot PROM must do an
 * unconditional jump to this vector.
 */
NESTED(except_vec_ejtag_debug, 0, sp)
        j       ejtag_debug_handler
#ifdef CONFIG_CPU_MICROMIPS
         nop
#endif
        END(except_vec_ejtag_debug)

> > Thanks.
> >
> > - Jiaxun
> >
> > >   
> > >   #ifdef CONFIG_SMP
> > >   1:	PTR_LA	k0, ejtag_debug_buffer_spinlock
> > > --
> > > 2.1.0
> > --
> > - Jiaxun