Hi,
On 7/2/11 12:29 AM, antonynpavlov wrote:
IIRC we will need to preserve ra for NMI case
How this preserved value can be used?
Not used right now.
We have no need to preserve ra because we have no full-grown exception handlers.
True.
The MIPS cpu reset entry point is 0xbfc00000 (KSEG1, unmapped and
uncached region).
So code after _start label run from 0xbfc00000.
Starting from 0xBFC00000 might be a primary boot scenario in MIPS
systems, but in my opinion it's just one of the use cases. Memory-
type flash devices are getting less used with newer MIPS SoCs.
---- Shinya-san's code start (arch/mips/cpu/start.S) ----
setup_c0_status_reset
/*
* Config: K0 should be set to the desired Cache Coherency
* Algorithm (CCA) prior to accessing Kseg0.
*/
mfc0 t0, CP0_CONFIG
/* T.B.D. */
mtc0 t0, CP0_CONFIG
/*
* Config: (4KEm and 4KEp cores only) KU and K23 should be set to
* the desired CCA for USeg/KUSeg and KSeg2/3 respectively prior to
* accessing those regions.
*/
mfc0 t0, CP0_CONFIG
/* T.B.D. */
mtc0 t0, CP0_CONFIG
---- Shinya-san's code end ----
does this code initialise KSEG0 cache mode?
does this code change CP0_CONFIG at all?
My answer is "no".
Correct. Please keep in mind I didn't do anything set up in start.S.
I was thinking around how to generalize the initialization procedures
how we could maintain/support various MIPS processors, and what shape
of code would be reasonable (and scalable).
But bellow, you can see switching to KSEG0 (label 1f linked to 0x9fc00xxx) ...
---- Shinya-san's code start (arch/mips/cpu/start.S) ----
/* Switch to CKSEG0 segment */
la t0, 1f
/* T.B.D. -- Convert an addree of the label '1f' into CKSEG0 */
jr t0
1:
---- Shinya-san's code end ----
It's intended to be converted to CKSEG0, but it's not necessarily
0x9fc00xxx.
Let's see relocate code:
---- Shinya-san's code start (arch/mips/cpu/start.S) ----
relocate:
ADR t0, _start, t1 # t0<- current position of code
PTR_LI t1, TEXT_BASE
beq t0, t1, stack_setup
nop
---- Shinya-san's code end ----
This code try to check if relocation needs. It try to compute _start<<current>>
address.
But
* _start link address is KSEG0 address 0x9fc00000;
* we have already switched to KSEG0, so _start<<current>> address
is 0x9fc00000 too.
No. ADR tries to compute 'PC-relative' _start address. If it starts
from physical 0xbfc0000, t0 is set up to 0xbfc0000 even if _start link
address is CKSEG0. If it starts from physical 0xA0100000, t0 is set
up to 0xA0100000, and in that case _start link address is supposed to
be set to 0x80100000. The 'link address' is supposed to be used
_after_ relocation. You see?
I'm afraid there might be something missing in my prototyoe, though.
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