在 2020/8/5 20:10, Thomas Bogendoerfer 写道:
On Sat, Aug 01, 2020 at 03:59:41PM +0800, Huacai Chen wrote:
On Wed, May 6, 2020 at 1:30 PM Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx> wrote:
于 2020年5月2日 GMT+08:00 下午12:55:43, Huacai Chen <chenhc@xxxxxxxxxx> 写到:
Loongson-3's COP2 is Multi-Media coprocessor, it is disabled in kernel
mode by default. However, gslq/gssq (16-bytes load/store instructions)
overrides the instruction format of lwc2/swc2. If we wan't to use gslq/
gssq for optimization in kernel, we should enable COP2 usage in kernel.
Please pay attention that in this patch we only enable COP2 in kernel,
which means it will lose ST0_CU2 when a process go to user space (try
to use COP2 in user space will trigger an exception and then grab COP2,
which is similar to FPU). And as a result, we need to modify the context
switching code because the new scheduled process doesn't contain ST0_CU2
in its THERAD_STATUS probably.
Signed-off-by: Huacai Chen <chenhc@xxxxxxxxxx>
Reviewed-by: Jiaxun Yang <jiaxun.yang@xxxxxxxxxxx>
Does this patch have some unresolved problems, or there is something unclear?
yes there is. Since this COP2 is a total black box to me, it would be
really helpfull to get some docs for it or at least some information what
it exactly does and how you want to use it in kernel code.
FYI:
Loongson doesn't have any CU2 register. It just reused LWC2 & LDC2 opcode
to define some load & store instructions (e.g. 128bit load to two GPRs).
I have a collection of these instructions here[1].
From GS464E (3A2000+), execuating these instruction won't produce COP2
unusable
exception. But older Loongson cores (GS464) will still produce COP2
exception, thus
we should have CU2 enabled in kernel. That would allow us use to these
instructions
to optimize kernel.
Looking closer at the patch I realized, that there is already support
for usage of COP2 in user land, which I thought isn't the case (at least
I understood that from one of your mails). So is there enough state
saving to support this ?
Actually there is no CU2 state for Loongson to save.
And finally what I stil don't like is the splittering of more
#ifdef LOONGSON into common code. I'd prefer a more generic way
to enable COPx for in kernel usage. Maybe a more generic config option
or a dynamic solution like the one for user land.
Agreed. some Kconfig options or cpuinfo_mips.options can be helpful.
Thanks.
- Jiaxun
[1]: https://github.com/FlyGoat/loongson-insn/
Thomas.
