[PATCH] ARC: Add a knob to control usage of dual-issue
Vineet.Gupta1 at synopsys.com
Thu Jan 18 14:50:22 PST 2018
On 01/18/2018 11:11 AM, Alexey Brodkin wrote:
> Hi Vineet,
> On Thu, 2018-01-18 at 10:54 -0800, Vineet Gupta wrote:
>> On 01/18/2018 05:48 AM, Alexey Brodkin wrote:
>>> HS48 core starts with dual-issue enabled but in some cases like
>>> debugging as well as benchmarking it might be useful to disable
>>> dual-issue for a particular run.
>>> 1. To disable dual-issue user has to change a value of a global variable
>>> in target's memory right before start of Linu kernel execution
>>> (most probably via JTAG)
>>> 2. Disabling happens very early on boot and to get it back enabled it's
>>> required to restart Linux kernel. I.e. with this change we don't allow
>>> toggling dual-issue state in random moments of run-time
>> But we need access to a debugger anyways to change this global variable.
>> If you already have that won't it be better to change the aux register itself from
>> the debugger itself.
>> I don't really see how the global variable way of toggle adds any value here ?
> I think there's some sense in a global variable.
> 1. Not sure if OpenOCD/GDB pair for ARC allows to read/write random AUX regs.
> Probably allows but I guess we'll need a new .xml description which lists EXEC_CTRL
> AUX reg. Correct me if I'm wrong here.
Not sure of this a strong argument ...
> 2. If somebody wants to disable dual-issue persistently (here I mean building an image
> which had dual-issue disabled and there's no need to alter anything after image loading)
Ok this is more reasonable !
But can we do this in early boot code then. There's a fair bit of code leading up
to where we are doing now so I'd prefer we do it just as we do for disabling L1
Please note that SLC / IOC are special - as much as I liked to do them super early
- we can't as there's fair bit of code bloat / mem size etc which we don't knwo
> then it's possible just to change a simple and very obvious line in arch/arc/kernel/setup.c
> other than trying to guess what could be good place in sources to stick that AUX reg write.
> Note we don't explicitly do reads of AUX_EXEC_CTRL instead we just use READ_BCR macro so
> for newcomers it might not be immediately obvious what to do and where.
> Add the fact that logic of the flag is reversed...
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