linux-kernel - Re: [PATCH] ARC: Add a knob to control usage of dual-issue

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Date:   Thu, 18 Jan 2018 10:54:56 -0800
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARC: Add a knob to control usage of dual-issue

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.
>
> Note:
>    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 ?

>
> Signed-off-by: Alexey Brodkin <abrodkin@...opsys.com>
> ---
>   arch/arc/kernel/setup.c | 13 +++++++++++++
>   1 file changed, 13 insertions(+)
>
> diff --git a/arch/arc/kernel/setup.c b/arch/arc/kernel/setup.c
> index 9d27331fe69a..cf97f7d88934 100644
> --- a/arch/arc/kernel/setup.c
> +++ b/arch/arc/kernel/setup.c
> @@ -31,6 +31,8 @@
>   
>   #define FIX_PTR(x)  __asm__ __volatile__(";" : "+r"(x))
>   
> +int dual_issue_enable = 1;
> +
>   unsigned int intr_to_DE_cnt;
>   
>   /* Part of U-boot ABI: see head.S */
> @@ -198,6 +200,17 @@ static void read_arc_build_cfg_regs(void)
>   		if (cpu->core.family >= 0x54) {
>   			unsigned int exec_ctrl;
>   
> +			if (!dual_issue_enable) {
> +				/*
> +				 * Note:
> +				 *   1) Reset value in AUX_EXEC_CTRL is 0
> +				 *   2) Reverse logic is used,
> +				 *      i.e. by default (AUX_EXEC_CTRL=0)
> +				 *      dual-issue is enabled.
> +				 */
> +				write_aux_reg(AUX_EXEC_CTRL, 1);
> +			}
> +
>   			READ_BCR(AUX_EXEC_CTRL, exec_ctrl);
>   			cpu->extn.dual_enb = !(exec_ctrl & 1);
>