Re: [PATCH] tty: mips_ejtag_fdc: Call cpu_relax() in registers polling busy loops

[Greg KH <gregkh@linuxfoundation.org>]

On Thu, Dec 19, 2024 at 08:42:54PM +0800, Zhongqiu Han wrote:
> It is considered good practice to call cpu_relax() in busy loops, see
> Documentation/process/volatile-considered-harmful.rst. This can lower CPU
> power consumption or yield to a hyperthreaded twin processor, or serve as
> a compiler barrier. In addition, if something goes wrong in the busy loop
> at least it can prevent things from getting worse.
> 
> Signed-off-by: Zhongqiu Han <quic_zhonhan@quicinc.com>
> ---
>  drivers/tty/mips_ejtag_fdc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/mips_ejtag_fdc.c b/drivers/tty/mips_ejtag_fdc.c
> index afbf7738c7c4..b17ead1e9698 100644
> --- a/drivers/tty/mips_ejtag_fdc.c
> +++ b/drivers/tty/mips_ejtag_fdc.c
> @@ -346,7 +346,7 @@ static void mips_ejtag_fdc_console_write(struct console *c, const char *s,
>  
>  		/* Busy wait until there's space in fifo */
>  		while (__raw_readl(regs + REG_FDSTAT) & REG_FDSTAT_TXF)
> -			;
> +			cpu_relax();
>  		__raw_writel(word.word, regs + REG_FDTX(c->index));
>  	}
>  out:
> @@ -1233,7 +1233,7 @@ static void kgdbfdc_push_one(void)
>  
>  	/* Busy wait until there's space in fifo */
>  	while (__raw_readl(regs + REG_FDSTAT) & REG_FDSTAT_TXF)
> -		;
> +		cpu_relax();

How did you test this?  Are you _sure_ it is needed at all?  I think you
just made these loops take a lot longer than before :(

Have you had problems with these tight loops doing anything bad to your
system?

thanks,

greg k-h