linux-kernel - Re: [PATCH v2 05/19] PM / devfreq: tegra: Replace write memory barrier with the read barrier

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Date:   Tue, 16 Apr 2019 17:00:01 +0900
From:   Chanwoo Choi <cw00.choi@...sung.com>
To:     Dmitry Osipenko <digetx@...il.com>,
        Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        MyungJoo Ham <myungjoo.ham@...sung.com>,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>
Cc:     linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 05/19] PM / devfreq: tegra: Replace write memory
 barrier with the read barrier

Hi,

On 19. 4. 15. 오후 11:54, Dmitry Osipenko wrote:
> The write memory barrier isn't needed because the BUS buffer is flushed
> by read after write that happens after the removed wmb(), we will also
> use readl() instead of the relaxed version to ensure that read is indeed
> completed.
> 
> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
> ---
>  drivers/devfreq/tegra-devfreq.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/devfreq/tegra-devfreq.c b/drivers/devfreq/tegra-devfreq.c
> index d62fb1b0d9bb..f0f0d78f6cbf 100644
> --- a/drivers/devfreq/tegra-devfreq.c
> +++ b/drivers/devfreq/tegra-devfreq.c
> @@ -243,8 +243,7 @@ static void tegra_devfreq_update_wmark(struct tegra_devfreq *tegra,
>  static void actmon_write_barrier(struct tegra_devfreq *tegra)
>  {
>  	/* ensure the update has reached the ACTMON */
> -	wmb();
> -	actmon_readl(tegra, ACTMON_GLB_STATUS);
> +	readl(tegra->regs + ACTMON_GLB_STATUS);

I think that this meaning of actmon_write_barrier() keeps
the order of 'store' assembly command without the execution change
from compiler optimization by using the wmb().

But, this patch edits it as following:
The result of the following two cases are same?

[original code]
	wmb()
	read_relaxed()

[new code by this patch]
	readl_relaxed()
	rmb()


>  }
>  
>  static void actmon_isr_device(struct tegra_devfreq *tegra,
> 


-- 
Best Regards,
Chanwoo Choi
Samsung Electronics