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

From: Chanwoo Choi <cw00.choi@samsung.com>
To: Dmitry Osipenko <digetx@gmail.com>,
	Thierry Reding <thierry.reding@gmail.com>,
	Jonathan Hunter <jonathanh@nvidia.com>,
	MyungJoo Ham <myungjoo.ham@samsung.com>,
	Kyungmin Park <kyungmin.park@samsung.com>,
	Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: linux-tegra@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-pm@vger.kernel.org
Subject: Re: [PATCH v2 05/19] PM / devfreq: tegra: Replace write memory barrier with the read barrier
Date: Tue, 16 Apr 2019 17:00:01 +0900	[thread overview]
Message-ID: <e0a9def6-e3ed-f134-e612-aa257aa3fa12@samsung.com> (raw)
In-Reply-To: <20190415145505.18397-6-digetx@gmail.com>

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@gmail.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