linux-kernel - Re: [PATCH v4 15/31] i2c: tegra: Remove bogus barrier()

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Date:   Sun, 6 Sep 2020 02:35:02 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Thierry Reding <thierry.reding@...il.com>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Laxman Dewangan <ldewangan@...dia.com>,
        Wolfram Sang <wsa@...-dreams.de>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     linux-i2c@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 15/31] i2c: tegra: Remove bogus barrier()

05.09.2020 23:41, Dmitry Osipenko пишет:
> Apparently barrier() was intended to reduce possibility of racing
> with the interrupt handler, but driver's code evolved significantly
> and today's driver enables interrupt only when it waits for completion
> notification. Hence barrier() has no good use anymore, let's remove it.
> 
> Signed-off-by: Dmitry Osipenko <digetx@...il.com>
> ---
>  drivers/i2c/busses/i2c-tegra.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c
> index 33d37a40fa83..f69587ca163b 100644
> --- a/drivers/i2c/busses/i2c-tegra.c
> +++ b/drivers/i2c/busses/i2c-tegra.c
> @@ -600,7 +600,6 @@ static int tegra_i2c_fill_tx_fifo(struct tegra_i2c_dev *i2c_dev)
>  		i2c_dev->msg_buf_remaining = buf_remaining;
>  		i2c_dev->msg_buf = buf +
>  			words_to_transfer * BYTES_PER_FIFO_WORD;
> -		barrier();
>  
>  		i2c_writesl(i2c_dev, buf, I2C_TX_FIFO, words_to_transfer);
>  
> @@ -624,7 +623,6 @@ static int tegra_i2c_fill_tx_fifo(struct tegra_i2c_dev *i2c_dev)
>  		/* Again update before writing to FIFO to make sure isr sees. */
>  		i2c_dev->msg_buf_remaining = 0;
>  		i2c_dev->msg_buf = NULL;
> -		barrier();
>  
>  		i2c_writel(i2c_dev, val, I2C_TX_FIFO);
>  	}
> 

It just caught my eye that there is actually a comment there saying that
barrier() was intended to mitigate the racing with the ISR. Hence that
comment is outdated now and needs to be removed. I'll correct it in v5.