linux-kernel - Re: [PATCH v1] i2c: tegra: Fix i2c_writesl() to use writel() instead of writesl()

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Message-ID: <X/wM+LBkqF5ixuMf@ulmo>
Date:   Mon, 11 Jan 2021 09:31:52 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Wolfram Sang <wsa@...-dreams.de>
Cc:     Sowjanya Komatineni <skomatineni@...dia.com>, jonathanh@...dia.com,
        digetx@...il.com, linux-tegra@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org
Subject: Re: [PATCH v1] i2c: tegra: Fix i2c_writesl() to use writel() instead
 of writesl()

On Mon, Oct 19, 2020 at 09:03:54PM -0700, Sowjanya Komatineni wrote:
> VI I2C don't have DMA support and uses PIO mode all the time.
> 
> Current driver uses writesl() to fill TX FIFO based on available
> empty slots and with this seeing strange silent hang during any I2C
> register access after filling TX FIFO with 8 words.
> 
> Using writel() followed by i2c_readl() in a loop to write all words
> to TX FIFO instead of using writesl() helps for large transfers in
> PIO mode.
> 
> So, this patch updates i2c_writesl() API to use writel() in a loop
> instead of writesl().
> 
> Signed-off-by: Sowjanya Komatineni <skomatineni@...dia.com>
> ---
>  drivers/i2c/busses/i2c-tegra.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)

Hi Wolfram,

after discussing a bit more with Sowjanya, I don't think we're going to
have a better solution than this. It should be fairly harmless to have
this additional flushing read of the interrupt status register because
the reads don't have any side-effects (the register is write-1-to-clear)
and these write loops don't happen very often (or when they do we tend
to use DMA anyway), so:

Acked-by: Thierry Reding <treding@...dia.com>

I did notice that for some reason Sowjanya hadn't listed you as a
recipient, so perhaps you don't have this anywhere in your inbox. I've
quoted the patch fully for reference below and the patchwork link for
this is:

	https://patchwork.ozlabs.org/project/linux-i2c/patch/1603166634-13639-1-git-send-email-skomatineni@nvidia.com/

If you'd prefer to have this in your inbox for proper review, please
let us know so that Sowjanya can resend this.

Thanks,
Thierry

> 
> diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c
> index 6f08c0c..274bf3a 100644
> --- a/drivers/i2c/busses/i2c-tegra.c
> +++ b/drivers/i2c/busses/i2c-tegra.c
> @@ -333,10 +333,13 @@ static u32 i2c_readl(struct tegra_i2c_dev *i2c_dev, unsigned int reg)
>  	return readl_relaxed(i2c_dev->base + tegra_i2c_reg_addr(i2c_dev, reg));
>  }
>  
> -static void i2c_writesl(struct tegra_i2c_dev *i2c_dev, void *data,
> +static void i2c_writesl(struct tegra_i2c_dev *i2c_dev, u32 *data,
>  			unsigned int reg, unsigned int len)
>  {
> -	writesl(i2c_dev->base + tegra_i2c_reg_addr(i2c_dev, reg), data, len);
> +	while (len--) {
> +		writel(*data++, i2c_dev->base + tegra_i2c_reg_addr(i2c_dev, reg));
> +		i2c_readl(i2c_dev, I2C_INT_STATUS);
> +	}
>  }
>  
>  static void i2c_readsl(struct tegra_i2c_dev *i2c_dev, void *data,
> @@ -811,7 +814,7 @@ 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;
>  
> -		i2c_writesl(i2c_dev, buf, I2C_TX_FIFO, words_to_transfer);
> +		i2c_writesl(i2c_dev, (u32 *)buf, I2C_TX_FIFO, words_to_transfer);
>  
>  		buf += words_to_transfer * BYTES_PER_FIFO_WORD;
>  	}
> -- 
> 2.7.4
> 

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