linux-kernel - Re: [PATCH v3 2/2] i2c: tegra: proper handling of error cases

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Date:	Mon, 18 Apr 2016 16:17:05 +0200
From:	Thierry Reding <thierry.reding@...il.com>
To:	Shardar Shariff Md <smohammed@...dia.com>
Cc:	ldewangan@...dia.com, swarren@...dotorg.org,
	linux-i2c@...r.kernel.org, linux-tegra@...r.kernel.org,
	linux-kernel@...r.kernel.org, wsa@...-dreams.de, gnurou@...il.com
Subject: Re: [PATCH v3 2/2] i2c: tegra: proper handling of error cases

On Mon, Apr 18, 2016 at 06:45:55PM +0530, Shardar Shariff Md wrote:
> To summarize the issue observed in error cases:
> 
> SW Flow: For i2c message transfer, packet header and data payload is
> posted and then required error/packet completion interrupts are enabled
> later.
> 
> HW flow: HW process the packet just after packet header is posted, if
> ARB lost/NACK error occurs (SW will not handle immediately when error
> happens as error interrupts are not enabled at this point). HW assumes
> error is acknowledged and clears current data in FIFO, But SW here posts
> the remaining data payload which still stays in FIFO as stale data
> (data without packet header).
> 
> Now once the interrupts are enabled, SW handles ARB lost/NACK error by
> clearing the ARB lost/NACK interrupt. Now HW assumes that SW attended
> the error and will parse/process stale data (data without packet header)
> present in FIFO which causes invalid NACK errors.
> 
> Fix: Enable the error interrupts before posting the packet into FIFO
> which make sure HW to not clear the fifo. Also disable the packet mode
> before acknowledging errors (ARB lost/NACK error) to not process any
> stale data. As error interrupts are enabled before posting the packet
> header use spinlock to avoid preempting.
> 
> Signed-off-by: Shardar Shariff Md <smohammed@...dia.com>
> 
> Changes since v1:
> - Align the commit message to 72 characters per line.
> - Removing unnecessary paranthesis.
> - Printing error if tegra_i2c_disable_packet_mode() fails
>   is already present and handling error is not taken cared
>   in ISR which was done in v2.

I think I didn't express myself very clearly, or gave insufficient
information. The changelog reads easier if you have one entry per
revision. The above would then read:

Changes in v3:
- Printing error if tegra_i2c_disable_packet_mode() fails
  is already present and handling error is not taken cared
  in ISR which was done in v2.

Changes in v2:
- Align the commit message to 72 characters per line.
- Removing unnecessary paranthesis.

And it should go below the --- separator.

One more nitpick below.

> ---
>  drivers/i2c/busses/i2c-tegra.c | 28 ++++++++++++++++++++++++++--
>  1 file changed, 26 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c
> index 6235f16..7922892 100644
> --- a/drivers/i2c/busses/i2c-tegra.c
> +++ b/drivers/i2c/busses/i2c-tegra.c
> @@ -191,6 +191,7 @@ struct tegra_i2c_dev {
>  	u16 clk_divisor_non_hs_mode;
>  	bool is_suspended;
>  	bool is_multimaster_mode;
> +	spinlock_t xfer_lock;
>  };
>  
>  static void dvc_writel(struct tegra_i2c_dev *i2c_dev, u32 val, unsigned long reg)
> @@ -510,14 +511,27 @@ err:
>  	return err;
>  }
>  
> +static int tegra_i2c_disable_packet_mode(struct tegra_i2c_dev *i2c_dev)
> +{
> +	u32 cnfg;
> +
> +	cnfg = i2c_readl(i2c_dev, I2C_CNFG);
> +	if (cnfg & I2C_CNFG_PACKET_MODE_EN)
> +		i2c_writel(i2c_dev, cnfg & ~I2C_CNFG_PACKET_MODE_EN, I2C_CNFG);
> +
> +	return tegra_i2c_wait_for_config_load(i2c_dev);
> +}

Since you no longer use the return value you might as well just make the
function return void. I don't feel very strongly about it, though, so
I'll defer to Wolfram.

Thierry

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