linux-kernel - Re: [PATCH V15 3/6] i2c: tegra: fix maximum transfer size

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Date:   Fri, 8 Feb 2019 01:09:47 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Sowjanya Komatineni <skomatineni@...dia.com>,
        thierry.reding@...il.com, jonathanh@...dia.com,
        mkarthik@...dia.com, smohammed@...dia.com, talho@...dia.com
Cc:     linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-i2c@...r.kernel.org
Subject: Re: [PATCH V15 3/6] i2c: tegra: fix maximum transfer size

08.02.2019 0:47, Sowjanya Komatineni пишет:
> Tegra194 supports maximum 64K bytes transfer per packet.
> Tegra186 and prior supports maximum 4K bytes transfer per packet.
> This includes 12 bytes of packet header.
> 
> This patch fixes max write length to account for packet header size
> for transfers.

The commit message isn't accurate. We are not actually fixing anything here, this patch is just a preparation for adding DMA support that require the packet-header size to be excluded from the overall TX size-quirk because the packet-header is the part of the DMA transfer.

Given how small this change is, it's probably not really worth to factor out it into a separate patch, let's just squash it to to the DMA-patch.

> Signed-off-by: Sowjanya Komatineni <skomatineni@...dia.com>
> ---
>  [V15]	: This is new patch in this series.
> 
>  drivers/i2c/busses/i2c-tegra.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/i2c/busses/i2c-tegra.c b/drivers/i2c/busses/i2c-tegra.c
> index 3758c7a2c781..ab474cc86e0a 100644
> --- a/drivers/i2c/busses/i2c-tegra.c
> +++ b/drivers/i2c/busses/i2c-tegra.c
> @@ -125,6 +125,9 @@
>  #define I2C_MST_FIFO_STATUS_TX_MASK		0xff0000
>  #define I2C_MST_FIFO_STATUS_TX_SHIFT		16
>  
> +/* Packet header size in bytes */
> +#define I2C_PACKET_HEADER_SIZE			12
> +
>  /*
>   * msg_end_type: The bus control which need to be send at end of transfer.
>   * @MSG_END_STOP: Send stop pulse at end of transfer.
> @@ -900,11 +903,13 @@ static const struct i2c_algorithm tegra_i2c_algo = {
>  static const struct i2c_adapter_quirks tegra_i2c_quirks = {
>  	.flags = I2C_AQ_NO_ZERO_LEN,
>  	.max_read_len = 4096,
> -	.max_write_len = 4096,
> +	.max_write_len = 4096 - I2C_PACKET_HEADER_SIZE,
>  };
>  
>  static const struct i2c_adapter_quirks tegra194_i2c_quirks = {
>  	.flags = I2C_AQ_NO_ZERO_LEN,
> +	.max_read_len = 65535,
> +	.max_write_len = 65535 - I2C_PACKET_HEADER_SIZE,
>  };

This is wrong, 65535 = 64 * 1024 - 1.

Let's just use size-constants provided by kernel:

static const struct i2c_adapter_quirks tegra_i2c_quirks = {
	.flags = I2C_AQ_NO_ZERO_LEN,
	.max_read_len = SZ_4K,
	.max_write_len = SZ_4K - I2C_PACKET_HEADER_SIZE,
};

static const struct i2c_adapter_quirks tegra194_i2c_quirks = {
	.flags = I2C_AQ_NO_ZERO_LEN,
	.max_read_len = SZ_64K,
	.max_write_len = SZ_64K - I2C_PACKET_HEADER_SIZE,
};


I'll take a look at other patches later today, no need to send out new version right now.