linux-kernel - Re: [PATCH v2 3/4] i2c: designware: Allow SMBus block reads up to 255 bytes in length

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Message-ID: <20200917205704.GA18027@kunai>
Date:   Thu, 17 Sep 2020 22:57:04 +0200
From:   Wolfram Sang <wsa@...nel.org>
To:     Sultan Alsawaf <sultan@...neltoast.com>
Cc:     linux-i2c@...r.kernel.org, jikos@...nel.org,
        aaron.ma@...onical.com, admin@...ma.net,
        andriy.shevchenko@...ux.intel.com, benjamin.tissoires@...hat.com,
        hdegoede@...hat.com, hn.chen@...dahitech.com,
        jarkko.nikula@...ux.intel.com, kai.heng.feng@...onical.com,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        mika.westerberg@...ux.intel.com, vicamo.yang@...onical.com
Subject: Re: [PATCH v2 3/4] i2c: designware: Allow SMBus block reads up to
 255 bytes in length

On Wed, Sep 16, 2020 at 10:22:55PM -0700, Sultan Alsawaf wrote:
> From: Sultan Alsawaf <sultan@...neltoast.com>
> 
> According to the SMBus 3.0 protocol specification, block transfer limits
> were increased from 32 bytes to 255 bytes. Remove the obsolete 32-byte
> limitation.

Sadly, it is not that easy. We are trying to extend BLOCK_MAX to 255
(SMBus 3 specs) but there are various things to be considered,
especially with buffers and when passing it to userspace. Check here for
the discussion (and you are welcome to join, of course):

http://patchwork.ozlabs.org/project/linux-i2c/list/?submitter=79741&state=*

> 
> Signed-off-by: Sultan Alsawaf <sultan@...neltoast.com>
> ---
>  drivers/i2c/busses/i2c-designware-master.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/i2c/busses/i2c-designware-master.c b/drivers/i2c/busses/i2c-designware-master.c
> index 22f28516bca7..5bd64bd17d94 100644
> --- a/drivers/i2c/busses/i2c-designware-master.c
> +++ b/drivers/i2c/busses/i2c-designware-master.c
> @@ -433,7 +433,7 @@ i2c_dw_read(struct dw_i2c_dev *dev)
>  			regmap_read(dev->map, DW_IC_DATA_CMD, &tmp);
>  			if (flags & I2C_M_RECV_LEN) {
>  				/* Ensure length byte is a valid value */
> -				if (tmp <= I2C_SMBUS_BLOCK_MAX && tmp > 0)
> +				if (tmp > 0)
>  					len = i2c_dw_recv_len(dev, tmp);
>  				else
>  					len = i2c_dw_recv_len(dev, len);
> -- 
> 2.28.0
> 

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