linux-kernel - Re: [PATCH v2] i2c: use void pointers for supplying data for reads and writes

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Message-ID: <f1bda5eb-b3ad-e7e1-f832-54a62e708d9c@lucaceresoli.net>
Date:   Tue, 12 Nov 2019 09:45:56 +0100
From:   Luca Ceresoli <luca@...aceresoli.net>
To:     Dmitry Torokhov <dmitry.torokhov@...il.com>,
        Wolfram Sang <wsa@...-dreams.de>
Cc:     Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-iio@...r.kernel.org
Subject: Re: [PATCH v2] i2c: use void pointers for supplying data for reads
 and writes

Hi Dmitry,

On 12/11/19 01:58, Dmitry Torokhov wrote:
> There is no need to force users of i2c_master_send()/i2c_master_recv()
> and other i2c read/write bulk data API to cast everything into u8
> pointers.  While everything can be considered byte stream, the drivers
> are usually work with more structured data.
> 
> Let's switch the APIs to accept [const] void pointers to cut amount of
> casting needed.
> 
> Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>

I agree on the principle, but I have a question, see below.

[...]
>  s32 i2c_smbus_read_i2c_block_data_or_emulated(const struct i2c_client *client,
> -					      u8 command, u8 length, u8 *values)
> +					      u8 command, u8 length, void *values)
>  {
>  	u8 i = 0;
>  	int status;
> @@ -647,8 +648,7 @@ s32 i2c_smbus_read_i2c_block_data_or_emulated(const struct i2c_client *client,
>  			status = i2c_smbus_read_word_data(client, command + i);
>  			if (status < 0)
>  				return status;
> -			values[i] = status & 0xff;
> -			values[i + 1] = status >> 8;
> +			put_unaligned_le16(status, values + i);

The switch to put_unaligned_le16() looks unrelated, is it?

>  			i += 2;
>  		}
>  	}
> @@ -657,7 +657,7 @@ s32 i2c_smbus_read_i2c_block_data_or_emulated(const struct i2c_client *client,
>  		status = i2c_smbus_read_byte_data(client, command + i);
>  		if (status < 0)
>  			return status;
> -		values[i] = status;
> +		*(u8 *)(values + i) = status;

My preference is to use an u8* helper variable in these cases:

s32 i2c_smbus_read_i2c_block_data_or_emulated(const struct i2c_client
*client,
-			      u8 command, u8 length, u8 *values)
+			      u8 command, u8 length, void *buf)
 {
+	u8 *bytes = buf;
@@
-		values[i] = status;
+		bytes[i] = status;

This clarifies we are accessing the raw bytes, avoids typecasts in the
middle of code for readability and avoids void pointer math.

PS: look, it's exactly what you do in the max1363.c file below! :)

> diff --git a/drivers/iio/adc/max1363.c b/drivers/iio/adc/max1363.c
> index 5c2cc61b666e7..48ed76a0e83d4 100644
> --- a/drivers/iio/adc/max1363.c
> +++ b/drivers/iio/adc/max1363.c
> @@ -182,9 +182,9 @@ struct max1363_state {
>  	struct regulator		*vref;
>  	u32				vref_uv;
>  	int				(*send)(const struct i2c_client *client,
> -						const char *buf, int count);
> +						const void *buf, int count);
>  	int				(*recv)(const struct i2c_client *client,
> -						char *buf, int count);
> +						void *buf, int count);
>  };
>  
>  #define MAX1363_MODE_SINGLE(_num, _mask) {				\
> @@ -310,27 +310,29 @@ static const struct max1363_mode
>  	return NULL;
>  }
>  
> -static int max1363_smbus_send(const struct i2c_client *client, const char *buf,
> +static int max1363_smbus_send(const struct i2c_client *client, const void *buf,
>  		int count)
>  {
> +	const u8 *data = buf;

Here it is! ^

-- 
Luca