linux-kernel - Re: [PATCH v3 1/3] i2c: use void pointers for supplying data for reads and writes

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Date:   Mon, 18 Nov 2019 00:04:46 -0800
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
Cc:     Wolfram Sang <wsa@...-dreams.de>, linux-iio@...r.kernel.orgi,
        Luca Ceresoli <luca@...aceresoli.net>,
        linux-i2c@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Hartmut Knaack <knaack.h@....de>,
        Jonathan Cameron <jic23@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-iio@...r.kernel.org
Subject: Re: [PATCH v3 1/3] i2c: use void pointers for supplying data for
 reads and writes

Hi Uwe,

On Mon, Nov 18, 2019 at 08:43:49AM +0100, Uwe Kleine-König wrote:
> Hello Dmitry,
> 
> On Tue, Nov 12, 2019 at 12:31:30PM -0800, 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.
> > 
> > Acked-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@...il.com>
> 
> Can you give an example where you save some casts? Given that i2c is a
> byte oriented protocol (as opposed to for example spi) I think it's a
> good idea to expose this in the API.

I see this at least:

dtor@...r-ws:~/kernel/work $ git grep "i2c_master.*(u8 \*)"
drivers/crypto/atmel-i2c.c:     ret = i2c_master_send(client, (u8 *)cmd, cmd->count + WORD_ADDR_SIZE);
drivers/iio/common/ms_sensors/ms_sensors_i2c.c:         ret = i2c_master_recv(client, (u8 *)&buf, 3);
drivers/iio/humidity/hdc100x.c: ret = i2c_master_recv(client, (u8 *)buf, 4);
drivers/iio/pressure/abp060mg.c:        ret = i2c_master_send(client, (u8 *)&buf, state->mreq_len);
drivers/iio/pressure/abp060mg.c:        ret = i2c_master_recv(client, (u8 *)&buf, sizeof(buf));
drivers/input/misc/ad714x-i2c.c:        error = i2c_master_send(client, (u8 *)chip->xfer_buf,
drivers/input/misc/ad714x-i2c.c:        error = i2c_master_send(client, (u8 *)chip->xfer_buf,
drivers/input/misc/ad714x-i2c.c:                error = i2c_master_recv(client, (u8 *)chip->xfer_buf,
drivers/input/touchscreen/sx8654.c:     len = i2c_master_recv(ts->client, (u8 *)data, readlen);
drivers/nfc/nfcmrvl/i2c.c:      ret = i2c_master_recv(drv_data->i2c, (u8 *)&nci_hdr, NCI_CTRL_HDR_SIZE);
drivers/nfc/nxp-nci/i2c.c:      r = i2c_master_recv(client, (u8 *) &header, NXP_NCI_FW_HDR_LEN);
drivers/nfc/nxp-nci/i2c.c:      r = i2c_master_recv(client, (u8 *) &header, NCI_CTRL_HDR_SIZE);
drivers/video/fbdev/ssd1307fb.c:        ret = i2c_master_send(client, (u8 *)array, len);

I am pretty sure there are more that my quick grep did not catch.

And I agree that I2C itself is a byte-oriented protocol, but the data from the
point of the driver (once transfer is done) is often more structured.

> 
> > 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
> 
> This change isn't motivated in the commit log. Is this here by mistake?

No, it is needed as signature of i2c_master_send/recv has changed.

> 
> > @@ -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;
> >  	int i, err;
> >  
> >  	for (i = err = 0; err == 0 && i < count; ++i)
> > -		err = i2c_smbus_write_byte(client, buf[i]);
> > +		err = i2c_smbus_write_byte(client, data[i]);
> 
> Isn't this hunk an indicator that keeping char (or u8) as type of the
> members of buf is a good idea?

Not necessarily, if you check the driver sometimes it deals with stream of
bytes, and sometimes it sends structured data, like little-endian words. I
did not make additional changes because I wanted to minimize amount of changes
to this driver in this patch.


>  
> >  	return err ? err : count;
> >  }
> >  
> > -static int max1363_smbus_recv(const struct i2c_client *client, char *buf,
> > +static int max1363_smbus_recv(const struct i2c_client *client, void *buf,
> >  		int count)
> >  {
> > +	u8 *data = buf;
> >  	int i, ret;
> >  
> >  	for (i = 0; i < count; ++i) {
> >  		ret = i2c_smbus_read_byte(client);
> >  		if (ret < 0)
> >  			return ret;
> > -		buf[i] = ret;
> > +		data[i] = ret;
> >  	}
> >  
> >  	return count;
> 

Thanks.

-- 
Dmitry