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SubjectRe: [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@huawei.com>
> > Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.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@dtor-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

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