linux-kernel - Re: [PATCH v3] iio: add driver for Microchip MCP413X/414X/415X/416X/423X/424X/425X/426X

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Date:	Sun, 20 Mar 2016 19:15:50 +0100
From:	Joachim Eastwood <manabian@...il.com>
To:	Jonathan Cameron <jic23@...nel.org>
Cc:	Slawomir Stepien <sst@...zta.fm>, Hartmut Knaack <knaack.h@....de>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Peter Meerwald-Stadler <pmeerw@...erw.net>,
	linux-iio@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3] iio: add driver for Microchip MCP413X/414X/415X/416X/423X/424X/425X/426X

Hi Jonathan,

On 20 March 2016 at 18:25, Jonathan Cameron <jic23@...nel.org> wrote:
> On 20/03/16 16:12, Joachim Eastwood wrote:
>>> +static int mcp4131_exec(struct mcp4131_data *data,
>>> +               u8 addr, u8 cmd,
>>> +               u16 val)
>>> +{
>>> +       int err;
>>> +       struct spi_device *spi = data->spi;
>>> +
>>> +       data->xfer.tx_buf = data->buf;
>>> +       data->xfer.rx_buf = data->buf;
>>> +
>>> +       switch (cmd) {
>>> +       case MCP4131_READ:
>>> +               data->xfer.len = 2; /* Two bytes transfer for this command */
>>> +               data->buf[0] = (addr << MCP4131_WIPER_SHIFT) | MCP4131_READ;
>>> +               data->buf[1] = 0;
>>> +               break;
>>> +
>>> +       case MCP4131_WRITE:
>>> +               data->xfer.len = 2;
>>> +               data->buf[0] = (addr << MCP4131_WIPER_SHIFT) |
>>> +                       MCP4131_WRITE | (val >> 8);
>>> +               data->buf[1] = val & 0xFF; /* 8 bits here */
>>> +               break;
>>> +
>>> +       default:
>>> +               return -EINVAL;
>>> +       }
>>> +
>>> +       dev_dbg(&spi->dev, "mcp4131_exec: tx0: 0x%x tx1: 0x%x\n",
>>> +                       data->buf[0], data->buf[1]);
>>> +
>>> +       spi_message_init(&data->msg);
>>> +       spi_message_add_tail(&data->xfer, &data->msg);
>>> +
>>> +       err = spi_sync(spi, &data->msg);
>>> +       if (err) {
>>> +               dev_err(&spi->dev, "spi_sync(): %d\n", err);
>>> +               return err;
>>> +       }
>>
>> Isn't this init, add, sync sequence basically open coding of what
>> spi_write/spi_read does?
>> If you could use those you could also get rid transfer/message structs
>> in priv data.
> I initially wrote the same comment, then realised it's more nuanced than
> that.  Whilst this initially looks like an w8r8 type cycle it's actually
> something like w4r12 in the read case anyway.  The write case could indeed
> be done with spi_write.

Indeed. I didn't notice that for the read case.

The read case could almost be copy of spi_read, though. One would only
need to add ".tx_buf = buf" when setting up the transfer struct, I
think. Having it in its a own function with a comment would make it
easier to spot the difference.


regards,
Joachim Eastwood