RE: [PATCH v8 2/2] iio: adc: max14001: New driver

[Paller, Kim Seer]

> -----Original Message-----
> From: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
> Sent: Sunday, July 2, 2023 6:04 PM
> To: Paller, Kim Seer <KimSeer.Paller@xxxxxxxxxx>
> Cc: jic23@xxxxxxxxxx; lars@xxxxxxxxxx; lgirdwood@xxxxxxxxx;
> broonie@xxxxxxxxxx; Hennerich, Michael <Michael.Hennerich@xxxxxxxxxx>;
> andy.shevchenko@xxxxxxxxx; robh@xxxxxxxxxx;
> krzysztof.kozlowski@xxxxxxxxxx; conor+dt@xxxxxxxxxx; linux-
> iio@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; devicetree@xxxxxxxxxxxxxxx;
> kernel test robot <lkp@xxxxxxxxx>
> Subject: Re: [PATCH v8 2/2] iio: adc: max14001: New driver
> 
> [External]
> 
> On Thu, 22 Jun 2023 22:32:27 +0800
> Kim Seer Paller <kimseer.paller@xxxxxxxxxx> wrote:
> 
> > The MAX14001 is configurable, isolated 10-bit ADCs for multi-range
> > binary inputs.
> >
> > Signed-off-by: Kim Seer Paller <kimseer.paller@xxxxxxxxxx>
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > Closes:
> > https://urldefense.com/v3/__https://lore.kernel.org/oe-kbuild-all/2023
> > 06211545.7b6CdqsL-
> lkp@xxxxxxxxx/__;!!A3Ni8CS0y2Y!4npD8X6TpKmeLcUf8QqQW
> >
> yEFp_Z1ORKb2dZNpuqfj0ZK74NiCYKQLNWEfKzVmuKTHJO0RW8n01vdXURqBvc
> ueb3V1Sb
> > GQdI$
> 
> Hi,
> 
> Two outstanding comments that I think I raised in earlier reviews..
> 
> Jonathan
> 
> > diff --git a/drivers/iio/adc/max14001.c b/drivers/iio/adc/max14001.c
> > new file mode 100644 index 000000000000..a21ebcde71fa
> > --- /dev/null
> > +++ b/drivers/iio/adc/max14001.c
> 
> ...
> 
> > +static int max14001_read(void *context, unsigned int reg_addr, unsigned int
> *data)
> > +{
> > +	struct max14001_state *st = context;
> > +	int ret;
> > +
> > +	struct spi_transfer xfers[] = {
> > +		{
> > +			.tx_buf = &st->spi_tx_buffer,
> > +			.len = sizeof(st->spi_tx_buffer),
> > +			.cs_change = 1,
> > +		}, {
> > +			.rx_buf = &st->spi_rx_buffer,
> > +			.len = sizeof(st->spi_rx_buffer),
> > +		},
> > +	};
> > +
> > +	/*
> > +	 * Convert transmit buffer to big-endian format and reverse transmit
> > +	 * buffer to align with the LSB-first input on SDI port.
> > +	 */
> > +	st->spi_tx_buffer =
> cpu_to_be16(bitrev16(FIELD_PREP(MAX14001_ADDR_MASK,
> > +								reg_addr)));
> > +
> > +	ret = spi_sync_transfer(st->spi, xfers, ARRAY_SIZE(xfers));
> > +	if (ret)
> > +		return ret;
> > +
> > +	/*
> > +	 * Align received data from the receive buffer, reversing and reordering
> > +	 * it to match the expected MSB-first format.
> > +	 */
> > +	*data = (__force u16)(be16_to_cpu(bitrev16(st->spi_rx_buffer))) &
> > +
> 	MAX14001_DATA_MASK;
> > +
> These sequences still confuse me a lot because I'd expect the values in tx
> to have the opposite operations applied to those for rx and that's not the
> case.
> 
> Let's take a le system.
> tx = cpu_to_be16(bitrev16(x))
>    = cpu_to_be16((__bitrev8(x & 0xff) << 8) | __bitrev8(x >> 8));
>    = __bitrev8(x & 0xff) | (__bitrev8(x >> 8) << 8)
> or swap all the bits in each byte, but don't swap the bytes.
> 
> rx = cpu_to_be16(bitrev16(x))
>    = be16_to_cpu(((__bitrev8(x & 0xff) << 8) | __bitrev8(x >> 8)_
>    = __bitrev8(x & 0xff) | __bitrev(x >> 8)
> 
> also swap all the bits in each byte, but don't swap the bytes.
> 
> So it is the reverse because the bytes swaps unwind themselves somewhat.
> For a be system cpu_to_be16 etc are noop.
> tx = (__bitrev8(x & 0xff) << 8) | __bitrev8(x >> 8)
> rx = (__bitrev8(x & 0xff) << 8) | __bitrev8(x >> 8)
> 
> So in this case swap all 16 bits.
> 
> Now, given I'd expected them to be reversed for the tx vs rx case.
> E.g.
> tx = cpu_to_be16(bitrev16(x))
> As above.
> For rx, le host
> rx = bitrev16(be16_to_cpu(x))
>    = __bitrev8((x >> 8) & 0xff) << 8) |  __bitrev8((((x & 0xff) << 8) >> 8)
> same as above (if you swap the two terms I think.
> 
> For be the be16_to_cpu is a noop again, so it's just bitrev16(x) as expected.
> 
> Hence if I've understood this correctly you could reverse the terms so that
> it was 'obvious' you were doing the opposite for the tx term vs the rx one
> without making the slightest bit of difference....
> 
> hmm. Might be worth doing simply to avoid questions.

Thank you for your feedback. I have tested the modifications based on your 
suggestions, taking the le system into account, and it appears that the code is 
functioning correctly. Before sending the new patch version, I would like to 
confirm if this aligns with your comments.

static int max14001_read(void *context, unsigned int reg_addr, unsigned int *data)
{
	struct max14001_state *st = context;
	int ret;

	struct spi_transfer xfers[] = {
		{
			.tx_buf = &st->spi_tx_buffer,
			.len = sizeof(st->spi_tx_buffer),
			.cs_change = 1,
		}, {
			.rx_buf = &st->spi_rx_buffer,
			.len = sizeof(st->spi_rx_buffer),
		},
	};

	st->spi_tx_buffer = cpu_to_be16(bitrev16(FIELD_PREP(MAX14001_ADDR_MASK, reg_addr)));

	ret = spi_sync_transfer(st->spi, xfers, ARRAY_SIZE(xfers));
	if (ret)
		return ret;

	*data = cpu_to_be16(bitrev16(st->spi_rx_buffer));

	return 0;
}

static int max14001_write(void *context, unsigned int reg_addr, unsigned int data)
{
	struct max14001_state *st = context;

	st->spi_tx_buffer = cpu_to_be16(bitrev16(
				FIELD_PREP(MAX14001_ADDR_MASK, reg_addr) |
				FIELD_PREP(MAX14001_SET_WRITE_BIT, 1) |
				FIELD_PREP(MAX14001_DATA_MASK, data)));

	return spi_write(st->spi, &st->spi_tx_buffer, sizeof(st->spi_tx_buffer));
}
 
> > +	return 0;
> > +}
> > +static int max14001_reg_update(struct max14001_state *st,
> > +				unsigned int reg_addr,
> > +				unsigned int mask,
> > +				unsigned int val)
> > +{
> > +	int ret;
> > +	unsigned int reg_data;
> > +
> > +	/* Enable SPI Registers Write */
> > +	ret = max14001_write(st, MAX14001_WEN, MAX14001_WRITE_WEN);
> > +	if (ret)
> > +		return ret;
> > +
> > +	ret = max14001_read(st, reg_addr, &reg_data);
> > +	if (ret)
> > +		return ret;
> > +
> > +	reg_data |= FIELD_PREP(mask, val);
> 
> This is still a problem if the compiler happens to fail to figure out
> that mask is a compile time constant.  Given it only ever takes one value
> I'd suggest either calling the FIELD_PREP at the caller, or just
> pushing all this code inline so that you can put the definition
> inline.

I would like to confirm including the 'static inline' keyword for the 
max14001_reg_update function.

> > +
> > +	ret = max14001_write(st, reg_addr, reg_data);
> > +	if (ret)
> > +		return ret;
> > +
> > +	/* Write Verification Register */
> > +	ret = max14001_write_verification_reg(st, reg_addr);
> > +	if (ret)
> > +		return ret;
> > +
> > +	/* Disable SPI Registers Write */
> > +	return max14001_write(st, MAX14001_WEN, 0);
> > +}
> 

Best Regards,
Kim Seer Paller