Re: [PATCH v2 2/7] iio: adc: ti-adc128s052: Simplify using be16_to_cpu()
From: Matti Vaittinen
Date: Mon Apr 07 2025 - 01:23:22 EST
On 05/04/2025 20:29, Jonathan Cameron wrote:
On Thu, 3 Apr 2025 08:16:43 +0300
Matti Vaittinen <mazziesaccount@xxxxxxxxx> wrote:
On 03/04/2025 00:04, David Lechner wrote:
On 4/2/25 1:09 AM, Matti Vaittinen wrote:
The register data is 12-bit big-endian data. Use be16_to_cpu() to do
the conversion, and simple bitwise AND for masking to make it more
obvious.
Signed-off-by: Matti Vaittinen <mazziesaccount@xxxxxxxxx>
---
Revision history:
v1 => v2:
- Fix commit msg to reflect the fact there was no bug
- Drop Fixes tag
- Use union for rx / tx buffer to avoid casting
- Keep the shared message protected by the mutex
---
drivers/iio/adc/ti-adc128s052.c | 18 ++++++++++--------
1 file changed, 10 insertions(+), 8 deletions(-)
diff --git a/drivers/iio/adc/ti-adc128s052.c b/drivers/iio/adc/ti-adc128s052.c
index a456ea78462f..3e69a5fce010 100644
--- a/drivers/iio/adc/ti-adc128s052.c
+++ b/drivers/iio/adc/ti-adc128s052.c
@@ -28,32 +28,34 @@ struct adc128 {
struct regulator *reg;
struct mutex lock;
- u8 buffer[2] __aligned(IIO_DMA_MINALIGN);
+ union {
+ __be16 rx_buffer;
+ u8 tx_buffer[2];
As below. Maybe
__be16 buffer16;
u8 buffer[2];
Ok.
+ } __aligned(IIO_DMA_MINALIGN);
};
static int adc128_adc_conversion(struct adc128 *adc, u8 channel)
{
int ret;
+ char *msg = &adc->tx_buffer[0];
mutex_lock(&adc->lock);
- adc->buffer[0] = channel << 3;
- adc->buffer[1] = 0;
+ msg[0] = channel << 3;
+ msg[1] = 0;
- ret = spi_write(adc->spi, &adc->buffer, 2);
+ ret = spi_write(adc->spi, msg, sizeof(adc->tx_buffer));
I'd get rid of msg as it's now just confusing given we are
using the sizeof() here.
Ok.
if (ret < 0) {
mutex_unlock(&adc->lock);
return ret;
}
- ret = spi_read(adc->spi, &adc->buffer, 2);
-
+ ret = spi_read(adc->spi, &adc->rx_buffer, 2);
sizeof(adc->rx_buffer)
I was thinking of this but went with raw 2 - because we need to read
exactly 2 bytes from the device. Sizeof buffer is matter of software
where as the 2 bytes is dictated by the device. (Sure the size of buffer
needs to be large enough).
I don't care it that much though, so I can go with the sizeof() if
that's what you prefer. Just explaining that the '2' here was a
conscious choice :)
mutex_unlock(&adc->lock);
-
if (ret < 0)
return ret;
- return ((adc->buffer[0] << 8 | adc->buffer[1]) & 0xFFF);
+ return be16_to_cpu(adc->rx_buffer) & 0xFFF;
The cast isn't exactly beautiful, but this would save a lot of
lines of diff and a few lines of code by avoiding the need for
the union and the local msg variable.
return be16_to_cpup((__be16 *)adc->buffer) & 0xFFF;
The cast only works because we have forced the alignment for DMA safety.
That to me is a little fragile.
You could do get_unaligned_be16() which doesn't need the cast then carry
on using the original buffer.
Thanks again for the review David :)
I am unsure which way to go. I kind of like having the __be16 in the
struct, as it immediately yells "data from device is big-endian". OTOH,
I've never loved unions (and, it silences the above "yelling" quite a
bit). I still think this might be the first time I really see a valid
use-case for an union :) And, you're right this adds more lines,
besides, the cast doesn't look that ugly to me. Yet, I originally had a
cast probably as simple as this (and I also had the __be16 in the
struct), and Jonathan suggested using union to avoid it...
At the end of the day, I suppose I am Okay with any of these 3
approaches. Original cast, union or this cast you suggest. Jonathan, any
preferences on your side?
Majority of the diff is really about renaming buffer to tx_buffer.
Could just not bother doing that and instead have buffer and buffer16
as the two union elements. With msg gone as suggested above, then the diff
becomes only a few lines and you get to keep the nicety of it being either
a pair of u8s or a __be16.
I was tempted to try using the spi_write_then_read() - but I suppose
this may be kind of a hot path.
I'll go with (not)renaming the buffer and dropping the msg, to squeeze
the diff.
Yours,
-- Matti