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Message-ID: <20250407194937.097b7709@jic23-huawei>
Date: Mon, 7 Apr 2025 19:49:37 +0100
From: Jonathan Cameron <jic23@...nel.org>
To: Matti Vaittinen <mazziesaccount@...il.com>
Cc: David Lechner <dlechner@...libre.com>, Matti Vaittinen
<matti.vaittinen@...rohmeurope.com>, Lars-Peter Clausen <lars@...afoo.de>,
Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
Conor Dooley <conor+dt@...nel.org>, Nuno Sa <nuno.sa@...log.com>, Javier
Carrasco <javier.carrasco.cruz@...il.com>, linux-iio@...r.kernel.org,
devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/7] iio: adc: ti-adc128s052: Simplify using
be16_to_cpu()
On Mon, 7 Apr 2025 08:23:07 +0300
Matti Vaittinen <mazziesaccount@...il.com> wrote:
> On 05/04/2025 20:29, Jonathan Cameron wrote:
> > On Thu, 3 Apr 2025 08:16:43 +0300
> > Matti Vaittinen <mazziesaccount@...il.com> 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@...il.com>
> >>>> ---
> >>>> 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 :)
Hmm. If we have a case where we read less than 2 bytes into that buffer
then fair enough. Otherwise it's correctly sized so having sizeof(buffer)
and having to check that size in only one place is a tiny bit preferable.
>
> >>>> 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.
Given it's small, I doubt it would make any noticeable difference in
performance.
>
> I'll go with (not)renaming the buffer and dropping the msg, to squeeze
> the diff.
Works for me.
J
>
> Yours,
> -- Matti
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