linux-kernel - Re: [PATCH v4 1/7] iio: introduce IIO_DECLARE_BUFFER_WITH

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Message-ID: <CAHp75VfcmvLhBDjbu6x46wGyzG+i7=rVypzSm11qzWN9Qq_rew@mail.gmail.com>
Date: Tue, 29 Apr 2025 22:36:24 +0300
From: Andy Shevchenko <andy.shevchenko@...il.com>
To: David Lechner <dlechner@...libre.com>
Cc: Jonathan Cameron <jic23@...nel.org>, Nuno Sá <nuno.sa@...log.com>, 
	Andy Shevchenko <andy@...nel.org>, Lars-Peter Clausen <lars@...afoo.de>, 
	Michael Hennerich <Michael.Hennerich@...log.com>, Eugen Hristev <eugen.hristev@...aro.org>, 
	Nicolas Ferre <nicolas.ferre@...rochip.com>, 
	Alexandre Belloni <alexandre.belloni@...tlin.com>, Claudiu Beznea <claudiu.beznea@...on.dev>, 
	linux-iio@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 1/7] iio: introduce IIO_DECLARE_BUFFER_WITH_TS macros

On Tue, Apr 29, 2025 at 10:31 PM David Lechner <dlechner@...libre.com> wrote:
> On 4/28/25 9:12 PM, David Lechner wrote:
> > On 4/28/25 3:23 PM, David Lechner wrote:
> >> Add new macros to help with the common case of declaring a buffer that
> >> is safe to use with iio_push_to_buffers_with_ts(). This is not trivial
> >> to do correctly because of the alignment requirements of the timestamp.
> >> This will make it easier for both authors and reviewers.
> >>
> >> To avoid double __align() attributes in cases where we also need DMA
> >> alignment, add a 2nd variant IIO_DECLARE_DMA_BUFFER_WITH_TS().

...

> >> +/**
> >> + * IIO_DECLARE_DMA_BUFFER_WITH_TS() - Declare a DMA-aligned buffer with timestamp
> >> + * @type: element type of the buffer
> >> + * @name: identifier name of the buffer
> >> + * @count: number of elements in the buffer
> >> + *
> >> + * Same as IIO_DECLARE_BUFFER_WITH_TS(), but is uses __aligned(IIO_DMA_MINALIGN)
> >> + * to ensure that the buffer doesn't share cachelines with anything that comes
> >> + * before it in a struct. This should not be used for stack-allocated buffers
> >> + * as stack memory cannot generally be used for DMA.
> >> + */
> >> +#define IIO_DECLARE_DMA_BUFFER_WITH_TS(type, name, count)   \
> >> +    __IIO_DECLARE_BUFFER_WITH_TS(type, name, count)         \
> >> +    /* IIO_DMA_MINALIGN may be 4 on some 32-bit arches. */  \
> >> +    __aligned(MAX(IIO_DMA_MINALIGN, sizeof(s64)))
> >
> > I just realized my logic behind this is faulty. It assumes sizeof(s64) ==
> > __alignof__(s64), but that isn't always true and that is what caused the builds
> > to hit the static_assert() on v3.
> >
> > We should be able to leave this as __aligned(IIO_DMA_MINALIGN)
> >
> > And have this (with better error message):
> >
> > static assert(IIO_DMA_MINALIGN % __alignof__(s64) == 0);
>
> I was working late yesterday and should have saved that reply until morning
> to think about it more!
>
> We do want to align to to sizeof(s64) instead of __alignof__(s64) to avoid
> issues with, e.g. 32-bit kernel and 64-bit userspace (same reason that
> aligned_s64 exists and always uses 8-byte alignment).
>
> So I think this patch is correct as-is after all.

I'm wondering, shouldn't it be better just to make sure that
IIO_DMA_MINALIGN is always bigger or equal to sizeof(s64)?

-- 
With Best Regards,
Andy Shevchenko