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

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Message-ID: <1d90fae5-9c58-4a77-b81c-2946e7cc74d4@baylibre.com>
Date: Mon, 28 Apr 2025 21:12:47 -0500
From: David Lechner <dlechner@...libre.com>
To: 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>
Cc: 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 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().
> 
> Signed-off-by: David Lechner <dlechner@...libre.com>
> ---

...

> +/**
> + * 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);