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

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Message-ID: <5c762653-b636-45bd-8800-e804ad8dfda5@baylibre.com>
Date: Tue, 29 Apr 2025 14:31:02 -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 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().
>>
>> 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);

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.