Re: [PATCH v4 1/7] iio: introduce IIO_DECLARE_BUFFER_WITH_TS macros

[Jonathan Cameron <jic23@kernel.org>]

On Tue, 29 Apr 2025 14:47:41 -0500
David Lechner <dlechner@baylibre.com> wrote:

> On 4/29/25 2:36 PM, Andy Shevchenko wrote:
> > On Tue, Apr 29, 2025 at 10:31 PM David Lechner <dlechner@baylibre.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)?
> >   
> 
> Sounds reasonable to me. From what I have seen while working on this is that
> there are quite a few drivers using IIO_DMA_MINALIGN expecting it to be
> sufficient for timestamp alignment, which as it seems is not always the case.
> 
> I'll wait for Jonathan to weigh in though before spinning up a new patch.
> 
It would be very odd if we ever see a platform with a DMA alignment requirement
that is not a multiple of sizeof(s64) just forcing IIO_DMA_MINALIGN to max
of the arch constraint and sizeof(s64) seems fine to me and fixes up
all those other drivers that were assuming this was true already...
I thought it was and only got fussy for this macro :(


Jonathan