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([2600:8803:e7e4:1d00:96ff:7f79:37f3:5c67]) by smtp.gmail.com with ESMTPSA id 5614622812f47-401ec977dafsm1329187b6e.41.2025.04.26.15.34.11 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 26 Apr 2025 15:34:13 -0700 (PDT) Message-ID: <8a57a332-c2a6-4825-a5f7-d765ea15bd6f@baylibre.com> Date: Sat, 26 Apr 2025 17:34:10 -0500 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 1/6] iio: introduce IIO_DECLARE_BUFFER_WITH_TS macros To: Jonathan Cameron Cc: =?UTF-8?Q?Nuno_S=C3=A1?= , Andy Shevchenko , Lars-Peter Clausen , Michael Hennerich , Eugen Hristev , Nicolas Ferre , Alexandre Belloni , Claudiu Beznea , linux-iio@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org References: <20250425-iio-introduce-iio_declare_buffer_with_ts-v3-0-f12df1bff248@baylibre.com> <20250425-iio-introduce-iio_declare_buffer_with_ts-v3-1-f12df1bff248@baylibre.com> <20250426123509.0b04f0f9@jic23-huawei> From: David Lechner Content-Language: en-US In-Reply-To: <20250426123509.0b04f0f9@jic23-huawei> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 4/26/25 6:35 AM, Jonathan Cameron wrote: > On Fri, 25 Apr 2025 16:08:43 -0500 > David Lechner wrote: > ... >> @@ -777,6 +779,42 @@ static inline void *iio_device_get_drvdata(const struct iio_dev *indio_dev) >> * them safe for use with non-coherent DMA. >> */ >> #define IIO_DMA_MINALIGN ARCH_DMA_MINALIGN >> + >> +#define __IIO_DECLARE_BUFFER_WITH_TS(type, name, count) \ >> + static_assert(count); \ > > Why do we care if count is 0? Or is intent to check if is constant? > If the thought is we don't care either way about 0 (as rather nonsensical) > and this will fail to compile if not constant, then perhaps a comment would > avoid future confusion? I would be inclined to just leave out the check. But yes, it is just checking that count is constant and we don't expect 0. > >> + type name[ALIGN((count), sizeof(s64) / sizeof(type)) + sizeof(s64) / sizeof(type)] >> + >> +/** >> + * IIO_DECLARE_BUFFER_WITH_TS() - Declare a buffer with timestamp >> + * @type: element type of the buffer >> + * @name: identifier name of the buffer >> + * @count: number of elements in the buffer >> + * >> + * Declares a buffer that is safe to use with iio_push_to_buffer_with_ts(). In >> + * addition to allocating enough space for @count elements of @type, it also >> + * allocates space for a s64 timestamp at the end of the buffer and ensures >> + * proper alignment of the timestamp. >> + */ >> +#define IIO_DECLARE_BUFFER_WITH_TS(type, name, count) \ >> + __IIO_DECLARE_BUFFER_WITH_TS(type, name, count) __aligned(sizeof(s64)) >> + >> +/** >> + * 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) __aligned(IIO_DMA_MINALIGN) >> + >> +static_assert(IIO_DMA_MINALIGN % sizeof(s64) == 0, > That message isn't super helpful if seen in a compile log as we aren't reading the code here > "IIO_DECLARE_DMA_BUFFER_WITH_TS() assumes that ... > >> + "macros above assume that IIO_DMA_MINALIGN also ensures s64 timestamp alignment"); >> + Seems we actually have an arch (openrisc) that triggers this [1]. This arch doesn't define ARCH_DMA_MINALIGN so it falls back to: #define ARCH_DMA_MINALIGN __alignof__(unsigned long long) Apparently this is only of those 32-bit arches that only does 4 byte alignment. >From the official docs [2]: Current OR32 implementations (OR1200) do not implement 8 byte alignment, but do require 4 byte alignment. Therefore the Application Binary Interface (chapter 16) uses 4 byte alignment for 8 byte types. Future extensions such as ORVDX64 may require natural alignment. [1]: https://lore.kernel.org/linux-iio/20250425-iio-introduce-iio_declare_buffer_with_ts-v3-0-f12df1bff248@baylibre.com/T/#m91e0332673438793ff76949037ff40a34765ca30 [2]: https://openrisc.io/or1k.html It looks like this could work (it compiles for me): __aligned(MAX(IIO_DMA_MINALIGN, sizeof(s64))) If that is OK we could leave out the static_assert(), unless we think there could be an arch with IIO_DMA_MINALIGN not a power of 2?!