counter: fix privdata alignment

Aligning to the L1 cache does not guarantee the same alignment as
kmallocing an object [1]. Furthermore, in some platforms, that
alignment is not sufficient for DMA safety (in case someone wants
to have a DMA safe buffer in privdata) [2].

Sometime ago, we had the same fixes in IIO.

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/base/devres.c#n35
[2]: https://lore.kernel.org/linux-iio/20220508175712.647246-2-jic23@kernel.org/

Fixes: c18e276 ("counter: Provide alternative counter registration functions")
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240209-counter-align-fix-v2-1-5777ea0a2722@analog.com
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>

Author: nunojsa

drivers/counter/counter-core.c

@@ -31,10 +31,11 @@ struct counter_device_allochelper {
 	struct counter_device counter;
 
 	/*
-	 * This is cache line aligned to ensure private data behaves like if it
-	 * were kmalloced separately.
+	 * This ensures private data behaves like if it were kmalloced
+	 * separately. Also ensures the minimum alignment for safe DMA
+	 * operations (which may or may not mean cache alignment).
 	 */
-	unsigned long privdata[] ____cacheline_aligned;
+	unsigned long privdata[] __aligned(ARCH_DMA_MINALIGN);
 };
 
 static void counter_device_release(struct device *dev)