Re: [PATCH 2/2] iio: adc: ti-adc161s626: use DMA-safe memory for spi_read()

[David Lechner <dlechner@baylibre.com>]

On 3/16/26 2:53 PM, Andy Shevchenko wrote:
> On Mon, Mar 16, 2026 at 06:31:17PM +0000, Jonathan Cameron wrote:
>> On Sat, 14 Mar 2026 18:13:32 -0500
>> David Lechner <dlechner@baylibre.com> wrote:
> 
> ...
> 
>>>  	u8 shift;
>>> +	u8 buf[3] __aligned(IIO_DMA_MINALIGN);
>> On this. There is new generic infrastructure for marking these.
>> https://elixir.bootlin.com/linux/v7.0-rc3/source/include/linux/dma-mapping.h#L720
>> https://lore.kernel.org/all/01ea88055ded4d70cac70ba557680fd5fa7d9ff5.1767601130.git.mst@redhat.com/
>>
>> Would look like
>> 	__dma_from_device_group_begin();
>> 	u8 buf[3];
>> 	__dma_from_device_group_end();
>>
>> Do you think we should adopt them rather than doing our own thing?
>> Slowly though I don't want the noise of a mass conversion.
>>
>> As normal, advantage of standard infrastructure is cutting down
>> in subsystem specific magic.
>>
>> I 'think' result is the same (though it also forces the trailing padding if anything
>> comes after this and needs it).
> 
> As I read it it will be an equivalent to
> 
> 	u8 shift; __aligned(IIO_DMA_MINALIGN);
> 	u8 buf[3] __aligned(IIO_DMA_MINALIGN);
> 
> 

It will be:

	u8 shift;
	__u8 __cacheline_group_begin__[0] __aligned(ARCH_DMA_MINALIGN);
	u8 buf[3];
	__u8 __cacheline_group_end__[0] __aligned(ARCH_DMA_MINALIGN);

Note that ARCH_DMA_MINALIGN is not always the same as IIO_DMA_MINALIGN.
IIO_DMA_MINALIGN has a minimum of 8 bytes to account for timestamp
alignment.

I wonder if this would add an extra ARCH_DMA_MINALIGN bytes to
the struct. Or if the compiler is smart enough to see that it has
0 size on the last array and have a special case for that.

And even if the 0 is handled, if someone added a new field after this,
I expect the struct would grow by ARCH_DMA_MINALIGN rather than sizeof(field)
bytes.