Re: [PATCH RFC 15/13] gpio: virtio: reorder fields to reduce struct padding

[Bartosz Golaszewski]

On Tue, 30 Dec 2025 17:40:33 +0100, "Michael S. Tsirkin" <mst@xxxxxxxxxx> said:
> Reorder struct virtio_gpio_line fields to place the DMA buffers (req/res)
> last. This eliminates the need for __dma_from_device_aligned_end padding
> after the DMA buffer, since struct tail padding naturally protects it,
> making the struct a bit smaller.
>
> Size reduction estimation when ARCH_DMA_MINALIGN=128:
> - request is 8 bytes
> - response is 2 bytes
> - removing _end saves up to 128-6=122 bytes padding to align rxlen field
>
> Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> ---
>  drivers/gpio/gpio-virtio.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpio/gpio-virtio.c b/drivers/gpio/gpio-virtio.c
> index 32b578b46df8..8b30a94e4625 100644
> --- a/drivers/gpio/gpio-virtio.c
> +++ b/drivers/gpio/gpio-virtio.c
> @@ -26,12 +26,11 @@ struct virtio_gpio_line {
>  	struct mutex lock; /* Protects line operation */
>  	struct completion completion;
>
> +	unsigned int rxlen;
> +
>  	__dma_from_device_aligned_begin
>  	struct virtio_gpio_request req;
>  	struct virtio_gpio_response res;
> -
> -	__dma_from_device_aligned_end
> -	unsigned int rxlen;
>  };
>
>  struct vgpio_irq_line {
> --
> MST
>
>

Acked-by: Bartosz Golaszewski <bartosz.golaszewski@xxxxxxxxxxxxxxxx>