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

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Message-ID: <55e9351282f530e2302e11497c6339c4a2e74471.1767112757.git.mst@redhat.com>
Date: Tue, 30 Dec 2025 11:40:33 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: Cong Wang <xiyou.wangcong@...il.com>, Jonathan Corbet <corbet@....net>,
	Olivia Mackall <olivia@...enic.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Jason Wang <jasowang@...hat.com>,
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	Leon Romanovsky <leon@...nel.org>, Jason Gunthorpe <jgg@...pe.ca>,
	linux-doc@...r.kernel.org, linux-crypto@...r.kernel.org,
	virtualization@...ts.linux.dev, linux-scsi@...r.kernel.org,
	iommu@...ts.linux.dev, kvm@...r.kernel.org, netdev@...r.kernel.org,
	"Enrico Weigelt, metux IT consult" <info@...ux.net>,
	Viresh Kumar <vireshk@...nel.org>,
	Linus Walleij <linusw@...nel.org>,
	Bartosz Golaszewski <brgl@...nel.org>, linux-gpio@...r.kernel.org
Subject: [PATCH RFC 15/13] gpio: virtio: reorder fields to reduce struct
 padding

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@...hat.com>
---
 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