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

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Message-ID: <20260102080135-mutt-send-email-mst@kernel.org>
Date: Fri, 2 Jan 2026 08:02:36 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Bartosz Golaszewski <brgl@...nel.org>
Cc: linux-kernel@...r.kernel.org, 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>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Stefan Hajnoczi <stefanha@...hat.com>,
	Eugenio Pérez <eperezma@...hat.com>,
	"James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Gerd Hoffmann <kraxel@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Robin Murphy <robin.murphy@....com>,
	Stefano Garzarella <sgarzare@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>, Petr Tesarik <ptesarik@...e.com>,
	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>, linux-gpio@...r.kernel.org
Subject: Re: [PATCH RFC 15/13] gpio: virtio: reorder fields to reduce struct
 padding

On Fri, Jan 02, 2026 at 12:47:04PM +0000, Bartosz Golaszewski wrote:
> On Tue, 30 Dec 2025 17:40:33 +0100, "Michael S. Tsirkin" <mst@...hat.com> 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@...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
> >
> >
> 
> Acked-by: Bartosz Golaszewski <bartosz.golaszewski@....qualcomm.com>

Thanks! There's a new API as suggested by Petr so these patches got changed,
but the same idea. Do you want me to carry your ack or you prefer to
re-review?

-- 
MST