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

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Message-ID: <CAMRc=MdWWQihgt8haFYxSh7MgUoBuf3ZkBA6cbErSVNmAtb8Mw@mail.gmail.com>
Date: Fri, 2 Jan 2026 14:27:50 +0100
From: Bartosz Golaszewski <brgl@...nel.org>
To: "Michael S. Tsirkin" <mst@...hat.com>
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 2, 2026 at 2:02 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> 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
>

I'll take a second look. Can you Cc me on all the key patches - like
the ones introducing new APIs? I needed to grab it from lore this
time.

Bart