linux-kernel - Re: [PATCH] uio: Provide UIO_MEM_PHYS

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Message-ID: <2026011415-portal-balsamic-a748@gregkh>
Date: Wed, 14 Jan 2026 12:24:43 +0100
From: Greg KH <gregkh@...uxfoundation.org>
To: Mike Looijmans <mike.looijmans@...ic.nl>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH] uio: Provide UIO_MEM_PHYS_WC for write-combine mappings

On Wed, Jan 14, 2026 at 11:51:49AM +0100, Mike Looijmans wrote:
> Introduce UIO_MEM_PHYS_WC which is identical to UIO_MEM_PHYS except
> that the mapping will enable write-combining. This allows the system to
> coalesce writes and greatly improves performance if the underlying
> hardware allows that. Typically used for video buffers, but also very
> well suited for filling FPGA (block)RAMs with reference data.
> 
> Signed-off-by: Mike Looijmans <mike.looijmans@...ic.nl>
> 
> ---
> 
>  drivers/uio/uio.c          | 3 +++
>  include/linux/uio_driver.h | 1 +
>  2 files changed, 4 insertions(+)
> 
> diff --git a/drivers/uio/uio.c b/drivers/uio/uio.c
> index fa0d4e6aee16..c33db04d2004 100644
> --- a/drivers/uio/uio.c
> +++ b/drivers/uio/uio.c
> @@ -757,6 +757,8 @@ static int uio_mmap_physical(struct vm_area_struct *vma)
>  	vma->vm_ops = &uio_physical_vm_ops;
>  	if (idev->info->mem[mi].memtype == UIO_MEM_PHYS)
>  		vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
> +	else if (idev->info->mem[mi].memtype == UIO_MEM_PHYS_WC)
> +		vma->vm_page_prot = pgprot_writecombine(vma->vm_page_prot);
>  
>  	/*
>  	 * We cannot use the vm_iomap_memory() helper here,
> @@ -858,6 +860,7 @@ static int uio_mmap(struct file *filep, struct vm_area_struct *vma)
>  	switch (idev->info->mem[mi].memtype) {
>  	case UIO_MEM_IOVA:
>  	case UIO_MEM_PHYS:
> +	case UIO_MEM_PHYS_WC:
>  		ret = uio_mmap_physical(vma);
>  		break;
>  	case UIO_MEM_LOGICAL:
> diff --git a/include/linux/uio_driver.h b/include/linux/uio_driver.h
> index 334641e20fb1..b925b8c17214 100644
> --- a/include/linux/uio_driver.h
> +++ b/include/linux/uio_driver.h
> @@ -171,6 +171,7 @@ extern int __must_check
>   * Do not use in new drivers.
>   */
>  #define UIO_MEM_DMA_COHERENT	5
> +#define UIO_MEM_PHYS_WC	6
>  
>  /* defines for uio_port->porttype */
>  #define UIO_PORT_NONE	0

But you have no in-kernel user for this, so why add it?  We really can't
take this unless there is such a user, sorry.

thanks,

greg k-h