linux-kernel - Re: [PATCH V2 4/4] misc: vop: mapping kernel memory to user space as noncached

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Date:   Tue, 29 Sep 2020 11:28:33 +0100
From:   Christoph Hellwig <hch@...radead.org>
To:     Sherry Sun <sherry.sun@....com>
Cc:     hch@...radead.org, sudeep.dutt@...el.com, ashutosh.dixit@...el.com,
        arnd@...db.de, gregkh@...uxfoundation.org, kishon@...com,
        lorenzo.pieralisi@....com, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, linux-imx@....com
Subject: Re: [PATCH V2 4/4] misc: vop: mapping kernel memory to user space as
 noncached

On Tue, Sep 29, 2020 at 04:44:25PM +0800, Sherry Sun wrote:
> Mapping kernel space memory to user space as noncached, since user space
> need check the updates of avail_idx and device page flags timely.
> 
> Signed-off-by: Joakim Zhang <qiangqing.zhang@....com>
> Signed-off-by: Sherry Sun <sherry.sun@....com>
> ---
>  drivers/misc/mic/vop/vop_vringh.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/misc/mic/vop/vop_vringh.c b/drivers/misc/mic/vop/vop_vringh.c
> index 4d5feb39aeb7..6e193bd64ef1 100644
> --- a/drivers/misc/mic/vop/vop_vringh.c
> +++ b/drivers/misc/mic/vop/vop_vringh.c
> @@ -1057,7 +1057,7 @@ static int vop_mmap(struct file *f, struct vm_area_struct *vma)
>  		}
>  		err = remap_pfn_range(vma, vma->vm_start + offset,
>  				      pa >> PAGE_SHIFT, size,
> -				      vma->vm_page_prot);
> +				      pgprot_noncached(vma->vm_page_prot));

You can't call remap_pfn_range on memory returned from
dma_alloc_coherent (which btw is not marked uncached on many platforms).

You need to use the dma_mmap_coherent helper instead.  And this also
needs to go into the first patch.