linux-kernel - Re: [PATCH 1/3] swiotlb: Export maximum allocation size

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Date:   Thu, 10 Jan 2019 12:02:05 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Joerg Roedel <joro@...tes.org>
Cc:     "Michael S . Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        virtualization@...ts.linux-foundation.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, jfehlig@...e.com,
        jon.grimm@....com, brijesh.singh@....com, hch@....de,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 1/3] swiotlb: Export maximum allocation size

On Thu, Jan 10, 2019 at 02:44:31PM +0100, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@...e.de>
> 
> The SWIOTLB implementation has a maximum size it can
> allocate dma-handles for. This needs to be exported so that
> device drivers don't try to allocate larger chunks.
> 
> This is especially important for block device drivers like
> virtio-blk, that might do DMA through SWIOTLB.

Why not use swiotlb_nr_tbl ? That is how drivers/gpu/drm use to figure if they
need to limit the size of pages.
> 
> Signed-off-by: Joerg Roedel <jroedel@...e.de>
> ---
>  include/linux/swiotlb.h | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index 7c007ed7505f..0bcc80a97036 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -72,6 +72,11 @@ static inline bool is_swiotlb_buffer(phys_addr_t paddr)
>  	return paddr >= io_tlb_start && paddr < io_tlb_end;
>  }
>  
> +static inline size_t swiotlb_max_alloc_size(void)
> +{
> +	return ((1UL << IO_TLB_SHIFT) * IO_TLB_SEGSIZE);
> +}
> +
>  bool swiotlb_map(struct device *dev, phys_addr_t *phys, dma_addr_t *dma_addr,
>  		size_t size, enum dma_data_direction dir, unsigned long attrs);
>  void __init swiotlb_exit(void);
> @@ -95,6 +100,13 @@ static inline unsigned int swiotlb_max_segment(void)
>  {
>  	return 0;
>  }
> +
> +static inline size_t swiotlb_max_alloc_size(void)
> +{
> +	/* There is no limit when SWIOTLB isn't used */
> +	return ~0UL;
> +}
> +
>  #endif /* CONFIG_SWIOTLB */
>  
>  extern void swiotlb_print_info(void);
> -- 
> 2.17.1
>