Re: [PATCH] Revert "swiotlb: remove SWIOTLB_MAP_ERROR"

[Konrad Rzeszutek Wilk]

On Mon, Mar 04, 2019 at 08:59:03PM +0100, Arnd Bergmann wrote:
> This reverts commit b907e20508d0 ("swiotlb: remove SWIOTLB_MAP_ERROR"), which
> introduced an overflow warning in configurations that have a larger
> dma_addr_t than phys_addr_t:
> 
> In file included from include/linux/dma-direct.h:5,
>                  from kernel/dma/swiotlb.c:23:
> kernel/dma/swiotlb.c: In function 'swiotlb_tbl_map_single':
> include/linux/dma-mapping.h:136:28: error: conversion from 'long long unsigned int' to 'phys_addr_t' {aka 'unsigned int'} changes value from '18446744073709551615' to '4294967295' [-Werror=overflow]
>  #define DMA_MAPPING_ERROR  (~(dma_addr_t)0)
>                             ^
> kernel/dma/swiotlb.c:544:9: note: in expansion of macro 'DMA_MAPPING_ERROR'
>   return DMA_MAPPING_ERROR;
> 
> The configuration that caused this is on 32-bit ARM, where the DMA address
> space depends on the enabled hardware platforms, while the physical
> address space depends on the type of MMU chosen (classic vs LPAE).
> 
> I tried a couple of alternative approaches, but the previous version
> seems as good as any other, so I went back to that.

That is really a bummer.

What about making the phys_addr_t and dma_addr_t have the same
width with some magic #ifdef hackery?

> 
> Fixes: b907e20508d0 ("swiotlb: remove SWIOTLB_MAP_ERROR")
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> ---
>  drivers/xen/swiotlb-xen.c | 4 ++--
>  include/linux/swiotlb.h   | 3 +++
>  kernel/dma/swiotlb.c      | 4 ++--
>  3 files changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> index 877baf2a94f4..57a98279bf4f 100644
> --- a/drivers/xen/swiotlb-xen.c
> +++ b/drivers/xen/swiotlb-xen.c
> @@ -406,7 +406,7 @@ static dma_addr_t xen_swiotlb_map_page(struct device *dev, struct page *page,
>  
>  	map = swiotlb_tbl_map_single(dev, start_dma_addr, phys, size, dir,
>  				     attrs);
> -	if (map == DMA_MAPPING_ERROR)
> +	if (map == SWIOTLB_MAP_ERROR)
>  		return DMA_MAPPING_ERROR;
>  
>  	dev_addr = xen_phys_to_bus(map);
> @@ -557,7 +557,7 @@ xen_swiotlb_map_sg_attrs(struct device *hwdev, struct scatterlist *sgl,
>  								 sg_phys(sg),
>  								 sg->length,
>  								 dir, attrs);
> -			if (map == DMA_MAPPING_ERROR) {
> +			if (map == SWIOTLB_MAP_ERROR) {
>  				dev_warn(hwdev, "swiotlb buffer is full\n");
>  				/* Don't panic here, we expect map_sg users
>  				   to do proper error handling. */
> diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
> index 361f62bb4a8e..a65a36551f58 100644
> --- a/include/linux/swiotlb.h
> +++ b/include/linux/swiotlb.h
> @@ -44,6 +44,9 @@ enum dma_sync_target {
>  	SYNC_FOR_DEVICE = 1,
>  };
>  
> +/* define the last possible byte of physical address space as a mapping error */
> +#define SWIOTLB_MAP_ERROR (~(phys_addr_t)0x0)
> +
>  extern phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
>  					  dma_addr_t tbl_dma_addr,
>  					  phys_addr_t phys, size_t size,
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index 12059b78b631..922880b84387 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -541,7 +541,7 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
>  	spin_unlock_irqrestore(&io_tlb_lock, flags);
>  	if (!(attrs & DMA_ATTR_NO_WARN) && printk_ratelimit())
>  		dev_warn(hwdev, "swiotlb buffer is full (sz: %zd bytes)\n", size);
> -	return DMA_MAPPING_ERROR;
> +	return SWIOTLB_MAP_ERROR;
>  found:
>  	io_tlb_used += nslots;
>  	spin_unlock_irqrestore(&io_tlb_lock, flags);
> @@ -659,7 +659,7 @@ bool swiotlb_map(struct device *dev, phys_addr_t *phys, dma_addr_t *dma_addr,
>  	/* Oh well, have to allocate and map a bounce buffer. */
>  	*phys = swiotlb_tbl_map_single(dev, __phys_to_dma(dev, io_tlb_start),
>  			*phys, size, dir, attrs);
> -	if (*phys == DMA_MAPPING_ERROR)
> +	if (*phys == SWIOTLB_MAP_ERROR)
>  		return false;
>  
>  	/* Ensure that the address returned is DMA'ble */
> -- 
> 2.20.0
>