Re: [PATCH 2/2] swiotlb: Return consistent SWIOTLB segments/nr_tbl

[Christoph Hellwig]

On Tue, Jun 11, 2019 at 10:58:25AM -0700, Florian Fainelli wrote:
> With a specifically contrived memory layout where there is no physical
> memory available to the kernel below the 4GB boundary, we will fail to
> perform the initial swiotlb_init() call and set no_iotlb_memory to true.
> 
> There are drivers out there that call into swiotlb_nr_tbl() to determine
> whether they can use the SWIOTLB. With the right DMA_BIT_MASK() value
> for these drivers (say 64-bit), they won't ever need to hit
> swiotlb_tbl_map_single() so this can go unoticed and we would be
> possibly lying about those drivers.
> 
> Signed-off-by: Florian Fainelli <f.fainelli@xxxxxxxxx>
> ---
>  kernel/dma/swiotlb.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> index b2b5c5df273c..e906ef2e6315 100644
> --- a/kernel/dma/swiotlb.c
> +++ b/kernel/dma/swiotlb.c
> @@ -129,15 +129,17 @@ setup_io_tlb_npages(char *str)
>  }
>  early_param("swiotlb", setup_io_tlb_npages);
>  
> +static bool no_iotlb_memory;
> +
>  unsigned long swiotlb_nr_tbl(void)
>  {
> -	return io_tlb_nslabs;
> +	return unlikely(no_iotlb_memory) ? 0 : io_tlb_nslabs;
>  }
>  EXPORT_SYMBOL_GPL(swiotlb_nr_tbl);
>  
>  unsigned int swiotlb_max_segment(void)
>  {
> -	return max_segment;
> +	return unlikely(no_iotlb_memory) ? 0 : max_segment;

I wouldn't bother with the unlikely here as anythign querying
swiotlb details should pretty much be a slow path already.

Otherwise looks good:

Reviewed-by: Christoph Hellwig <hch@xxxxxx>