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Date:   Sun, 21 Jun 2020 14:12:00 -0700
From:   Guenter Roeck <linux@roeck-us.net>
To:     David Rientjes <rientjes@google.com>
Cc:     Christoph Hellwig <hch@lst.de>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Linux IOMMU <iommu@lists.linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [patch] dma-pool: warn when coherent pool is depleted
Message-ID: <20200621211200.GA158319@roeck-us.net>
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On Sun, Jun 21, 2020 at 01:43:02PM -0700, David Rientjes wrote:
> When a DMA coherent pool is depleted, allocation failures may or may not
> get reported in the kernel log depending on the allocator.
> 
> The admin does have a workaround, however, by using coherent_pool= on the
> kernel command line.
> 
> Provide some guidance on the failure and a recommended minimum size for
> the pools (double the size).
> 
> Signed-off-by: David Rientjes <rientjes@google.com>

Tested-by: Guenter Roeck <linux@roeck-us.net>

Also confirmed that coherent_pool=256k works around the crash
I had observed.

Guenter

> ---
>  kernel/dma/pool.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/dma/pool.c b/kernel/dma/pool.c
> --- a/kernel/dma/pool.c
> +++ b/kernel/dma/pool.c
> @@ -239,12 +239,16 @@ void *dma_alloc_from_pool(struct device *dev, size_t size,
>  	}
>  
>  	val = gen_pool_alloc(pool, size);
> -	if (val) {
> +	if (likely(val)) {
>  		phys_addr_t phys = gen_pool_virt_to_phys(pool, val);
>  
>  		*ret_page = pfn_to_page(__phys_to_pfn(phys));
>  		ptr = (void *)val;
>  		memset(ptr, 0, size);
> +	} else {
> +		WARN_ONCE(1, "DMA coherent pool depleted, increase size "
> +			     "(recommended min coherent_pool=%zuK)\n",
> +			  gen_pool_size(pool) >> 9);
>  	}
>  	if (gen_pool_avail(pool) < atomic_pool_size)
>  		schedule_work(&atomic_pool_work);