netdev - Re: [PATCH net-next 1/7] page_pool: disable dma mapping support for 32-bit arch with 64-bit DMA

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Message-ID: <0ffa15a1-742d-a05d-3ea6-04ff25be6a29@redhat.com>
Date:   Thu, 23 Sep 2021 11:33:27 +0200
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>, davem@...emloft.net,
        kuba@...nel.org
Cc:     brouer@...hat.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linuxarm@...neuler.org,
        hawk@...nel.org, ilias.apalodimas@...aro.org,
        jonathan.lemon@...il.com, alobakin@...me, willemb@...gle.com,
        cong.wang@...edance.com, pabeni@...hat.com, haokexin@...il.com,
        nogikh@...gle.com, elver@...gle.com, memxor@...il.com,
        edumazet@...gle.com, alexander.duyck@...il.com, dsahern@...il.com
Subject: Re: [PATCH net-next 1/7] page_pool: disable dma mapping support for
 32-bit arch with 64-bit DMA


On 22/09/2021 11.41, Yunsheng Lin wrote:
> diff --git a/net/core/page_pool.c b/net/core/page_pool.c
> index 1a6978427d6c..a65bd7972e37 100644
> --- a/net/core/page_pool.c
> +++ b/net/core/page_pool.c
> @@ -49,6 +49,12 @@ static int page_pool_init(struct page_pool *pool,
>   	 * which is the XDP_TX use-case.
>   	 */
>   	if (pool->p.flags & PP_FLAG_DMA_MAP) {
> +		/* DMA-mapping is not supported on 32-bit systems with
> +		 * 64-bit DMA mapping.
> +		 */
> +		if (sizeof(dma_addr_t) > sizeof(unsigned long))
> +			return -EINVAL;

As I said before, can we please use another error than EINVAL.
We should give drivers a chance/ability to detect this error, and e.g. 
fallback to doing DMA mappings inside driver instead.

I suggest using EOPNOTSUPP 95 (Operation not supported).

-Jesper