linux-kernel - Re: [PATCH net-next v9 05/14] netdev: netdevice devmem allocator

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Message-ID: <090be3c0-42e6-4b97-8b03-eb64b06a2911@davidwei.uk>
Date: Fri, 17 May 2024 17:39:25 -0700
From: David Wei <dw@...idwei.uk>
To: Mina Almasry <almasrymina@...gle.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org,
 linux-parisc@...r.kernel.org, sparclinux@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
 bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
 linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org
Cc: Donald Hunter <donald.hunter@...il.com>, Jakub Kicinski
 <kuba@...nel.org>, "David S. Miller" <davem@...emloft.net>,
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Subject: Re: [PATCH net-next v9 05/14] netdev: netdevice devmem allocator

On 2024-05-10 16:21, Mina Almasry wrote:
> +/* This returns the absolute dma_addr_t calculated from
> + * net_iov_owner(niov)->owner->base_dma_addr, not the page_pool-owned
> + * niov->dma_addr.
> + *
> + * The absolute dma_addr_t is a dma_addr_t that is always uncompressed.
> + *
> + * The page_pool-owner niov->dma_addr is the absolute dma_addr compressed into
> + * an unsigned long. Special handling is done when the unsigned long is 32-bit
> + * but the dma_addr_t is 64-bit.
> + *
> + * In general code looking for the dma_addr_t should use net_iov_dma_addr(),
> + * while page_pool code looking for the unsigned long dma_addr which mirrors
> + * the field in struct page should use niov->dma_addr.
> + */
> +static inline dma_addr_t net_iov_dma_addr(const struct net_iov *niov)
> +{
> +	struct dmabuf_genpool_chunk_owner *owner = net_iov_owner(niov);
> +
> +	return owner->base_dma_addr +
> +	       ((dma_addr_t)net_iov_idx(niov) << PAGE_SHIFT);
> +}

This part feels like devmem TCP specific, yet the function is in
netmem.h. Please consider moving it into devmem.{h,c} which makes it
less likely that people not reading your comment will try using it.

> +
> +static inline struct net_devmem_dmabuf_binding *
> +net_iov_binding(const struct net_iov *niov)
> +{
> +	return net_iov_owner(niov)->binding;
> +}
> +
>  /* netmem */
>  
>  /**
> diff --git a/net/core/devmem.c b/net/core/devmem.c
> index d82f92d7cf9ce..1f90e23a81441 100644
> --- a/net/core/devmem.c
> +++ b/net/core/devmem.c
> @@ -54,6 +54,42 @@ void __net_devmem_dmabuf_binding_free(struct net_devmem_dmabuf_binding *binding)
>  	kfree(binding);
>  }
>  
> +struct net_iov *
> +net_devmem_alloc_dmabuf(struct net_devmem_dmabuf_binding *binding)
> +{
> +	struct dmabuf_genpool_chunk_owner *owner;
> +	unsigned long dma_addr;
> +	struct net_iov *niov;
> +	ssize_t offset;
> +	ssize_t index;
> +
> +	dma_addr = gen_pool_alloc_owner(binding->chunk_pool, PAGE_SIZE,
> +					(void **)&owner);
> +	if (!dma_addr)
> +		return NULL;
> +
> +	offset = dma_addr - owner->base_dma_addr;
> +	index = offset / PAGE_SIZE;
> +	niov = &owner->niovs[index];
> +
> +	niov->dma_addr = 0;
> +
> +	net_devmem_dmabuf_binding_get(binding);
> +
> +	return niov;
> +}
> +
> +void net_devmem_free_dmabuf(struct net_iov *niov)
> +{
> +	struct net_devmem_dmabuf_binding *binding = net_iov_binding(niov);
> +	unsigned long dma_addr = net_iov_dma_addr(niov);
> +
> +	if (gen_pool_has_addr(binding->chunk_pool, dma_addr, PAGE_SIZE))
> +		gen_pool_free(binding->chunk_pool, dma_addr, PAGE_SIZE);
> +
> +	net_devmem_dmabuf_binding_put(binding);
> +}
> +
>  /* Protected by rtnl_lock() */
>  static DEFINE_XARRAY_FLAGS(net_devmem_dmabuf_bindings, XA_FLAGS_ALLOC1);
>