netdev - Re: [PATCH net-next v2 1/2] docs: net: page_pool: document PP_FLAG_DMA_SYNC

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Date: Thu, 3 Aug 2023 15:05:21 +0530
From: Somnath Kotur <somnath.kotur@...adcom.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: davem@...emloft.net, netdev@...r.kernel.org, edumazet@...gle.com, 
	pabeni@...hat.com, aleksander.lobakin@...el.com, hawk@...nel.org, 
	ilias.apalodimas@...aro.org, corbet@....net, linux-doc@...r.kernel.org, 
	Michael Chan <michael.chan@...adcom.com>, Lorenzo Bianconi <lorenzo@...nel.org>, 
	Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH net-next v2 1/2] docs: net: page_pool: document
 PP_FLAG_DMA_SYNC_DEV parameters

On Wed, Aug 2, 2023 at 10:03 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> Using PP_FLAG_DMA_SYNC_DEV is a bit confusing. It was perhaps
> more obvious when it was introduced but the page pool use
> has grown beyond XDP and beyond packet-per-page so now
> making the heads and tails out of this feature is not
> trivial.
>
> Obviously making the API more user friendly would be
> a better fix, but until someone steps up to do that
> let's at least document what the parameters are.
>
> Relevant discussion in the first Link.
>
> Link: https://lore.kernel.org/all/20230731114427.0da1f73b@kernel.org/
> Signed-off-by: Jakub Kicinski <kuba@...nel.org>
> ---
> v2:
>  - s/sync'/sync/ and other fixes from Randy
> v1: https://lore.kernel.org/all/20230801203124.980703-1-kuba@kernel.org/
>
> CC: hawk@...nel.org
> CC: ilias.apalodimas@...aro.org
> CC: corbet@....net
> CC: linux-doc@...r.kernel.org
> CC: Michael Chan <michael.chan@...adcom.com>
> CC: Lorenzo Bianconi <lorenzo@...nel.org>
> CC: Randy Dunlap <rdunlap@...radead.org>
> ---
>  Documentation/networking/page_pool.rst | 34 ++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
>
> diff --git a/Documentation/networking/page_pool.rst b/Documentation/networking/page_pool.rst
> index 0aa850cf4447..eb96a592ec6b 100644
> --- a/Documentation/networking/page_pool.rst
> +++ b/Documentation/networking/page_pool.rst
> @@ -109,6 +109,40 @@ a page will cause no race conditions is enough.
>    caller can then report those stats to the user (perhaps via ethtool,
>    debugfs, etc.). See below for an example usage of this API.
>
> +DMA sync
> +--------
> +Driver is always responsible for syncing the pages for the CPU.
> +Drivers may choose to take care of syncing for the device as well
> +or set the ``PP_FLAG_DMA_SYNC_DEV`` flag to request that pages
> +allocated from the page pool are already synced for the device.
> +
> +If ``PP_FLAG_DMA_SYNC_DEV`` is set, the driver must inform the core what portion
> +of the buffer has to be synced. This allows the core to avoid syncing the entire
> +page when the drivers knows that the device only accessed a portion of the page.
> +
> +Most drivers will reserve headroom in front of the frame. This part
> +of the buffer is not touched by the device, so to avoid syncing
> +it drivers can set the ``offset`` field in struct page_pool_params
> +appropriately.
> +
> +For pages recycled on the XDP xmit and skb paths the page pool will
> +use the ``max_len`` member of struct page_pool_params to decide how
> +much of the page needs to be synced (starting at ``offset``).
> +When directly freeing pages in the driver (page_pool_put_page())
> +the ``dma_sync_size`` argument specifies how much of the buffer needs
> +to be synced.
> +
> +If in doubt set ``offset`` to 0, ``max_len`` to ``PAGE_SIZE`` and
> +pass -1 as ``dma_sync_size``. That combination of arguments is always
> +correct.
> +
> +Note that the syncing parameters are for the entire page.
> +This is important to remember when using fragments (``PP_FLAG_PAGE_FRAG``),
> +where allocated buffers may be smaller than a full page.
> +Unless the driver author really understands page pool internals
> +it's recommended to always use ``offset = 0``, ``max_len = PAGE_SIZE``
> +with fragmented page pools.
> +
>  Stats API and structures
>  ------------------------
>  If the kernel is configured with ``CONFIG_PAGE_POOL_STATS=y``, the API
> --
> 2.41.0
>
>
Reviewed-by: Somnath Kotur <somnath.kotur@...adcom.com>

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