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Subject: Re: [PATCH net-next v5 03/14] page_pool: avoid calling no-op
 externals when possible
To:     Alexander Lobakin <aleksander.lobakin@intel.com>,
        Christoph Hellwig <hch@lst.de>
CC:     Maciej Fijalkowski <maciej.fijalkowski@intel.com>,
        Michal Kubiak <michal.kubiak@intel.com>,
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        Alexander Duyck <alexanderduyck@fb.com>,
        David Christensen <drc@linux.vnet.ibm.com>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        Paul Menzel <pmenzel@molgen.mpg.de>, <netdev@vger.kernel.org>,
        <intel-wired-lan@lists.osuosl.org>, <linux-kernel@vger.kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>
References: <20231124154732.1623518-1-aleksander.lobakin@intel.com>
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From:   Yunsheng Lin <linyunsheng@huawei.com>
Message-ID: <534e7752-38a9-3e7e-cb04-65789712fb66@huawei.com>
Date:   Wed, 29 Nov 2023 11:17:50 +0800
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On 2023/11/27 22:32, Alexander Lobakin wrote:
> 
> Chris, any thoughts on a global flag for skipping DMA syncs ladder?

It seems there was one already in the past:

https://lore.kernel.org/netdev/7c55a4d7-b4aa-25d4-1917-f6f355bd722e@arm.com/T/

> 
>>
>>

>>> +static inline bool page_pool_set_dma_addr(const struct page_pool *pool,
>>> +					  struct page *page,
>>> +					  dma_addr_t addr)
>>>  {
>>> +	unsigned long val = addr;
>>> +
>>> +	if (unlikely(!addr)) {
>>> +		page->dma_addr = 0;
>>> +		return true;
>>> +	}
>>
>> The above seems unrelated change?
> 
> Related. We use page_put_set_dma_addr() to clear ::dma_addr as well
> (grep for it in page_pool.c). In this case, we don't want
> dma_need_sync() to be called as we explicitly pass zero. This check
> zeroes the field and exits as quickly as possible.

The question seems to be about if we need to ensure the LSB of
page->dma_addr is not set when page_pool releases a page back to page
allocator?

> In case with the call mentioned above, zero is a compile-time constant
> there, so that this little branch will be inlined with the rest dropped.
>