linux-kernel - Re: [Intel-wired-lan] [PATCH net-next 07/11] net: page_pool: add DMA-sync-for-CPU inline helpers

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Date:   Thu, 18 May 2023 15:45:33 +0200
From:   Alexander Lobakin <aleksander.lobakin@...el.com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     Jesper Dangaard Brouer <hawk@...nel.org>,
        Larysa Zaremba <larysa.zaremba@...el.com>,
        <netdev@...r.kernel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        <linux-kernel@...r.kernel.org>, "Christoph Hellwig" <hch@....de>,
        Eric Dumazet <edumazet@...gle.com>,
        Michal Kubiak <michal.kubiak@...el.com>,
        <intel-wired-lan@...ts.osuosl.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Magnus Karlsson <magnus.karlsson@...el.com>
Subject: Re: [Intel-wired-lan] [PATCH net-next 07/11] net: page_pool: add
 DMA-sync-for-CPU inline helpers

From: Jakub Kicinski <kuba@...nel.org>
Date: Wed, 17 May 2023 21:12:11 -0700

> On Tue, 16 May 2023 18:18:37 +0200 Alexander Lobakin wrote:
>> Each driver is responsible for syncing buffers written by HW for CPU
>> before accessing them. Almost each PP-enabled driver uses the same
>> pattern, which could be shorthanded into a static inline to make driver
>> code a little bit more compact.
>> Introduce a couple such functions. The first one takes the actual size
>> of the data written by HW and is the main one to be used on Rx. The
>> second does the same, but only if the PP performs DMA synchronizations
>> at all. The last one picks max_len from the PP params and is designed
>> for more extreme cases when the size is unknown, but the buffer still
>> needs to be synced.
>> Also constify pointer arguments of page_pool_get_dma_dir() and
>> page_pool_get_dma_addr() to give a bit more room for optimization,
>> as both of them are read-only.
> 
> Very neat.
> 
>> diff --git a/include/net/page_pool.h b/include/net/page_pool.h
>> index 8435013de06e..f740c50b661f 100644
>> --- a/include/net/page_pool.h
>> +++ b/include/net/page_pool.h
>> @@ -32,7 +32,7 @@
>>  
>>  #include <linux/mm.h> /* Needed by ptr_ring */
>>  #include <linux/ptr_ring.h>
>> -#include <linux/dma-direction.h>
>> +#include <linux/dma-mapping.h>
> 
> highly nit picky - but isn't dma-mapping.h pretty heavy?
> And we include page_pool.h in skbuff.h. Not that it matters
> today, but maybe one day we'll succeed putting skbuff.h
> on a diet -- so perhaps it's better to put "inline helpers
> with non-trivial dependencies" into a new header?

Maybe we could rather stop including page_pool.h into skbuff.h? It's
used there only for  1 external, which could be declared directly in
skbuff.h. When Matteo was developing PP recycling, he was storing
mem_info in skb as well, but then it was optimized and we don't do that
anymore.
It annoys sometimes to see the whole kernel rebuilt each time I edit
pag_pool.h :D In fact, only PP-enabled drivers and core code need it.

> 
>>  #define PP_FLAG_DMA_MAP		BIT(0) /* Should page_pool do the DMA
>>  					* map/unmap
> 
>> +/**
>> + * page_pool_dma_sync_for_cpu - sync Rx page for CPU after it's written by HW
>> + * @pool: page_pool which this page belongs to
>> + * @page: page to sync
>> + * @dma_sync_size: size of the data written to the page
>> + *
>> + * Can be used as a shorthand to sync Rx pages before accessing them in the
>> + * driver. Caller must ensure the pool was created with %PP_FLAG_DMA_MAP.
>> + */
>> +static inline void page_pool_dma_sync_for_cpu(const struct page_pool *pool,
>> +					      const struct page *page,
>> +					      u32 dma_sync_size)
>> +{
>> +	dma_sync_single_range_for_cpu(pool->p.dev,
>> +				      page_pool_get_dma_addr(page),
>> +				      pool->p.offset, dma_sync_size,
>> +				      page_pool_get_dma_dir(pool));
> 
> Likely a dumb question but why does this exist?
> Is there a case where the "maybe" version is not safe?

If the driver doesn't set DMA_SYNC_DEV flag, then the "maybe" version
will never do anything. But we may want to use these helpers in such
drivers too?

> 
>> +}
>> +
>> +/**
>> + * page_pool_dma_maybe_sync_for_cpu - sync Rx page for CPU if needed
>> + * @pool: page_pool which this page belongs to
>> + * @page: page to sync
>> + * @dma_sync_size: size of the data written to the page
>> + *
>> + * Performs DMA sync for CPU, but only when required (swiotlb, IOMMU etc.).
>> + */
>> +static inline void
>> +page_pool_dma_maybe_sync_for_cpu(const struct page_pool *pool,
>> +				 const struct page *page, u32 dma_sync_size)
>> +{
>> +	if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
>> +		page_pool_dma_sync_for_cpu(pool, page, dma_sync_size);
>> +}
Thanks,
Olek