netdev - RE: [Intel-wired-lan] [PATCH iwl-net v3 4/6] idpf: replace flow scheduling buffer ring with buffer pool

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Message-ID: <SJ1PR11MB629738204A166F965040C0D59B2BA@SJ1PR11MB6297.namprd11.prod.outlook.com>
Date: Tue, 12 Aug 2025 16:45:30 +0000
From: "Salin, Samuel" <samuel.salin@...el.com>
To: "Hay, Joshua A" <joshua.a.hay@...el.com>,
	"intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "Hay, Joshua A"
	<joshua.a.hay@...el.com>, Luigi Rizzo <lrizzo@...gle.com>, Brian Vazquez
	<brianvv@...gle.com>, "Chittim, Madhu" <madhu.chittim@...el.com>, "Loktionov,
 Aleksandr" <aleksandr.loktionov@...el.com>
Subject: RE: [Intel-wired-lan] [PATCH iwl-net v3 4/6] idpf: replace flow
 scheduling buffer ring with buffer pool



> -----Original Message-----
> From: Intel-wired-lan <intel-wired-lan-bounces@...osl.org> On Behalf Of
> Joshua Hay
> Sent: Friday, July 25, 2025 11:42 AM
> To: intel-wired-lan@...ts.osuosl.org
> Cc: netdev@...r.kernel.org; Hay, Joshua A <joshua.a.hay@...el.com>; Luigi
> Rizzo <lrizzo@...gle.com>; Brian Vazquez <brianvv@...gle.com>; Chittim,
> Madhu <madhu.chittim@...el.com>; Loktionov, Aleksandr
> <aleksandr.loktionov@...el.com>
> Subject: [Intel-wired-lan] [PATCH iwl-net v3 4/6] idpf: replace flow scheduling
> buffer ring with buffer pool
> 
> Replace the TxQ buffer ring with one large pool/array of buffers (only for flow
> scheduling). This eliminates the tag generation and makes it impossible for a
> tag to be associated with more than one packet.
> 
> The completion tag passed to HW through the descriptor is the index into the
> array. That same completion tag is posted back to the driver in the completion
> descriptor, and used to index into the array to quickly retrieve the buffer
> during cleaning.  In this way, the tags are treated as a fix sized resource. If all
> tags are in use, no more packets can be sent on that particular queue (until
> some are freed up). The tag pool size is 64K since the completion tag width is
> 16 bits.
> 
> For each packet, the driver pulls a free tag from the refillq to get the next free
> buffer index. When cleaning is complete, the tag is posted back to the refillq. A
> multi-frag packet spans multiple buffers in the driver, therefore it uses multiple
> buffer indexes/tags from the pool.
> Each frag pulls from the refillq to get the next free buffer index.
> These are tracked in a next_buf field that replaces the completion tag field in
> the buffer struct. This chains the buffers together so that the packet can be
> cleaned from the starting completion tag taken from the completion
> descriptor, then from the next_buf field for each subsequent buffer.
> 
> In case of a dma_mapping_error occurs or the refillq runs out of free buf_ids,
> the packet will execute the rollback error path. This unmaps any buffers
> previously mapped for the packet. Since several free buf_ids could have
> already been pulled from the refillq, we need to restore its original state as
> well. Otherwise, the buf_ids/tags will be leaked and not used again until the
> queue is reallocated.
> 
> Descriptor completions only advance the descriptor ring index to "clean"
> the descriptors. The packet completions only clean the buffers associated with
> the given packet completion tag and do not update the descriptor ring index.
> 
> When operating in queue based scheduling mode, the array still acts as a ring
> and will only have TxQ descriptor count entries. The tx_bufs are still associated
> 1:1 with the descriptor ring entries and we can use the conventional indexing
> mechanisms.
> 
> Fixes: c2d548cad150 ("idpf: add TX splitq napi poll support")
> Signed-off-by: Luigi Rizzo <lrizzo@...gle.com>
> Signed-off-by: Brian Vazquez <brianvv@...gle.com>
> Signed-off-by: Joshua Hay <joshua.a.hay@...el.com>
> Reviewed-by: Madhu Chittim <madhu.chittim@...el.com>
> Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@...el.com>
> ---
> v3:
> - remove unreachable code
> 
> v2:
> - removed unused buf_size
> - miscellaneous cleanup based on changes to prior patches and addition
>   of packet rollback changes patch
> - refactor packet rollback logic to iterate through chained bufs
> - add refillq state restore if rollback occurs
> ---
> 2.39.2

Tested-by: Samuel Salin <Samuel.salin@...el.com>