linux-kernel - Re: [PATCH net v4 4/5] net: macb: single dma_alloc

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Message-Id: <DCP9B9VNMQVW.2XKBF1MH15N91@bootlin.com>
Date: Wed, 10 Sep 2025 18:22:03 +0200
From: Théo Lebrun <theo.lebrun@...tlin.com>
To: "Nicolas Ferre" <nicolas.ferre@...rochip.com>, "Andrew Lunn"
 <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, "Eric
 Dumazet" <edumazet@...gle.com>, "Jakub Kicinski" <kuba@...nel.org>, "Paolo
 Abeni" <pabeni@...hat.com>, "Rob Herring" <robh@...nel.org>, "Krzysztof
 Kozlowski" <krzk+dt@...nel.org>, "Conor Dooley" <conor+dt@...nel.org>,
 "Claudiu Beznea" <claudiu.beznea@...on.dev>, "Geert Uytterhoeven"
 <geert@...ux-m68k.org>, "Harini Katakam" <harini.katakam@...inx.com>,
 "Richard Cochran" <richardcochran@...il.com>, "Russell King"
 <linux@...linux.org.uk>
Cc: <netdev@...r.kernel.org>, <devicetree@...r.kernel.org>,
 <linux-kernel@...r.kernel.org>, "Thomas Petazzoni"
 <thomas.petazzoni@...tlin.com>, "Tawfik Bayouk"
 <tawfik.bayouk@...ileye.com>, "Sean Anderson" <sean.anderson@...ux.dev>
Subject: Re: [PATCH net v4 4/5] net: macb: single dma_alloc_coherent() for
 DMA descriptors

Hello Nicolas,

On Tue Aug 26, 2025 at 5:23 PM CEST, Nicolas Ferre wrote:
> On 20/08/2025 at 16:55, Théo Lebrun wrote:
>> Move from 2*NUM_QUEUES dma_alloc_coherent() for DMA descriptor rings to
>> 2 calls overall.
>> 
>> Issue is with how all queues share the same register for configuring the
>> upper 32-bits of Tx/Rx descriptor rings. Taking Tx, notice how TBQPH
>> does *not* depend on the queue index:
>> 
>>          #define GEM_TBQP(hw_q)          (0x0440 + ((hw_q) << 2))
>>          #define GEM_TBQPH(hw_q)         (0x04C8)
>> 
>>          queue_writel(queue, TBQP, lower_32_bits(queue->tx_ring_dma));
>>          #ifdef CONFIG_ARCH_DMA_ADDR_T_64BIT
>>          if (bp->hw_dma_cap & HW_DMA_CAP_64B)
>>                  queue_writel(queue, TBQPH, upper_32_bits(queue->tx_ring_dma));
>>          #endif
>> 
>> To maximise our chances of getting valid DMA addresses, we do a single
>> dma_alloc_coherent() across queues. This improves the odds because
>> alloc_pages() guarantees natural alignment. Other codepaths (IOMMU or
>> dev/arch dma_map_ops) don't give high enough guarantees
>> (even page-aligned isn't enough).
>> 
>> Two consideration:
>> 
>>   - dma_alloc_coherent() gives us page alignment. Here we remove this
>>     constraint meaning each queue's ring won't be page-aligned anymore.
>
> However... We must guarantee alignement depending on the controller's 
> bus width (32 or 64 bits)... but being page aligned and having 
> descriptors multiple of 64 bits anyway, we're good for each descriptor 
> (might be worth explicitly adding).

Sorry, your comment was unclear to me.

 - I don't see how we can guarantee bus alignment using
   dma_alloc_coherent() which doesn't ask for desired alignment. In
   what case can the DMA APIs return something with less than the
   tolerated bus alignment?

 - What does "having descriptors multiple of 64 bits anyway" mean?

Thanks for your review and acks! V5 got published here:
https://lore.kernel.org/lkml/20250910-macb-fixes-v5-0-f413a3601ce4@bootlin.com/

Regards,

--
Théo Lebrun, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com