Re: [net] net: macb: Shuffle the tx ring before enabling tx

[Kevin Hao <haokexin@gmail.com>]

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On Fri, Mar 06, 2026 at 06:56:38PM -0800, Jakub Kicinski wrote:
> Does this memcpy corrupt the descriptor fields on systems with 64-bit DMA?
> The macb_tx_desc() macro uses macb_adj_dma_desc_idx() to properly space the
> descriptor pointers in the ring according to the descriptor size reported by
> macb_dma_desc_get_size(bp), which can be 8 bytes (base), 16 bytes (with
> DMA64), or 24 bytes (with DMA64 + PTP). However, all three memcpy calls in
> this function use sizeof(struct macb_dma_desc), which is only 8 bytes and
> covers only the addr and ctrl fields.
> 
> On systems where macb_dma64(bp) == true, such as the AMD ZynqMp platform
> this patch targets, each descriptor has an additional macb_dma_desc_64
> struct containing the upper 32 bits of the DMA address (addrh field).
> 
> When the shuffle copies descriptors using only 8 bytes, it moves the lower
> 32-bit address (desc->addr) and control word (desc->ctrl) to new positions
> but leaves the upper 32-bit address (addrh) unmoved in the old location.
> After the shuffle, hardware reads a composite 64-bit DMA address from
> mismatched descriptor slots: the old slot's addrh combined with the new
> slot's addr.

This is indeed an issue. Coincidentally, all the upper 32-bit addresses in the
tx ring were identical, which is why it went undetected during testing.
My apologies for this oversight; I will address it in the v2 version.

Thanks,
Kevin

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