Re: [PATCH v4 1/7] dmaengine: dw: Fix XFER bit set, but channel not idle error

[Frank Li]

On Thu, Oct 29, 2026 at 04:29:35PM -0600, James Hilliard wrote:
> From: Serge Semin <fancer.lancer@xxxxxxxxx>
>
> If a client driver gets to use the DW DMAC engine device tougher
> than usual, with occasional DMA-transfers termination and restart, then
> the next error can be randomly spotted in the system log:
>
> > dma dma0chan0: BUG: XFER bit set, but channel not idle!
>
> For instance that happens in case of the 8250 UART port driver handling
> the looped back high-speed traffic (in my case > 1.5Mbaud) by means of the
> DMA-engine interface.
>
> The error happens due to the two-staged nature of the DW DMAC IRQs
> handling procedure and due to the critical section break in the meantime.
> In particular in case if the DMA-transfer is terminated and restarted:
> 1. after the IRQ-handler submitted the tasklet but before the tasklet
>    started handling the DMA-descriptors in dwc_scan_descriptors();
> 2. after the XFER completion flag was detected in the
>    dwc_scan_descriptors() method, but before the dwc_complete_all() method
>    is called
> the error denoted above is printed due to the overlap of the last transfer
> completion and the new transfer execution stages.
>
> Clear the channel interrupt status when stopping it, and keep the XFER
> check, descriptor detachment and queued-transfer restart in one locked
> section. An empty active list needs no completion processing.
>
> When the locked XFER recheck finds no event, only update residue. A stale
> tasklet snapshot may belong to a terminated transfer, and a zero residue
> only means the source has been read, not that destination writes have
> finished. Requiring RAW.XFER also prevents the fallback scan from freeing
> a descriptor still linked on active_list.
>
> Fixes: 69cea5a00d31 ("dmaengine/dw_dmac: Replace spin_lock* with irqsave variants and enable submission from callback")
> Fixes: 3bfb1d20b547 ("dmaengine: Driver for the Synopsys DesignWare DMA controller")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Serge Semin <fancer.lancer@xxxxxxxxx>
> [james: require a locked XFER event and handle an empty active list]
> Signed-off-by: James Hilliard <james.hilliard1@xxxxxxxxx>
> Reviewed-by: Frank Li <Frank.Li@xxxxxxx>

Please follow b4 order. your signed-off should be last one.

Frank