Re: [PATCH v3 3/4] serial: 8250: drain canceled TX DMA before reusing its buffer

[sashiko-bot@kernel.org]

> uart_flush_buffer() resets the transmit kfifo before calling the driver.
> serial8250_tx_dma_flush() terminates DMA asynchronously and clears
> tx_running, allowing a new write to replace tx_size while the old callback
> is still waiting for the port lock. That callback can then consume bytes
> from the new transmission. Reusing the reset kfifo can also overwrite the
> source of a transfer that has not stopped yet.
> 
> Reserve the flushed FIFO storage through serial core before returning
> from the flush callback. Keep tx_running set until a worker synchronizes
> the terminated channel, suppressing old callbacks and new submissions
> during that interval. Return the mapping to CPU ownership before releasing
> the reservation and waking writers. Normal transfers continue to use the
> FIFO directly.
> 
> Reuse the locked completion processing after retirement, including OMAP
> delayed register restoration and transmitter-empty handling. Take a runtime
> PM reference around worker UART accesses and drain the worker before
> releasing the channels. Shutdown releases the reservation after stopping
> DMA and unmapping its source.
> 
> Track a flushed FIFO separately from the transfer length. If termination
> fails, retain the reservation until normal completion and account the
> completed bytes without advancing the reset FIFO. After successful
> cancellation, process a zero-length completion once callbacks have drained.
> 
> The race was found by inspecting cancellation and restart while preparing
> H616 UART DMA support.
> 
> Fixes: a424a34b8fad ("serial: 8250: Fix TX deadlock when using DMA")
> Signed-off-by: James Hilliard <james.hilliard1@gmail.com>

Sashiko has reviewed this patch and found no issues.

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