Re: [PATCH 1/2] 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's
> flush callback. serial8250_tx_dma_flush() terminates DMA asynchronously and
> clears tx_running, allowing a new write to replace tx_size before the old
> completion callback has finished. That callback can then consume bytes
> from the new transmission. The serial core can also overwrite the old
> DMA source as soon as it reuses the reset kfifo.
> 
> Give DMA a private transmit buffer, keeping the kfifo offsets used by
> FIFO-window providers. Copy only the queued ranges before handing the
> buffer to the device. Flushing the kfifo can then no longer change an
> in-flight transfer's source.
> 
> Keep tx_running set while a worker synchronizes a successfully terminated
> channel. Suppress callbacks and new submissions during that interval.
> The DesignWare idle helpers must not pause or resume a retiring channel.
> [ ... ]
> 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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