[PATCH 0/2] serial: keep the flushed TX buffer mapped until DMA has stopped

[Samarth Narang <samarth.colleges@gmail.com>]

This series addresses the DMA buffer lifetime issues in amba-pl011 and
fsl_lpuart reported in the parent thread, where flush_buffer() unmaps
the TX buffer before the terminated DMA transfer is guaranteed to have
stopped accessing it. In fsl_lpuart, the buffer is unmapped even before
termination is requested.

Since flush_buffer() runs under the port lock and cannot wait for DMA
termination, both patches request termination while holding the lock and
defer unmapping to a work item that first calls dmaengine_synchronize().
The TX completion callback leaves the cleanup of flushed transfers to
this work item, while shutdown, and suspend in the case of fsl_lpuart,
waits for the work to finish before releasing the TX DMA state.

Both patches have only been compile-tested with arm64 defconfig, the
relevant drivers enabled, and W=1, with no new warnings. I do not have
access to PL011 or LPUART hardware with DMA support, so testing by
someone with suitable hardware would be appreciated.

The imx driver also needs a fix for the issue reported in the same
thread, but requires a different approach because its TX completion
callback unmaps the scatterlist unconditionally. I will send that patch
separately.

Samarth Narang (2):
  serial: amba-pl011: keep the flushed TX buffer mapped until DMA stops
  tty: serial: fsl_lpuart: don't unmap TX buffer before DMA has stopped

 drivers/tty/serial/amba-pl011.c | 54 +++++++++++++++++++++++++++++++--
 drivers/tty/serial/fsl_lpuart.c | 53 ++++++++++++++++++++++++++++----
 2 files changed, 98 insertions(+), 9 deletions(-)


base-commit: 36aad21d50d1af3f6954bb0c988257a908202477
-- 
2.34.1