dmaengine: xilinx_dma: Fix CPU stall in xilinx_dma_poll_timeout

Currently when calling xilinx_dma_poll_timeout with delay_us=0 and a
condition that is never fulfilled, the CPU busy-waits for prolonged time
and the timeout triggers only with a massive delay causing a CPU stall.

This happens due to a huge underestimation of wall clock time in
poll_timeout_us_atomic. Commit 7349a69cf312 ("iopoll: Do not use
timekeeping in read_poll_timeout_atomic()") changed the behavior to no
longer use ktime_get at the expense of underestimation of wall clock
time which appears to be very large for delay_us=0. Instead of timing
out after approximately XILINX_DMA_LOOP_COUNT microseconds, the timeout
takes XILINX_DMA_LOOP_COUNT * 1000 * (time that the overhead of the for
loop in poll_timeout_us_atomic takes) which is in the range of several
minutes for XILINX_DMA_LOOP_COUNT=1000000. Fix this by using a non-zero
value for delay_us. Use delay_us=10 to keep the delay in the hot path of
starting DMA transfers minimal but still avoid CPU stalls in case of
unexpected hardware failures.

One-off measurement with delay_us=0 causes the cpu to busy wait around 7
minutes in the timeout case. After applying this patch with delay_us=10
the measured timeout was 1053428 microseconds which is roughly
equivalent to the expected 1000000 microseconds specified in
XILINX_DMA_LOOP_COUNT.

Add a constant XILINX_DMA_POLL_DELAY_US for delay_us value.

Fixes: 9495f2648287 ("dmaengine: xilinx_vdma: Use readl_poll_timeout instead of do while loop's")
Fixes: 7349a69cf312 ("iopoll: Do not use timekeeping in read_poll_timeout_atomic()")
Reviewed-by: Suraj Gupta <suraj.gupta2@amd.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Alex Bereza <alex@bereza.email>
Link: https://patch.msgid.link/20260402-fix-atomic-poll-timeout-regression-v4-1-f30d6a6c13cb@bereza.email
Signed-off-by: Vinod Koul <vkoul@kernel.org>

0 files changed, 0 insertions, 0 deletions