[PATCH] dmaengine: tegra: Fix burst size calculation

[Kartik Rajput <kkartik@nvidia.com>]

Currently, the Tegra GPC DMA hardware requires the transfer length to
be a multiple of the max burst size configured for the channel. When a
client requests a transfer where the length is not evenly divisible by
the configured max burst size, the DMA hangs with partial burst at
the end.

Fix this by reducing the burst size to the largest power-of-2 value
that evenly divides the transfer length. For example, a 40-byte
transfer with a 16-byte max burst will now use an 8-byte burst
(40 / 8 = 5 complete bursts) instead of causing a hang.

This issue was observed with the PL011 UART driver where TX DMA
transfers of arbitrary lengths were stuck.

Fixes: ee17028009d4 ("dmaengine: tegra: Add tegra gpcdma driver")
Cc: stable@vger.kernel.org
Signed-off-by: Kartik Rajput <kkartik@nvidia.com>
---
 drivers/dma/tegra186-gpc-dma.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/dma/tegra186-gpc-dma.c b/drivers/dma/tegra186-gpc-dma.c
index 4d6fe0efa76e..7df0a745e7b8 100644
--- a/drivers/dma/tegra186-gpc-dma.c
+++ b/drivers/dma/tegra186-gpc-dma.c
@@ -825,6 +825,13 @@ static unsigned int get_burst_size(struct tegra_dma_channel *tdc,
 	 * len to calculate the optimum burst size
 	 */
 	burst_byte = burst_size ? burst_size * slave_bw : len;
+
+	/*
+	 * Find the largest burst size that evenly divides the transfer length.
+	 * The hardware requires the transfer length to be a multiple of the
+	 * burst size - partial bursts are not supported.
+	 */
+	burst_byte = min(burst_byte, 1U << __ffs(len));
 	burst_mmio_width = burst_byte / 4;
 
 	if (burst_mmio_width < TEGRA_GPCDMA_MMIOSEQ_BURST_MIN)
-- 
2.43.0