From 56c1723426d3cfd4723bfbfce531d7b38bae6266 Mon Sep 17 00:00:00 2001 From: Lukas Wunner Date: Thu, 8 Nov 2018 08:06:10 +0100 Subject: [PATCH] spi: bcm2835: Avoid finishing transfer prematurely in IRQ mode MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The IRQ handler bcm2835_spi_interrupt() first reads as much as possible from the RX FIFO, then writes as much as possible to the TX FIFO. Afterwards it decides whether the transfer is finished by checking if the TX FIFO is empty. If very few bytes were written to the TX FIFO, they may already have been transmitted by the time the FIFO's emptiness is checked. As a result, the transfer will be declared finished and the chip will be reset without reading the corresponding received bytes from the RX FIFO. The odds of this happening increase with a high clock frequency (such that the TX FIFO drains quickly) and either passing "threadirqs" on the command line or enabling CONFIG_PREEMPT_RT_BASE (such that the IRQ handler may be preempted between filling the TX FIFO and checking its emptiness). Fix by instead checking whether rx_len has reached zero, which means that the transfer has been received in full. This is also more efficient as it avoids one bus read access per interrupt. Note that bcm2835_spi_transfer_one_poll() likewise uses rx_len to determine whether the transfer has finished. Signed-off-by: Lukas Wunner Fixes: e34ff011c70e ("spi: bcm2835: move to the transfer_one driver model") Cc: stable@vger.kernel.org # v4.1+ Cc: Mathias Duckeck Cc: Frank Pavlic Cc: Martin Sperl Cc: Noralf Trønnes Signed-off-by: Mark Brown --- drivers/spi/spi-bcm2835.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/drivers/spi/spi-bcm2835.c b/drivers/spi/spi-bcm2835.c index f35cc10772f6..fa7844a956c7 100644 --- a/drivers/spi/spi-bcm2835.c +++ b/drivers/spi/spi-bcm2835.c @@ -155,8 +155,7 @@ static irqreturn_t bcm2835_spi_interrupt(int irq, void *dev_id) /* Write as many bytes as possible to FIFO */ bcm2835_wr_fifo(bs); - /* based on flags decide if we can finish the transfer */ - if (bcm2835_rd(bs, BCM2835_SPI_CS) & BCM2835_SPI_CS_DONE) { + if (!bs->rx_len) { /* Transfer complete - reset SPI HW */ bcm2835_spi_reset_hw(master); /* wake up the framework */