4.20-stable review patch. If anyone has any objections, please let me know.
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From: Lukas Wunner lukas@wunner.de
commit 9e528c799d17a4ac37d788c81440b50377dd592d upstream.
There are multiple issues with bcm2835_dma_abort() (which is called on termination of a transaction):
* The algorithm to abort the transaction first pauses the channel by clearing the ACTIVE flag in the CS register, then waits for the PAUSED flag to clear. Page 49 of the spec documents the latter as follows:
"Indicates if the DMA is currently paused and not transferring data. This will occur if the active bit has been cleared [...]" https://www.raspberrypi.org/app/uploads/2012/02/BCM2835-ARM-Peripherals.pdf
So the function is entering an infinite loop because it is waiting for PAUSED to clear which is always set due to the function having cleared the ACTIVE flag. The only thing that's saving it from itself is the upper bound of 10000 loop iterations.
The code comment says that the intention is to "wait for any current AXI transfer to complete", so the author probably wanted to check the WAITING_FOR_OUTSTANDING_WRITES flag instead. Amend the function accordingly.
* The CS register is only read at the beginning of the function. It needs to be read again after pausing the channel and before checking for outstanding writes, otherwise writes which were issued between the register read at the beginning of the function and pausing the channel may not be waited for.
* The function seeks to abort the transfer by writing 0 to the NEXTCONBK register and setting the ABORT and ACTIVE flags. Thereby, the 0 in NEXTCONBK is sought to be loaded into the CONBLK_AD register. However experimentation has shown this approach to not work: The CONBLK_AD register remains the same as before and the CS register contains 0x00000030 (PAUSED | DREQ_STOPS_DMA). In other words, the control block is not aborted but merely paused and it will be resumed once the next DMA transaction is started. That is absolutely not the desired behavior.
A simpler approach is to set the channel's RESET flag instead. This reliably zeroes the NEXTCONBK as well as the CS register. It requires less code and only a single MMIO write. This is also what popular user space DMA drivers do, e.g.: https://github.com/metachris/RPIO/blob/master/source/c_pwm/pwm.c
Note that the spec is contradictory whether the NEXTCONBK register is writeable at all. On the one hand, page 41 claims:
"The value loaded into the NEXTCONBK register can be overwritten so that the linked list of Control Block data structures can be dynamically altered. However it is only safe to do this when the DMA is paused."
On the other hand, page 40 specifies:
"Only three registers in each channel's register set are directly writeable (CS, CONBLK_AD and DEBUG). The other registers (TI, SOURCE_AD, DEST_AD, TXFR_LEN, STRIDE & NEXTCONBK), are automatically loaded from a Control Block data structure held in external memory."
Fixes: 96286b576690 ("dmaengine: Add support for BCM2835") Signed-off-by: Lukas Wunner lukas@wunner.de Cc: stable@vger.kernel.org # v3.14+ Cc: Frank Pavlic f.pavlic@kunbus.de Cc: Martin Sperl kernel@martin.sperl.org Cc: Florian Meier florian.meier@koalo.de Cc: Clive Messer clive.m.messer@gmail.com Cc: Matthias Reichl hias@horus.com Tested-by: Stefan Wahren stefan.wahren@i2se.com Acked-by: Florian Kauer florian.kauer@koalo.de Signed-off-by: Vinod Koul vkoul@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- drivers/dma/bcm2835-dma.c | 41 +++++++++-------------------------------- 1 file changed, 9 insertions(+), 32 deletions(-)
--- a/drivers/dma/bcm2835-dma.c +++ b/drivers/dma/bcm2835-dma.c @@ -415,13 +415,11 @@ static void bcm2835_dma_fill_cb_chain_wi } }
-static int bcm2835_dma_abort(void __iomem *chan_base) +static int bcm2835_dma_abort(struct bcm2835_chan *c) { - unsigned long cs; + void __iomem *chan_base = c->chan_base; long int timeout = 10000;
- cs = readl(chan_base + BCM2835_DMA_CS); - /* * A zero control block address means the channel is idle. * (The ACTIVE flag in the CS register is not a reliable indicator.) @@ -433,25 +431,16 @@ static int bcm2835_dma_abort(void __iome writel(0, chan_base + BCM2835_DMA_CS);
/* Wait for any current AXI transfer to complete */ - while ((cs & BCM2835_DMA_ISPAUSED) && --timeout) { + while ((readl(chan_base + BCM2835_DMA_CS) & + BCM2835_DMA_WAITING_FOR_WRITES) && --timeout) cpu_relax(); - cs = readl(chan_base + BCM2835_DMA_CS); - }
- /* We'll un-pause when we set of our next DMA */ + /* Peripheral might be stuck and fail to signal AXI write responses */ if (!timeout) - return -ETIMEDOUT; - - if (!(cs & BCM2835_DMA_ACTIVE)) - return 0; - - /* Terminate the control block chain */ - writel(0, chan_base + BCM2835_DMA_NEXTCB); - - /* Abort the whole DMA */ - writel(BCM2835_DMA_ABORT | BCM2835_DMA_ACTIVE, - chan_base + BCM2835_DMA_CS); + dev_err(c->vc.chan.device->dev, + "failed to complete outstanding writes\n");
+ writel(BCM2835_DMA_RESET, chan_base + BCM2835_DMA_CS); return 0; }
@@ -796,7 +785,6 @@ static int bcm2835_dma_terminate_all(str struct bcm2835_chan *c = to_bcm2835_dma_chan(chan); struct bcm2835_dmadev *d = to_bcm2835_dma_dev(c->vc.chan.device); unsigned long flags; - int timeout = 10000; LIST_HEAD(head);
spin_lock_irqsave(&c->vc.lock, flags); @@ -810,18 +798,7 @@ static int bcm2835_dma_terminate_all(str if (c->desc) { vchan_terminate_vdesc(&c->desc->vd); c->desc = NULL; - bcm2835_dma_abort(c->chan_base); - - /* Wait for stopping */ - while (--timeout) { - if (!readl(c->chan_base + BCM2835_DMA_ADDR)) - break; - - cpu_relax(); - } - - if (!timeout) - dev_err(d->ddev.dev, "DMA transfer could not be terminated\n"); + bcm2835_dma_abort(c); }
vchan_get_all_descriptors(&c->vc, &head);