On 02/08/2018 09:58 AM, sean.wang@xxxxxxxxxxxx wrote:
> From: Sean Wang <sean.wang@xxxxxxxxxxxx>
>
> MediaTek High-Speed DMA controller (HSDMA) on MT7622 and MT7623 SoC has
> a single ring is dedicated to memory-to-memory transfer through ring based
> descriptor management.
>
> Even though there is only one physical ring available inside HSDMA, the
> driver can be easily extended to the support of multiple virtual channels
> processing simultaneously by means of DMA_VIRTUAL_CHANNELS effort.
>
> Signed-off-by: Sean Wang <sean.wang@xxxxxxxxxxxx>
> ---
> drivers/dma/Kconfig | 2 +
> drivers/dma/Makefile | 1 +
> drivers/dma/mediatek/Kconfig | 13 +
> drivers/dma/mediatek/Makefile | 1 +
> drivers/dma/mediatek/mtk-hsdma.c | 1046 ++++++++++++++++++++++++++++++++++++++
> 5 files changed, 1063 insertions(+)
> create mode 100644 drivers/dma/mediatek/Kconfig
> create mode 100644 drivers/dma/mediatek/Makefile
> create mode 100644 drivers/dma/mediatek/mtk-hsdma.c
>
> diff --git a/drivers/dma/mediatek/Kconfig b/drivers/dma/mediatek/Kconfig
> new file mode 100644
> index 0000000..27bac0b
> --- /dev/null
> +++ b/drivers/dma/mediatek/Kconfig
> @@ -0,0 +1,13 @@
> +
> +config MTK_HSDMA
> + tristate "MediaTek High-Speed DMA controller support"
> + depends on ARCH_MEDIATEK || COMPILE_TEST
> + select DMA_ENGINE
> + select DMA_VIRTUAL_CHANNELS
> + ---help---
> + Enable support for High-Speed DMA controller on MediaTek
> + SoCs.
> +
> + This controller provides the channels which is dedicated to
are dedicated to
> + memory-to-memory transfer to offload from CPU through ring-
> + based descriptor management.
> diff --git a/drivers/dma/mediatek/mtk-hsdma.c b/drivers/dma/mediatek/mtk-hsdma.c
> new file mode 100644
> index 0000000..d394fc6
> --- /dev/null
> +++ b/drivers/dma/mediatek/mtk-hsdma.c
> @@ -0,0 +1,1046 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Driver for MediaTek High-Speed DMA Controller
> + *
> + * Copyright (c) 2017-2018 MediaTek Inc.
> + * Author: Sean Wang <sean.wang@xxxxxxxxxxxx>
> + *
> + */
> +
#include <linux/bitops.h>
for BIT() macro usage.
> +#include <linux/clk.h>
> +#include <linux/delay.h>
> +#include <linux/dmaengine.h>
> +#include <linux/dma-mapping.h>
> +#include <linux/err.h>
> +#include <linux/init.h>
> +#include <linux/iopoll.h>
> +#include <linux/jiffies.h>
> +#include <linux/list.h>
> +#include <linux/module.h>
> +#include <linux/of.h>
> +#include <linux/of_device.h>
> +#include <linux/of_dma.h>
> +#include <linux/platform_device.h>
> +#include <linux/pm_runtime.h>
> +#include <linux/refcount.h>
> +#include <linux/slab.h>
> +
> +#include "../virt-dma.h"
> +
> +#define MTK_DMA_DEV KBUILD_MODNAME
> +
> +#define MTK_HSDMA_USEC_POLL 20
> +#define MTK_HSDMA_TIMEOUT_POLL 200000
> +#define MTK_HSDMA_DMA_BUSWIDTHS BIT(DMA_SLAVE_BUSWIDTH_UNDEFINED)
> +
> +/* The default number of virtual channel */
> +#define MTK_HSDMA_NR_VCHANS 3
> +
> +/* Only one physical channel supported */
> +#define MTK_HSDMA_NR_MAX_PCHANS 1
> +
> +/* Macro for physical descriptor (PD) manipulation */
> +/* The number of PD which must be 2 of power */
> +#define MTK_DMA_SIZE 64
> +#define MTK_HSDMA_NEXT_DESP_IDX(x, y) (((x) + 1) & ((y) - 1))
> +#define MTK_HSDMA_LAST_DESP_IDX(x, y) (((x) - 1) & ((y) - 1))
> +#define MTK_HSDMA_MAX_LEN 0x3f80
> +#define MTK_HSDMA_ALIGN_SIZE 4
> +#define MTK_HSDMA_PLEN_MASK 0x3fff
> +#define MTK_HSDMA_DESC_PLEN(x) (((x) & MTK_HSDMA_PLEN_MASK) << 16)
> +#define MTK_HSDMA_DESC_PLEN_GET(x) (((x) >> 16) & MTK_HSDMA_PLEN_MASK)
> +
> +/* Registers for underlying ring manipulation */
> +#define MTK_HSDMA_TX_BASE 0x0
> +#define MTK_HSDMA_TX_CNT 0x4
> +#define MTK_HSDMA_TX_CPU 0x8
> +#define MTK_HSDMA_TX_DMA 0xc
> +#define MTK_HSDMA_RX_BASE 0x100
> +#define MTK_HSDMA_RX_CNT 0x104
> +#define MTK_HSDMA_RX_CPU 0x108
> +#define MTK_HSDMA_RX_DMA 0x10c
> +
> +/* Registers for global setup */
> +#define MTK_HSDMA_GLO 0x204
> +#define MTK_HSDMA_GLO_MULTI_DMA BIT(10)
> +#define MTK_HSDMA_TX_WB_DDONE BIT(6)
> +#define MTK_HSDMA_BURST_64BYTES (0x2 << 4)
> +#define MTK_HSDMA_GLO_RX_BUSY BIT(3)
> +#define MTK_HSDMA_GLO_RX_DMA BIT(2)
> +#define MTK_HSDMA_GLO_TX_BUSY BIT(1)
> +#define MTK_HSDMA_GLO_TX_DMA BIT(0)
> +#define MTK_HSDMA_GLO_DMA (MTK_HSDMA_GLO_TX_DMA | \
> + MTK_HSDMA_GLO_RX_DMA)
> +#define MTK_HSDMA_GLO_BUSY (MTK_HSDMA_GLO_RX_BUSY | \
> + MTK_HSDMA_GLO_TX_BUSY)
> +#define MTK_HSDMA_GLO_DEFAULT (MTK_HSDMA_GLO_TX_DMA | \
> + MTK_HSDMA_GLO_RX_DMA | \
> + MTK_HSDMA_TX_WB_DDONE | \
> + MTK_HSDMA_BURST_64BYTES | \
> + MTK_HSDMA_GLO_MULTI_DMA)
> +
> +/* Registers for reset */
> +#define MTK_HSDMA_RESET 0x208
> +#define MTK_HSDMA_RST_TX BIT(0)
> +#define MTK_HSDMA_RST_RX BIT(16)
> +
> +/* Registers for interrupt control */
> +#define MTK_HSDMA_DLYINT 0x20c
> +#define MTK_HSDMA_RXDLY_INT_EN BIT(15)
> +/* Interrupt fires when the pending number's more than the specified */
> +#define MTK_HSDMA_RXMAX_PINT(x) (((x) & 0x7f) << 8)
> +/* Interrupt fires when the pending time's more than the specified in 20 us */
> +#define MTK_HSDMA_RXMAX_PTIME(x) ((x) & 0x7f)
> +#define MTK_HSDMA_DLYINT_DEFAULT (MTK_HSDMA_RXDLY_INT_EN | \
> + MTK_HSDMA_RXMAX_PINT(20) | \
> + MTK_HSDMA_RXMAX_PTIME(20))
> +#define MTK_HSDMA_INT_STATUS 0x220
> +#define MTK_HSDMA_INT_ENABLE 0x228
> +#define MTK_HSDMA_INT_RXDONE BIT(16)
> +
> +enum mtk_hsdma_vdesc_flag {
> + MTK_HSDMA_VDESC_FINISHED = 0x01,
> +};
> +
> +#define IS_MTK_HSDMA_VDESC_FINISHED(x) ((x) == MTK_HSDMA_VDESC_FINISHED)
> +
> +/*
> + * struct mtk_hsdma_pdesc - This is the struct holding info describing physical
> + * descriptor (PD) and its placement must be kept at
> + * 4-bytes alignment in little endian order.
> + * @des[1-4]: The control pad used to indicate hardware how to
s/des/desc/
> + * deal with the descriptor such as source and
> + * destination address and data length. The maximum
> + * data length each pdesc can handle is 0x3f80 bytes
> + */
> +struct mtk_hsdma_pdesc {
> + __le32 desc1;
> + __le32 desc2;
> + __le32 desc3;
> + __le32 desc4;
> +} __packed __aligned(4);
> +
> +/*
/**
> + * struct mtk_hsdma_vdesc - This is the struct holding info describing virtual
> + * descriptor (VD)
> + * @vd: An instance for struct virt_dma_desc
> + * @len: The total data size device wants to move
> + * @residue: The remaining data size device will move
> + * @dest: The destination address device wants to move to
> + * @src: The source address device wants to move from
> + */
> +struct mtk_hsdma_vdesc {
> + struct virt_dma_desc vd;
> + size_t len;
> + size_t residue;
> + dma_addr_t dest;
> + dma_addr_t src;
> +};
> +
> +/*
> + * struct mtk_hsdma_cb - This is the struct holding extra info required for RX
> + * ring to know what relevant VD the the PD is being
> + * mapped to.
> + * @vd: Pointer to the relevant VD.
add * @flag: here, please
> + */
> +struct mtk_hsdma_cb {
> + struct virt_dma_desc *vd;
> + enum mtk_hsdma_vdesc_flag flag;
> +};
> +
> +/*
/** for kernel-doc comments.
> + * struct mtk_hsdma_ring - This struct holds info describing underlying ring
> + * space
> + * @txd: The descriptor TX ring which describes DMA source
> + * information
> + * @rxd: The descriptor RX ring which describes DMA
> + * destination information
> + * @cb: The extra information pointed at by RX ring
> + * @tphys: The physical addr of TX ring
> + * @rphys: The physical addr of RX ring
> + * @cur_tptr: Pointer to the next free descriptor used by the host
> + * @cur_rptr: Pointer to the last done descriptor by the device
> + */
> +struct mtk_hsdma_ring {
> + struct mtk_hsdma_pdesc *txd;
> + struct mtk_hsdma_pdesc *rxd;
> + struct mtk_hsdma_cb *cb;
> + dma_addr_t tphys;
> + dma_addr_t rphys;
> + u16 cur_tptr;
> + u16 cur_rptr;
> +};
> +
> +/*
s:/*:/**:
> + * struct mtk_hsdma_pchan - This is the struct holding info describing physical
> + * channel (PC)
> + * @ring: An instance for the underlying ring
> + * @sz_ring: Total size allocated for the ring
> + * @nr_free: Total number of free rooms in the ring. It would
> + * be accessed and updated frequently between IRQ
> + * context and user context to reflect whether ring
> + * can accept requests from VD.
> + */
> +struct mtk_hsdma_pchan {
> + struct mtk_hsdma_ring ring;
> + size_t sz_ring;
> + atomic_t nr_free;
> +};
> +
> +/*
Use /** for kernel-doc comments.
> + * struct mtk_hsdma_vchan - This is the struct holding info describing virtual
> + * channel (VC)
> + * @vc: An instance for struct virt_dma_chan
> + * @issue_completion: The wait for all issued descriptors completited
> + * @issue_synchronize: Bool indicating channel synchronization starts
> + * @desc_hw_processing: List those descriptors the hardware is processing
> + */
> +struct mtk_hsdma_vchan {
> + struct virt_dma_chan vc;
> + struct completion issue_completion;
> + bool issue_synchronize;
> + /* protected by vc.lock */
> + struct list_head desc_hw_processing;
> +};
> +
> +/*
Use /** to begin a kernel-doc comment block.
> + * struct mtk_hsdma_soc - This is the struct holding differences among SoCs
> + * @ddone: Bit mask for DDONE
> + * @ls0: Bit mask for LS0
> + */
> +struct mtk_hsdma_soc {
> + __le32 ddone;
> + __le32 ls0;
> +};
> +
> +/*
You can use /** to begin this kernel-doc comment block.
> + * struct mtk_hsdma_device - This is the struct holding info describing HSDMA
> + * device
> + * @ddev: An instance for struct dma_device
> + * @base: The mapped register I/O base
> + * @clk: The clock that device internal is using
> + * @irq: The IRQ that device are using
> + * @dma_requests: The number of VCs the device supports to
> + * @vc: The pointer to all available VCs
> + * @pc: The pointer to the underlying PC
> + * @pc_refcnt: Track how many VCs are using the PC
> + * @lock: Lock protect agaisting multiple VCs access PC
> + * @soc: The pointer to area holding differences among
> + * vaious platform
> + */
> +struct mtk_hsdma_device {
> + struct dma_device ddev;
> + void __iomem *base;
> + struct clk *clk;
> + u32 irq;
> +
> + u32 dma_requests;
> + struct mtk_hsdma_vchan *vc;
> + struct mtk_hsdma_pchan *pc;
> + refcount_t pc_refcnt;
> +
> + /* Lock used to protect against multiple VCs access PC */
> + spinlock_t lock;
> +
> + const struct mtk_hsdma_soc *soc;
> +};
[snip]
> +
> +static void mtk_hsdma_issue_vchan_pending(struct mtk_hsdma_device *hsdma,
> + struct mtk_hsdma_vchan *hvc)
> +{
> + struct virt_dma_desc *vd, *vd2;
> + int err;
> +
> + lockdep_assert_held(&hvc->vc.lock);
> +
> + list_for_each_entry_safe(vd, vd2, &hvc->vc.desc_issued, node) {
> + struct mtk_hsdma_vdesc *hvd;
> +
> + hvd = to_hsdma_vdesc(vd);
> +
> + /* Map VD into PC and all VCs shares a single PC */
> + err = mtk_hsdma_issue_pending_vdesc(hsdma, hsdma->pc, hvd);
> +
> + /* Move VD from desc_issued to desc_hw_processing when entire
> + * VD is fit into available PDs. Otherwise, the uncompleted
> + * VDs would stay in list desc_issued and then restart the
> + * processing as soon as possible once underlying ring space
> + * got freed.
> + */
Please fix multi-line comment style.
> + if (err == -ENOSPC)
> + break;
> +
> + /*
> + * The extra list desc_hw_processing is used because
> + * hardware can't provide sufficient information allowing us
> + * to know what VDs are still working on the underlying ring.
> + * Through the additional list, it can help us to implement
> + * terminate_all, residue calculation and such thing needed
> + * to know detail descriptor status on the hardware.
> + */
> + list_move_tail(&vd->node, &hvc->desc_hw_processing);
> + }
> +}
> +
> +static void mtk_hsdma_free_rooms_in_ring(struct mtk_hsdma_device *hsdma)
> +{
> + struct mtk_hsdma_vchan *hvc;
> + struct mtk_hsdma_pdesc *rxd;
> + struct mtk_hsdma_vdesc *hvd;
> + struct mtk_hsdma_pchan *pc;
> + struct mtk_hsdma_cb *cb;
> + __le32 desc2;
> + u32 status;
> + u16 next;
> + int i;
> +
> + pc = hsdma->pc;
> +
> + /* Read IRQ status */
> + status = mtk_dma_read(hsdma, MTK_HSDMA_INT_STATUS);
> +
> + /*
> + * Ack the pending IRQ all to let hardware know software is handling
> + * those finished physical descriptors. Otherwise, the hardware would
> + * keep the used IRQ line in certain trigger state.
> + */
> + mtk_dma_write(hsdma, MTK_HSDMA_INT_STATUS, status);
> +
> + while (1) {
> + next = MTK_HSDMA_NEXT_DESP_IDX(pc->ring.cur_rptr,
> + MTK_DMA_SIZE);
> + rxd = &pc->ring.rxd[next];
> + /*
> + * If MTK_HSDMA_DESC_DDONE is no specified, that means data
> + * moving for the PD is still under going.
> + */
> + desc2 = READ_ONCE(rxd->desc2);
> + if (!(desc2 & hsdma->soc->ddone))
> + break;
> +
> + cb = &pc->ring.cb[next];
> + if (unlikely(!cb->vd)) {
> + dev_err(hsdma2dev(hsdma), "cb->vd cannot be null\n");
> + break;
> + }
> +
> + /* Update residue of VD the associated PD belonged to */
> + hvd = to_hsdma_vdesc(cb->vd);
> + hvd->residue -= MTK_HSDMA_DESC_PLEN_GET(rxd->desc2);
> +
> + /* Complete VD until the relevant last PD is finished */
> + if (IS_MTK_HSDMA_VDESC_FINISHED(cb->flag)) {
> + hvc = to_hsdma_vchan(cb->vd->tx.chan);
> +
> + spin_lock(&hvc->vc.lock);
> + /* Remove VD from list desc_hw_processing */
> + list_del(&cb->vd->node);
> + /* Add VD into list desc_completed */
> + vchan_cookie_complete(cb->vd);
> +
> + if (hvc->issue_synchronize &&
> + list_empty(&hvc->desc_hw_processing)) {
> + complete(&hvc->issue_completion);
> + hvc->issue_synchronize = false;
> + }
> + spin_unlock(&hvc->vc.lock);
> +
> + cb->flag = 0;
> + }
> +
> + cb->vd = 0;
> +
> + /*
> + * Recycle the RXD with the helper WRITE_ONCE that can ensure
> + * data written into RAM would really happens.
> + */
> + WRITE_ONCE(rxd->desc1, 0);
> + WRITE_ONCE(rxd->desc2, 0);
> + pc->ring.cur_rptr = next;
> +
> + /* Release rooms */
> + atomic_inc(&pc->nr_free);
> + }
> +
> + /* Ensure all changes indeed done before we're going on */
> + wmb();
> +
> + /* Update CPU pointer for those completed PDs */
> + mtk_dma_write(hsdma, MTK_HSDMA_RX_CPU, pc->ring.cur_rptr);
> +
> + /* ASAP handles pending VDs in all VCs after freeing some rooms */
> + for (i = 0; i < hsdma->dma_requests; i++) {
> + hvc = &hsdma->vc[i];
> + spin_lock(&hvc->vc.lock);
> + mtk_hsdma_issue_vchan_pending(hsdma, hvc);
> + spin_unlock(&hvc->vc.lock);
> + }
> +
> + /* All completed PDs are cleaned up, so enable interrupt again */
> + mtk_dma_set(hsdma, MTK_HSDMA_INT_ENABLE, MTK_HSDMA_INT_RXDONE);
> +}
> +
> +static irqreturn_t mtk_hsdma_irq(int irq, void *devid)
> +{
> + struct mtk_hsdma_device *hsdma = devid;
> +
> + /*
> + * Disable interrupt until all completed PDs are cleaned up in
> + * mtk_hsdma_free_rooms call.
> + */
> + mtk_dma_clr(hsdma, MTK_HSDMA_INT_ENABLE, MTK_HSDMA_INT_RXDONE);
> +
> + mtk_hsdma_free_rooms_in_ring(hsdma);
> +
> + return IRQ_HANDLED;
> +}
> +
> +struct virt_dma_desc *mtk_hsdma_find_active_desc(struct dma_chan *c,
> + dma_cookie_t cookie)
> +{
> + struct mtk_hsdma_vchan *hvc = to_hsdma_vchan(c);
> + struct virt_dma_desc *vd;
> +
> + list_for_each_entry(vd, &hvc->desc_hw_processing, node)
> + if (vd->tx.cookie == cookie)
> + return vd;
> +
> + list_for_each_entry(vd, &hvc->vc.desc_issued, node)
> + if (vd->tx.cookie == cookie)
> + return vd;
> +
> + return NULL;
> +}
> +
> +static enum dma_status mtk_hsdma_tx_status(struct dma_chan *c,
> + dma_cookie_t cookie,
> + struct dma_tx_state *txstate)
> +{
> + struct mtk_hsdma_vchan *hvc = to_hsdma_vchan(c);
> + struct mtk_hsdma_vdesc *hvd;
> + struct virt_dma_desc *vd;
> + enum dma_status ret;
> + unsigned long flags;
> + size_t bytes = 0;
> +
> + ret = dma_cookie_status(c, cookie, txstate);
> + if (ret == DMA_COMPLETE || !txstate)
> + return ret;
> +
> + spin_lock_irqsave(&hvc->vc.lock, flags);
> + vd = mtk_hsdma_find_active_desc(c, cookie);
> + spin_unlock_irqrestore(&hvc->vc.lock, flags);
> +
> + if (vd) {
> + hvd = to_hsdma_vdesc(vd);
> + bytes = hvd->residue;
> + }
> +
> + dma_set_residue(txstate, bytes);
> +
> + return ret;
> +}
> +
> +static void mtk_hsdma_issue_pending(struct dma_chan *c)
> +{
> + struct mtk_hsdma_device *hsdma = to_hsdma_dev(c);
> + struct mtk_hsdma_vchan *hvc = to_hsdma_vchan(c);
> + unsigned long flags;
> +
> + spin_lock_irqsave(&hvc->vc.lock, flags);
> +
> + if (vchan_issue_pending(&hvc->vc))
> + mtk_hsdma_issue_vchan_pending(hsdma, hvc);
> +
> + spin_unlock_irqrestore(&hvc->vc.lock, flags);
> +}
> +
> +static struct dma_async_tx_descriptor *
> +mtk_hsdma_prep_dma_memcpy(struct dma_chan *c, dma_addr_t dest,
> + dma_addr_t src, size_t len, unsigned long flags)
> +{
> + struct mtk_hsdma_vdesc *hvd;
> +
> + hvd = kzalloc(sizeof(*hvd), GFP_NOWAIT);
> + if (!hvd)
> + return NULL;
> +
> + hvd->len = len;
> + hvd->residue = len;
> + hvd->src = src;
> + hvd->dest = dest;
> +
> + return vchan_tx_prep(to_virt_chan(c), &hvd->vd, flags);
> +}
> +
> +static int mtk_hsdma_free_inactive_desc(struct dma_chan *c, bool reuse_clr)
> +{
> + struct virt_dma_chan *vc = to_virt_chan(c);
> + unsigned long flags;
> + LIST_HEAD(head);
> +
> + /* Terminate pending descriptors that have previously been submitted
Begin multi-line comments with:
/*
then real comments start on the next line.
> + * to the channel. However, the DMA engine doesn't provide any way to
> + * stop these descriptors being processed by hardware. The only way is
> + * just waiting until these descriptors are all processing done. Thus,
> + * users must synchronize to the DMA channel termination to guarantee
> + * that all transfers for previously issued descriptors have stopped.
> + */
> + spin_lock_irqsave(&vc->lock, flags);
> + list_splice_tail_init(&vc->desc_allocated, &head);
> + list_splice_tail_init(&vc->desc_submitted, &head);
> + list_splice_tail_init(&vc->desc_issued, &head);
> +
> + if (reuse_clr) {
> + struct virt_dma_desc *vd;
> +
> + list_for_each_entry(vd, &head, node)
> + dmaengine_desc_clear_reuse(&vd->tx);
> + }
> + spin_unlock_irqrestore(&vc->lock, flags);
> +
> + /* At the point, we don't expect users put descriptor into VC again */
> + vchan_dma_desc_free_list(vc, &head);
> +
> + return 0;
> +}
> +
> +static int mtk_hsdma_terminate_all(struct dma_chan *c)
> +{
> + mtk_hsdma_free_inactive_desc(c, false);
> +
> + return 0;
> +}
> +
> +static void mtk_hsdma_synchronize(struct dma_chan *c)
> +{
> + struct mtk_hsdma_vchan *hvc = to_hsdma_vchan(c);
> + bool sync_needed = false;
> +
> + /*
> + * Once issue_synchronize is being set, which means once the hardware
> + * consumes all descriptors for the channel in the ring, the
> + * synchronization must be be notified immediately it is completed.
> + */
> + spin_lock(&hvc->vc.lock);
> + if (!list_empty(&hvc->desc_hw_processing)) {
> + hvc->issue_synchronize = true;
> + sync_needed = true;
> + }
> + spin_unlock(&hvc->vc.lock);
> +
> + if (sync_needed)
> + wait_for_completion(&hvc->issue_completion);
> + /*
> + * At the point, we expect that all remaining descriptors in the ring
> + * for the channel should be all processing done.
> + */
> + WARN_ONCE(!list_empty(&hvc->desc_hw_processing),
> + "Desc pending still in list desc_hw_processing\n");
> +
> + /* Free all descriptors in list desc_completed */
> + vchan_synchronize(&hvc->vc);
> +
> + WARN_ONCE(!list_empty(&hvc->vc.desc_completed),
> + "Desc pending still in list desc_completed\n");
> +}
thanks,
--
~Randy
--
To unsubscribe from this list: send the line "unsubscribe dmaengine" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
