kern/drivers/dev/cbdma.c - akaros - Git at Google

 /* Copyright (c) 2019-2020 Google Inc
  * Aditya Basu <mitthu@google.com>
  * Barret Rhoden <brho@google.com>
  * See LICENSE for details.
  *
  * Useful resources:
  *   - Intel Xeon E7 2800/4800/8800 Datasheet Vol. 2
  *   - Purley Programmer's Guide
  *
  * Acronyms:
  *   - IOAT: (Intel) I/O Acceleration Technology
  *   - CDMA: Crystal Beach DMA
  *
  * TODO
  * - Consider something lighter-weight than the qlock for ensuring the device
  * doesn't get detached during operation.  kref, perhaps.  There's also an
  * element of "stop new people from coming in", like we do with closing FDs.
  * There's also stuff that the dmaengine does in linux.  See dma_chan_get().
  * - Freeze or handle faults with VA->PA page mappings, till DMA is completed.
  * Right now, we could get iommu faults, which was the purpose of this whole
  * thing.
  *	- The dmaengine has helpers for some of this.  dma_set_unmap() is a
  *	"unmap all these things when you're done" approach, called by __cleanup
  *	-> dma_descriptor_unmap().  the unmap struct is basically a todo list.
  * - There's a lot of stuff we could do with the DMA engine to reduce the
  * amount of device touches, contention, and other inefficiencies.
  * issue_dma() is a minimalist one.  No batching, etc.  And with the pdev
  * qlock, we have only a single request per PCI device, though there may be
  * numerous channels.
  */

 #include <kmalloc.h>
 #include <string.h>
 #include <stdio.h>
 #include <assert.h>
 #include <error.h>
 #include <net/ip.h>
 #include <linux_compat.h>
 #include <arch/pci.h>
 #include <page_alloc.h>
 #include <pmap.h>
 #include <arch/pci_regs.h>

 #include <linux/dmaengine.h>

 /* QID Path */
 enum {
 	Qdir           = 0,
 	Qcbdmaktest    = 1,
 	Qcbdmaucopy    = 2,
 };

 static struct dirtab cbdmadir[] = {
 	{".",         {Qdir, 0, QTDIR}, 0, 0555},
 	{"ktest",     {Qcbdmaktest, 0, QTFILE}, 0, 0555},
 	{"ucopy",     {Qcbdmaucopy, 0, QTFILE}, 0, 0755},
 };

 /* TODO: this is a device/kernel ABI.  ucbdma.c has a copy.  It's probably not
  * worth putting in its own header, since this is really cheap test code. */
 struct ucbdma {
 	uint64_t		dst_addr;
 	uint64_t		src_addr;
 	uint32_t		xfer_size;
 	char			bdf_str[10];
 } __attribute__((packed));

 #define KTEST_SIZE 64
 static struct {
 	char    src[KTEST_SIZE];
 	char    dst[KTEST_SIZE];
 	char    srcfill;
 	char    dstfill;
 } ktest = {.srcfill = '0', .dstfill = 'X'};

 static inline struct pci_device *dma_chan_to_pci_dev(struct dma_chan *dc)
 {
 	return container_of(dc->device->dev, struct pci_device, linux_dev);
 }

 /* Filter function for finding a particular PCI device.  If
  * __dma_request_channel() asks for a particular device, we'll only give it that
  * chan.  If you don't care, pass NULL, and you'll get any free chan. */
 static bool filter_pci_dev(struct dma_chan *dc, void *arg)
 {
 	struct pci_device *pdev = dma_chan_to_pci_dev(dc);

 	if (arg)
 		return arg == pdev;
 	return true;
 }

 /* Addresses are device-physical.  Caller holds the pdev qlock. */
 static void issue_dma(struct pci_device *pdev, physaddr_t dst, physaddr_t src,
 		      size_t len, bool async)
 {
 	ERRSTACK(1);
 	struct dma_chan *dc;
 	dma_cap_mask_t mask;
 	struct dma_async_tx_descriptor *tx;
 	int flags;

 	struct completion cmp;
 	unsigned long tmo;
 	dma_cookie_t cookie;

 	/* dmaengine_get works for the non-DMA_PRIVATE devices.  A lot
 	 * of devices turn on DMA_PRIVATE, in which case they won't be in the
 	 * general pool available to the dmaengine.  Instead, we directly
 	 * request DMA channels - particularly since we want specific devices to
 	 * use with the IOMMU. */

 	dma_cap_zero(mask);
 	dma_cap_set(DMA_MEMCPY, mask);
 	dc = __dma_request_channel(&mask, filter_pci_dev, pdev);
 	if (!dc)
 		error(EFAIL, "Couldn't get a DMA channel");
 	if (waserror()) {
 		dma_release_channel(dc);
 		nexterror();
 	}

 	flags = 0;
 	if (async)
 		flags |= DMA_PREP_INTERRUPT;

 	if (!is_dma_copy_aligned(dc->device, dst, src, len))
 		error(EINVAL, "Bad copy alignment: %p %p %lu", dst, src, len);

 	tx = dmaengine_prep_dma_memcpy(dc, dst, src, len, flags);
 	if (!tx)
 		error(EFAIL, "Couldn't prep the memcpy!\n");

 	if (async) {
 		async_tx_ack(tx);
 		init_completion(&cmp);
 		tx->callback = (dma_async_tx_callback)complete;
 		tx->callback_param = &cmp;
 	}

 	cookie = dmaengine_submit(tx);
 	if (cookie < 0)
 		error(EFAIL, "Failed to submit the DMA...");

 	/* You can poke this.  dma_sync_wait() also calls this. */
 	dma_async_issue_pending(dc);

 	if (async) {
 		/* Giant warning: the polling methods, like
 		 * dmaengine_tx_status(), might actually trigger the
 		 * tx->callback.  At least the IOAT driver does this. */
 		tmo = wait_for_completion_timeout(&cmp, msecs_to_jiffies(3000));
 		if (tmo == 0 || dmaengine_tx_status(dc, cookie, NULL)
 					!= DMA_COMPLETE) {
 			error(ETIMEDOUT, "timeout or related spurious failure");
 		}
 	} else {
 		dma_wait_for_async_tx(tx);
 	}

 	dma_release_channel(dc);
 	poperror();
 }

 static void issue_dma_ucbdma(struct ucbdma *u)
 {
 	ERRSTACK(1);
 	struct pci_device *pdev;

 	pdev = pci_match_string(u->bdf_str);
 	if (!pdev)
 		error(ENODEV, "No device %s", u->bdf_str);
 	/* The qlock prevents unassignment from happening during an operation.
 	 * If that happened, the driver's reset method would be called while the
 	 * op is ongoing.  The driver might be able to handle that.  Though when
 	 * the iommu mappings are destroyed, the driver is likely to get wedged.
 	 *
 	 * A kref or something else might work better here, to allow multiple
 	 * DMAs at a time. */
 	qlock(&pdev->qlock);
 	if (waserror()) {
 		qunlock(&pdev->qlock);
 		nexterror();
 	}
 	if (pdev->proc_owner != current)
 		error(EINVAL, "wrong proc_owner");
 	issue_dma(pdev, u->dst_addr, u->src_addr, u->xfer_size, true);
 	qunlock(&pdev->qlock);
 	poperror();
 }

 /* Runs a basic test from within the kernel on 0:4.3.
  *
  * One option would be to have write() set the sza buffer.  It won't be static
  * through the chan's lifetime (so you'd need to deal with syncing), but it'd
  * let you set things.  Another would be to have another chan/file for the BDF
  * (and you'd sync on that). */
 static struct sized_alloc *open_ktest(void)
 {
 	ERRSTACK(2);
 	struct pci_device *pdev = pci_match_tbdf(MKBUS(0, 0, 4, 3));
 	struct sized_alloc *sza;
 	physaddr_t dst, src;	/* device addrs */
 	char *dst_d, *src_d;	/* driver addrs */
 	uintptr_t prev;

 	if (!pdev)
 		error(EINVAL, "no 00:04.3");

 	qlock(&pdev->qlock);
 	/* We need to get into the address space of the device, which might be
 	 * NULL if it's the kernel's or unassigned. */
 	prev = switch_to(pdev->proc_owner);
 	if (waserror()) {
 		switch_back(pdev->proc_owner, prev);
 		qunlock(&pdev->qlock);
 		nexterror();
 	}

 	if (pdev->state != DEV_STATE_ASSIGNED_KERNEL &&
 	    pdev->state != DEV_STATE_ASSIGNED_USER)
 		error(EINVAL, "00:04.3 is unassigned (%d)", pdev->state);

 	dst_d = dma_alloc_coherent(&pdev->linux_dev, KTEST_SIZE, &dst,
 				   MEM_WAIT);
 	src_d = dma_alloc_coherent(&pdev->linux_dev, KTEST_SIZE, &src,
 				   MEM_WAIT);

 	if (waserror()) {
 		dma_free_coherent(&pdev->linux_dev, KTEST_SIZE, dst_d, dst);
 		dma_free_coherent(&pdev->linux_dev, KTEST_SIZE, src_d, src);
 		nexterror();
 	}

 	ktest.srcfill += 1;
 	/* initialize src and dst address */
 	memset(src_d, ktest.srcfill, KTEST_SIZE);
 	memset(dst_d, ktest.dstfill, KTEST_SIZE);
 	src_d[KTEST_SIZE-1] = '\0';
 	dst_d[KTEST_SIZE-1] = '\0';

 	issue_dma(pdev, dst, src, KTEST_SIZE, true);

 	sza = sized_kzmalloc(1024, MEM_WAIT);
 	sza_printf(sza, "\tCopy Size: %d (0x%x)\n", KTEST_SIZE, KTEST_SIZE);
 	sza_printf(sza, "\tsrcfill: %c (0x%x)\n", ktest.srcfill, ktest.srcfill);
 	sza_printf(sza, "\tdstfill: %c (0x%x)\n", ktest.dstfill, ktest.dstfill);

 	/* %s on a uptr causes a printfmt warning.  stop at 20 too.  sanity.*/
 	sza_printf(sza, "\tsrc_str (after copy): ");
 	for (int i = 0; i < 20; i++)
 		sza_printf(sza, "%c", src_d[i]);
 	sza_printf(sza, "\n");

 	sza_printf(sza, "\tdst_str (after copy): ");
 	for (int i = 0; i < 20; i++)
 		sza_printf(sza, "%c", dst_d[i]);
 	sza_printf(sza, "\n");

 	dma_free_coherent(&pdev->linux_dev, KTEST_SIZE, dst_d, dst);
 	dma_free_coherent(&pdev->linux_dev, KTEST_SIZE, src_d, src);
 	poperror();

 	switch_back(pdev->proc_owner, prev);
 	qunlock(&pdev->qlock);
 	poperror();

 	return sza;
 }

 struct dev cbdmadevtab;

 static char *devname(void)
 {
 	return cbdmadevtab.name;
 }

 static struct chan *cbdmaattach(char *spec)
 {
 	return devattach(devname(), spec);
 }

 struct walkqid *cbdmawalk(struct chan *c, struct chan *nc, char **name,
 			 unsigned int nname)
 {
 	return devwalk(c, nc, name, nname, cbdmadir,
 		       ARRAY_SIZE(cbdmadir), devgen);
 }

 static size_t cbdmastat(struct chan *c, uint8_t *dp, size_t n)
 {
 	return devstat(c, dp, n, cbdmadir, ARRAY_SIZE(cbdmadir), devgen);
 }

 static struct chan *cbdmaopen(struct chan *c, int omode)
 {
 	switch (c->qid.path) {
 	case Qcbdmaktest:
 		c->synth_buf = open_ktest();
 		break;
 	case Qdir:
 	case Qcbdmaucopy:
 		break;
 	default:
 		error(EIO, "cbdma: qid 0x%x is impossible", c->qid.path);
 	}

 	return devopen(c, omode, cbdmadir, ARRAY_SIZE(cbdmadir), devgen);
 }

 static void cbdmaclose(struct chan *c)
 {
 	switch (c->qid.path) {
 	case Qcbdmaktest:
 		kfree(c->synth_buf);
 		c->synth_buf = NULL;
 		break;
 	case Qdir:
 	case Qcbdmaucopy:
 		break;
 	default:
 		error(EIO, "cbdma: qid 0x%x is impossible", c->qid.path);
 	}
 }

 static size_t cbdmaread(struct chan *c, void *va, size_t n, off64_t offset)
 {
 	struct sized_alloc *sza = c->synth_buf;

 	switch (c->qid.path) {
 	case Qcbdmaktest:
 		return readstr(offset, va, n, sza->buf);
 	case Qcbdmaucopy:
 		return readstr(offset, va, n,
 			"Write address of struct ucopy to issue DMA\n");
 	case Qdir:
 		return devdirread(c, va, n, cbdmadir, ARRAY_SIZE(cbdmadir),
 					devgen);
 	default:
 		error(EIO, "cbdma: qid 0x%x is impossible", c->qid.path);
 	}

 	return -1;      /* not reached */
 }

 static size_t cbdmawrite(struct chan *c, void *va, size_t n, off64_t offset)
 {
 	struct ucbdma ucbdma[1];

 	switch (c->qid.path) {
 	case Qdir:
 		error(EPERM, "writing not permitted");
 	case Qcbdmaktest:
 		error(EPERM, ERROR_FIXME);
 	case Qcbdmaucopy:
 		if (n != sizeof(struct ucbdma))
 			error(EINVAL, "Bad ucbdma size %u (%u)", n,
 			      sizeof(struct ucbdma));
 		if (copy_from_user(ucbdma, va, sizeof(struct ucbdma)))
 			error(EINVAL, "Bad ucbdma pointer");
 		issue_dma_ucbdma(ucbdma);
 		return n;
 	default:
 		error(EIO, "cbdma: qid 0x%x is impossible", c->qid.path);
 	}

 	return -1;      /* not reached */
 }

 struct dev cbdmadevtab __devtab = {
 	.name       = "cbdma",
 	.reset      = devreset,
 	.init       = devinit,
 	.shutdown   = devshutdown,
 	.attach     = cbdmaattach,
 	.walk       = cbdmawalk,
 	.stat       = cbdmastat,
 	.open       = cbdmaopen,
 	.create     = devcreate,
 	.close      = cbdmaclose,
 	.read       = cbdmaread,
 	.bread      = devbread,
 	.write      = cbdmawrite,
 	.bwrite     = devbwrite,
 	.remove     = devremove,
 	.wstat      = devwstat,
 };
	/* Copyright (c) 2019-2020 Google Inc
	* Aditya Basu <mitthu@google.com>
	* Barret Rhoden <brho@google.com>
	* See LICENSE for details.
	*
	* Useful resources:
	* - Intel Xeon E7 2800/4800/8800 Datasheet Vol. 2
	* - Purley Programmer's Guide
	*
	* Acronyms:
	* - IOAT: (Intel) I/O Acceleration Technology
	* - CDMA: Crystal Beach DMA
	*
	* TODO
	* - Consider something lighter-weight than the qlock for ensuring the device
	* doesn't get detached during operation. kref, perhaps. There's also an
	* element of "stop new people from coming in", like we do with closing FDs.
	* There's also stuff that the dmaengine does in linux. See dma_chan_get().
	* - Freeze or handle faults with VA->PA page mappings, till DMA is completed.
	* Right now, we could get iommu faults, which was the purpose of this whole
	* thing.
	* - The dmaengine has helpers for some of this. dma_set_unmap() is a
	* "unmap all these things when you're done" approach, called by __cleanup
	* -> dma_descriptor_unmap(). the unmap struct is basically a todo list.
	* - There's a lot of stuff we could do with the DMA engine to reduce the
	* amount of device touches, contention, and other inefficiencies.
	* issue_dma() is a minimalist one. No batching, etc. And with the pdev
	* qlock, we have only a single request per PCI device, though there may be
	* numerous channels.
	*/

	#include <kmalloc.h>
	#include <string.h>
	#include <stdio.h>
	#include <assert.h>
	#include <error.h>
	#include <net/ip.h>
	#include <linux_compat.h>
	#include <arch/pci.h>
	#include <page_alloc.h>
	#include <pmap.h>
	#include <arch/pci_regs.h>

	#include <linux/dmaengine.h>

	/* QID Path */
	enum {
	Qdir = 0,
	Qcbdmaktest = 1,
	Qcbdmaucopy = 2,
	};

	static struct dirtab cbdmadir[] = {
	{".", {Qdir, 0, QTDIR}, 0, 0555},
	{"ktest", {Qcbdmaktest, 0, QTFILE}, 0, 0555},
	{"ucopy", {Qcbdmaucopy, 0, QTFILE}, 0, 0755},
	};

	/* TODO: this is a device/kernel ABI. ucbdma.c has a copy. It's probably not
	* worth putting in its own header, since this is really cheap test code. */
	struct ucbdma {
	uint64_t dst_addr;
	uint64_t src_addr;
	uint32_t xfer_size;
	char bdf_str[10];
	} __attribute__((packed));

	#define KTEST_SIZE 64
	static struct {
	char src[KTEST_SIZE];
	char dst[KTEST_SIZE];
	char srcfill;
	char dstfill;
	} ktest = {.srcfill = '0', .dstfill = 'X'};

	static inline struct pci_device dma_chan_to_pci_dev(struct dma_chan dc)
	{
	return container_of(dc->device->dev, struct pci_device, linux_dev);
	}

	/* Filter function for finding a particular PCI device. If
	* __dma_request_channel() asks for a particular device, we'll only give it that
	* chan. If you don't care, pass NULL, and you'll get any free chan. */
	static bool filter_pci_dev(struct dma_chan dc, void arg)
	{
	struct pci_device *pdev = dma_chan_to_pci_dev(dc);

	if (arg)
	return arg == pdev;
	return true;
	}

	/* Addresses are device-physical. Caller holds the pdev qlock. */
	static void issue_dma(struct pci_device *pdev, physaddr_t dst, physaddr_t src,
	size_t len, bool async)
	{
	ERRSTACK(1);
	struct dma_chan *dc;
	dma_cap_mask_t mask;
	struct dma_async_tx_descriptor *tx;
	int flags;

	struct completion cmp;
	unsigned long tmo;
	dma_cookie_t cookie;

	/* dmaengine_get works for the non-DMA_PRIVATE devices. A lot
	* of devices turn on DMA_PRIVATE, in which case they won't be in the
	* general pool available to the dmaengine. Instead, we directly
	* request DMA channels - particularly since we want specific devices to
	* use with the IOMMU. */

	dma_cap_zero(mask);
	dma_cap_set(DMA_MEMCPY, mask);
	dc = __dma_request_channel(&mask, filter_pci_dev, pdev);
	if (!dc)
	error(EFAIL, "Couldn't get a DMA channel");
	if (waserror()) {
	dma_release_channel(dc);
	nexterror();
	}

	flags = 0;
	if (async)
	flags \|= DMA_PREP_INTERRUPT;

	if (!is_dma_copy_aligned(dc->device, dst, src, len))
	error(EINVAL, "Bad copy alignment: %p %p %lu", dst, src, len);

	tx = dmaengine_prep_dma_memcpy(dc, dst, src, len, flags);
	if (!tx)
	error(EFAIL, "Couldn't prep the memcpy!\n");

	if (async) {
	async_tx_ack(tx);
	init_completion(&cmp);
	tx->callback = (dma_async_tx_callback)complete;
	tx->callback_param = &cmp;
	}

	cookie = dmaengine_submit(tx);
	if (cookie < 0)
	error(EFAIL, "Failed to submit the DMA...");

	/* You can poke this. dma_sync_wait() also calls this. */
	dma_async_issue_pending(dc);

	if (async) {
	/* Giant warning: the polling methods, like
	* dmaengine_tx_status(), might actually trigger the
	* tx->callback. At least the IOAT driver does this. */
	tmo = wait_for_completion_timeout(&cmp, msecs_to_jiffies(3000));
	if (tmo == 0 \|\| dmaengine_tx_status(dc, cookie, NULL)
	!= DMA_COMPLETE) {
	error(ETIMEDOUT, "timeout or related spurious failure");
	}
	} else {
	dma_wait_for_async_tx(tx);
	}

	dma_release_channel(dc);
	poperror();
	}

	static void issue_dma_ucbdma(struct ucbdma *u)
	{
	ERRSTACK(1);
	struct pci_device *pdev;

	pdev = pci_match_string(u->bdf_str);
	if (!pdev)
	error(ENODEV, "No device %s", u->bdf_str);
	/* The qlock prevents unassignment from happening during an operation.
	* If that happened, the driver's reset method would be called while the
	* op is ongoing. The driver might be able to handle that. Though when
	* the iommu mappings are destroyed, the driver is likely to get wedged.
	*
	* A kref or something else might work better here, to allow multiple
	* DMAs at a time. */
	qlock(&pdev->qlock);
	if (waserror()) {
	qunlock(&pdev->qlock);
	nexterror();
	}
	if (pdev->proc_owner != current)
	error(EINVAL, "wrong proc_owner");
	issue_dma(pdev, u->dst_addr, u->src_addr, u->xfer_size, true);
	qunlock(&pdev->qlock);
	poperror();
	}

	/* Runs a basic test from within the kernel on 0:4.3.
	*
	* One option would be to have write() set the sza buffer. It won't be static
	* through the chan's lifetime (so you'd need to deal with syncing), but it'd
	* let you set things. Another would be to have another chan/file for the BDF
	* (and you'd sync on that). */
	static struct sized_alloc *open_ktest(void)
	{
	ERRSTACK(2);
	struct pci_device *pdev = pci_match_tbdf(MKBUS(0, 0, 4, 3));
	struct sized_alloc *sza;
	physaddr_t dst, src; /* device addrs */
	char dst_d, src_d; /* driver addrs */
	uintptr_t prev;

	if (!pdev)
	error(EINVAL, "no 00:04.3");

	qlock(&pdev->qlock);
	/* We need to get into the address space of the device, which might be
	* NULL if it's the kernel's or unassigned. */
	prev = switch_to(pdev->proc_owner);
	if (waserror()) {
	switch_back(pdev->proc_owner, prev);
	qunlock(&pdev->qlock);
	nexterror();
	}

	if (pdev->state != DEV_STATE_ASSIGNED_KERNEL &&
	pdev->state != DEV_STATE_ASSIGNED_USER)
	error(EINVAL, "00:04.3 is unassigned (%d)", pdev->state);

	dst_d = dma_alloc_coherent(&pdev->linux_dev, KTEST_SIZE, &dst,
	MEM_WAIT);
	src_d = dma_alloc_coherent(&pdev->linux_dev, KTEST_SIZE, &src,
	MEM_WAIT);

	if (waserror()) {
	dma_free_coherent(&pdev->linux_dev, KTEST_SIZE, dst_d, dst);
	dma_free_coherent(&pdev->linux_dev, KTEST_SIZE, src_d, src);
	nexterror();
	}

	ktest.srcfill += 1;
	/* initialize src and dst address */
	memset(src_d, ktest.srcfill, KTEST_SIZE);
	memset(dst_d, ktest.dstfill, KTEST_SIZE);
	src_d[KTEST_SIZE-1] = '\0';
	dst_d[KTEST_SIZE-1] = '\0';

	issue_dma(pdev, dst, src, KTEST_SIZE, true);

	sza = sized_kzmalloc(1024, MEM_WAIT);
	sza_printf(sza, "\tCopy Size: %d (0x%x)\n", KTEST_SIZE, KTEST_SIZE);
	sza_printf(sza, "\tsrcfill: %c (0x%x)\n", ktest.srcfill, ktest.srcfill);
	sza_printf(sza, "\tdstfill: %c (0x%x)\n", ktest.dstfill, ktest.dstfill);

	/* %s on a uptr causes a printfmt warning. stop at 20 too. sanity.*/
	sza_printf(sza, "\tsrc_str (after copy): ");
	for (int i = 0; i < 20; i++)
	sza_printf(sza, "%c", src_d[i]);
	sza_printf(sza, "\n");

	sza_printf(sza, "\tdst_str (after copy): ");
	for (int i = 0; i < 20; i++)
	sza_printf(sza, "%c", dst_d[i]);
	sza_printf(sza, "\n");

	dma_free_coherent(&pdev->linux_dev, KTEST_SIZE, dst_d, dst);
	dma_free_coherent(&pdev->linux_dev, KTEST_SIZE, src_d, src);
	poperror();

	switch_back(pdev->proc_owner, prev);
	qunlock(&pdev->qlock);
	poperror();

	return sza;
	}

	struct dev cbdmadevtab;

	static char *devname(void)
	{
	return cbdmadevtab.name;
	}

	static struct chan cbdmaattach(char spec)
	{
	return devattach(devname(), spec);
	}

	struct walkqid cbdmawalk(struct chan c, struct chan nc, char *name,
	unsigned int nname)
	{
	return devwalk(c, nc, name, nname, cbdmadir,
	ARRAY_SIZE(cbdmadir), devgen);
	}

	static size_t cbdmastat(struct chan c, uint8_t dp, size_t n)
	{
	return devstat(c, dp, n, cbdmadir, ARRAY_SIZE(cbdmadir), devgen);
	}

	static struct chan cbdmaopen(struct chan c, int omode)
	{
	switch (c->qid.path) {
	case Qcbdmaktest:
	c->synth_buf = open_ktest();
	break;
	case Qdir:
	case Qcbdmaucopy:
	break;
	default:
	error(EIO, "cbdma: qid 0x%x is impossible", c->qid.path);
	}

	return devopen(c, omode, cbdmadir, ARRAY_SIZE(cbdmadir), devgen);
	}

	static void cbdmaclose(struct chan *c)
	{
	switch (c->qid.path) {
	case Qcbdmaktest:
	kfree(c->synth_buf);
	c->synth_buf = NULL;
	break;
	case Qdir:
	case Qcbdmaucopy:
	break;
	default:
	error(EIO, "cbdma: qid 0x%x is impossible", c->qid.path);
	}
	}

	static size_t cbdmaread(struct chan c, void va, size_t n, off64_t offset)
	{
	struct sized_alloc *sza = c->synth_buf;

	switch (c->qid.path) {
	case Qcbdmaktest:
	return readstr(offset, va, n, sza->buf);
	case Qcbdmaucopy:
	return readstr(offset, va, n,
	"Write address of struct ucopy to issue DMA\n");
	case Qdir:
	return devdirread(c, va, n, cbdmadir, ARRAY_SIZE(cbdmadir),
	devgen);
	default:
	error(EIO, "cbdma: qid 0x%x is impossible", c->qid.path);
	}

	return -1; /* not reached */
	}

	static size_t cbdmawrite(struct chan c, void va, size_t n, off64_t offset)
	{
	struct ucbdma ucbdma[1];

	switch (c->qid.path) {
	case Qdir:
	error(EPERM, "writing not permitted");
	case Qcbdmaktest:
	error(EPERM, ERROR_FIXME);
	case Qcbdmaucopy:
	if (n != sizeof(struct ucbdma))
	error(EINVAL, "Bad ucbdma size %u (%u)", n,
	sizeof(struct ucbdma));
	if (copy_from_user(ucbdma, va, sizeof(struct ucbdma)))
	error(EINVAL, "Bad ucbdma pointer");
	issue_dma_ucbdma(ucbdma);
	return n;
	default:
	error(EIO, "cbdma: qid 0x%x is impossible", c->qid.path);
	}

	return -1; /* not reached */
	}

	struct dev cbdmadevtab __devtab = {
	.name = "cbdma",
	.reset = devreset,
	.init = devinit,
	.shutdown = devshutdown,
	.attach = cbdmaattach,
	.walk = cbdmawalk,
	.stat = cbdmastat,
	.open = cbdmaopen,
	.create = devcreate,
	.close = cbdmaclose,
	.read = cbdmaread,
	.bread = devbread,
	.write = cbdmawrite,
	.bwrite = devbwrite,
	.remove = devremove,
	.wstat = devwstat,
	};