* [PATCH 2/2] virtio: use DMA coherent APIs
2026-01-15 12:05 [PATCH 1/2] ARM: mmu: optimize dma_alloc_coherent for cache-coherent DMA masters Ahmad Fatoum
@ 2026-01-15 12:05 ` Ahmad Fatoum
2026-01-19 10:20 ` [PATCH 1/2] ARM: mmu: optimize dma_alloc_coherent for cache-coherent DMA masters Sascha Hauer
1 sibling, 0 replies; 3+ messages in thread
From: Ahmad Fatoum @ 2026-01-15 12:05 UTC (permalink / raw)
To: barebox; +Cc: Ahmad Fatoum
This requires that barebox can actually tell apart DMA coherent devices
from non-coherent ones, so select OF_DMA_COHERENCY where applicable.
Signed-off-by: Ahmad Fatoum <a.fatoum@barebox.org>
---
drivers/virtio/Kconfig | 1 +
drivers/virtio/virtio.c | 4 +++
drivers/virtio/virtio_ring.c | 62 +++++++-----------------------------
3 files changed, 16 insertions(+), 51 deletions(-)
diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
index ecf66987b3ed..e39ec863f003 100644
--- a/drivers/virtio/Kconfig
+++ b/drivers/virtio/Kconfig
@@ -1,6 +1,7 @@
# SPDX-License-Identifier: GPL-2.0-only
config VIRTIO
bool
+ select OF_DMA_COHERENCY if OF
help
This option is selected by any driver which implements the virtio
bus, such as CONFIG_VIRTIO_MMIO, CONFIG_VIRTIO_PCI.
diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
index 4abf551a2834..262c09c9fe58 100644
--- a/drivers/virtio/virtio.c
+++ b/drivers/virtio/virtio.c
@@ -138,6 +138,10 @@ int virtio_finalize_features(struct virtio_device *dev)
return -ENODEV;
}
+ /* When this changes in future with support for IOMMUs in
+ * emulation, make sure to adapt vring_alloc_queue(), so
+ * it ignores IOMMUs if virtio_has_dma_quirk()
+ */
if (!virtio_has_feature(dev, VIRTIO_F_ACCESS_PLATFORM)) {
dev_warn(&dev->dev,
"device must provide VIRTIO_F_ACCESS_PLATFORM\n");
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 8b6469f54d2a..8953d43c440c 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -23,9 +23,14 @@
#define vq_info(vq, fmt, ...) \
dev_info(&vq->vdev->dev, fmt, ##__VA_ARGS__)
+static inline struct device *virtio_dma_dev(const struct virtio_device *vdev)
+{
+ return vdev->dev.parent;
+}
+
static inline struct device *vring_dma_dev(const struct virtqueue *vq)
{
- return vq->vdev->dev.parent;
+ return virtio_dma_dev(vq->vdev);
}
/* Map one sg entry. */
@@ -330,11 +335,6 @@ static struct virtqueue *__vring_new_virtqueue(unsigned int index,
* On most systems with virtio, physical addresses match bus addresses,
* and it _shouldn't_ particularly matter whether we use the DMA API.
*
- * However, barebox' dma_alloc_coherent doesn't yet take a device pointer
- * as argument, so even for dma-coherent devices, the virtqueue is mapped
- * uncached on ARM. This has considerable impact on the Virt I/O performance,
- * so we really want to avoid using the DMA API if possible for the time being.
- *
* On some systems, including Xen and any system with a physical device
* that speaks virtio behind a physical IOMMU, we must use the DMA API
* for virtio DMA to work at all.
@@ -344,60 +344,20 @@ static struct virtqueue *__vring_new_virtqueue(unsigned int index,
* ignores the IOMMU, so we must either pretend that the IOMMU isn't
* there or somehow map everything as the identity.
*
- * For the time being, we preserve historic behavior and bypass the DMA
- * API.
- *
- * TODO: install a per-device DMA ops structure that does the right thing
- * taking into account all the above quirks, and use the DMA API
- * unconditionally on data path.
+ * As we do not support IOMMUs yet amd dma_alloc_cohrent takes a device
+ * pointer that enables us to do cached DMA, just use the DMA API
+ * unconditionally for now.
*/
-
-static bool vring_use_dma_api(const struct virtio_device *vdev)
-{
- return !virtio_has_dma_quirk(vdev);
-}
-
static void *vring_alloc_queue(struct virtio_device *vdev,
size_t size, dma_addr_t *dma_handle)
{
- if (vring_use_dma_api(vdev)) {
- return dma_alloc_coherent(DMA_DEVICE_BROKEN, size, dma_handle);
- } else {
- void *queue = memalign(PAGE_SIZE, PAGE_ALIGN(size));
-
- if (queue) {
- phys_addr_t phys_addr = virt_to_phys(queue);
- *dma_handle = (dma_addr_t)phys_addr;
-
- memset(queue, 0x00, PAGE_ALIGN(size));
-
- /*
- * Sanity check: make sure we dind't truncate
- * the address. The only arches I can find that
- * have 64-bit phys_addr_t but 32-bit dma_addr_t
- * are certain non-highmem MIPS and x86
- * configurations, but these configurations
- * should never allocate physical pages above 32
- * bits, so this is fine. Just in case, throw a
- * warning and abort if we end up with an
- * unrepresentable address.
- */
- if (WARN_ON_ONCE(*dma_handle != phys_addr)) {
- free(queue);
- return NULL;
- }
- }
- return queue;
- }
+ return dma_alloc_coherent(virtio_dma_dev(vdev), size, dma_handle);
}
static void vring_free_queue(struct virtio_device *vdev,
size_t size, void *queue, dma_addr_t dma_handle)
{
- if (vring_use_dma_api(vdev))
- dma_free_coherent(DMA_DEVICE_BROKEN, queue, dma_handle, size);
- else
- free(queue);
+ dma_free_coherent(virtio_dma_dev(vdev), queue, dma_handle, size);
}
struct virtqueue *vring_create_virtqueue(unsigned int index, unsigned int num,
--
2.47.3
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