Re: [PATCH] media: venus: use contig vb2 ops

From: Robin Murphy
Date: Tue Dec 15 2020 - 06:35:49 EST

On 2020-12-15 11:16, Stanimir Varbanov wrote:
Hi,

Cc: Robin

On 12/14/20 2:57 PM, Alexandre Courbot wrote:
This driver uses the SG vb2 ops, but effectively only ever accesses the
first entry of the SG table, indicating that it expects a flat layout.
Switch it to use the contiguous ops to make sure this expected invariant

Under what circumstances the sg table will has nents > 1? I came down to
[1] but not sure I got it right.

I'm afraid that for systems with low amount of system memory and when
the memory become fragmented, the driver will not work. That's why I
started with sg allocator.

Despite what videobuf-dma-contig seems to assume, dma_alloc_coherent() makes no guarantee of providing physically contiguous memory. What it provides is *DMA contiguous* memory, i.e. from the point of view of the device. When an IOMMU is present and managed by the DMA API, such buffers may be assembled out of physically-scattered pages (particularly under memory pressure/fragmentation).

Robin.


[1]
https://elixir.bootlin.com/linux/v5.10.1/source/drivers/iommu/dma-iommu.c#L782

is always enforced. Since the device is supposed to be behind an IOMMU
this should have little to none practical consequences beyond making the
driver not rely on a particular behavior of the SG implementation.

Reported-by: Tomasz Figa <tfiga@xxxxxxxxxxxx>
Signed-off-by: Alexandre Courbot <acourbot@xxxxxxxxxxxx>
---
Hi everyone,

It probably doesn't hurt to fix this issue before some actual issue happens.
I have tested this patch on Chrome OS and playback was just as fine as with
the SG ops.

drivers/media/platform/Kconfig | 2 +-
drivers/media/platform/qcom/venus/helpers.c | 9 ++-------
drivers/media/platform/qcom/venus/vdec.c | 6 +++---
drivers/media/platform/qcom/venus/venc.c | 6 +++---
4 files changed, 9 insertions(+), 14 deletions(-)

diff --git a/drivers/media/platform/Kconfig b/drivers/media/platform/Kconfig
index 35a18d388f3f..d9d7954111f2 100644
--- a/drivers/media/platform/Kconfig
+++ b/drivers/media/platform/Kconfig
@@ -533,7 +533,7 @@ config VIDEO_QCOM_VENUS
depends on INTERCONNECT || !INTERCONNECT
select QCOM_MDT_LOADER if ARCH_QCOM
select QCOM_SCM if ARCH_QCOM
- select VIDEOBUF2_DMA_SG
+ select VIDEOBUF2_DMA_CONTIG
select V4L2_MEM2MEM_DEV
help
This is a V4L2 driver for Qualcomm Venus video accelerator
diff --git a/drivers/media/platform/qcom/venus/helpers.c b/drivers/media/platform/qcom/venus/helpers.c
index 50439eb1ffea..859d260f002b 100644
--- a/drivers/media/platform/qcom/venus/helpers.c
+++ b/drivers/media/platform/qcom/venus/helpers.c
@@ -7,7 +7,7 @@
#include <linux/mutex.h>
#include <linux/slab.h>
#include <linux/kernel.h>
-#include <media/videobuf2-dma-sg.h>
+#include <media/videobuf2-dma-contig.h>
#include <media/v4l2-mem2mem.h>
#include <asm/div64.h>
@@ -1284,14 +1284,9 @@ int venus_helper_vb2_buf_init(struct vb2_buffer *vb)
struct venus_inst *inst = vb2_get_drv_priv(vb->vb2_queue);
struct vb2_v4l2_buffer *vbuf = to_vb2_v4l2_buffer(vb);
struct venus_buffer *buf = to_venus_buffer(vbuf);
- struct sg_table *sgt;
-
- sgt = vb2_dma_sg_plane_desc(vb, 0);
- if (!sgt)
- return -EFAULT;
buf->size = vb2_plane_size(vb, 0);
- buf->dma_addr = sg_dma_address(sgt->sgl);

Can we do it:

if (WARN_ON(sgt->nents > 1))
return -EFAULT;

I understand that logically using dma-sg when the flat layout is
expected by the hardware is wrong, but I haven't seen issues until now.

+ buf->dma_addr = vb2_dma_contig_plane_dma_addr(vb, 0);
if (vb->type == V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE)
list_add_tail(&buf->reg_list, &inst->registeredbufs);
diff --git a/drivers/media/platform/qcom/venus/vdec.c b/drivers/media/platform/qcom/venus/vdec.c
index 8488411204c3..3fb277c81aca 100644
--- a/drivers/media/platform/qcom/venus/vdec.c
+++ b/drivers/media/platform/qcom/venus/vdec.c
@@ -13,7 +13,7 @@
#include <media/v4l2-event.h>
#include <media/v4l2-ctrls.h>
#include <media/v4l2-mem2mem.h>
-#include <media/videobuf2-dma-sg.h>
+#include <media/videobuf2-dma-contig.h>
#include "hfi_venus_io.h"
#include "hfi_parser.h"
@@ -1461,7 +1461,7 @@ static int m2m_queue_init(void *priv, struct vb2_queue *src_vq,
src_vq->io_modes = VB2_MMAP | VB2_DMABUF;
src_vq->timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_COPY;
src_vq->ops = &vdec_vb2_ops;
- src_vq->mem_ops = &vb2_dma_sg_memops;
+ src_vq->mem_ops = &vb2_dma_contig_memops;
src_vq->drv_priv = inst;
src_vq->buf_struct_size = sizeof(struct venus_buffer);
src_vq->allow_zero_bytesused = 1;
@@ -1475,7 +1475,7 @@ static int m2m_queue_init(void *priv, struct vb2_queue *src_vq,
dst_vq->io_modes = VB2_MMAP | VB2_DMABUF;
dst_vq->timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_COPY;
dst_vq->ops = &vdec_vb2_ops;
- dst_vq->mem_ops = &vb2_dma_sg_memops;
+ dst_vq->mem_ops = &vb2_dma_contig_memops;
dst_vq->drv_priv = inst;
dst_vq->buf_struct_size = sizeof(struct venus_buffer);
dst_vq->allow_zero_bytesused = 1;
diff --git a/drivers/media/platform/qcom/venus/venc.c b/drivers/media/platform/qcom/venus/venc.c
index 1c61602c5de1..a09550cd1dba 100644
--- a/drivers/media/platform/qcom/venus/venc.c
+++ b/drivers/media/platform/qcom/venus/venc.c
@@ -10,7 +10,7 @@
#include <linux/pm_runtime.h>
#include <linux/slab.h>
#include <media/v4l2-mem2mem.h>
-#include <media/videobuf2-dma-sg.h>
+#include <media/videobuf2-dma-contig.h>
#include <media/v4l2-ioctl.h>
#include <media/v4l2-event.h>
#include <media/v4l2-ctrls.h>
@@ -1001,7 +1001,7 @@ static int m2m_queue_init(void *priv, struct vb2_queue *src_vq,
src_vq->io_modes = VB2_MMAP | VB2_USERPTR | VB2_DMABUF;
src_vq->timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_COPY;
src_vq->ops = &venc_vb2_ops;
- src_vq->mem_ops = &vb2_dma_sg_memops;
+ src_vq->mem_ops = &vb2_dma_contig_memops;
src_vq->drv_priv = inst;
src_vq->buf_struct_size = sizeof(struct venus_buffer);
src_vq->allow_zero_bytesused = 1;
@@ -1017,7 +1017,7 @@ static int m2m_queue_init(void *priv, struct vb2_queue *src_vq,
dst_vq->io_modes = VB2_MMAP | VB2_USERPTR | VB2_DMABUF;
dst_vq->timestamp_flags = V4L2_BUF_FLAG_TIMESTAMP_COPY;
dst_vq->ops = &venc_vb2_ops;
- dst_vq->mem_ops = &vb2_dma_sg_memops;
+ dst_vq->mem_ops = &vb2_dma_contig_memops;
dst_vq->drv_priv = inst;
dst_vq->buf_struct_size = sizeof(struct venus_buffer);
dst_vq->allow_zero_bytesused = 1;