[PATCH 2/2] dmabuf/heaps: implement DMA_BUF_IOCTL_RW_FILE for system_heap

wangtao tao.wangtao at honor.com
Wed May 14 11:02:09 UTC 2025 


> -----Original Message-----
> From: Christian König <christian.koenig at amd.com>
> Sent: Tuesday, May 13, 2025 9:18 PM
> To: wangtao <tao.wangtao at honor.com>; sumit.semwal at linaro.org;
> benjamin.gaignard at collabora.com; Brian.Starkey at arm.com;
> jstultz at google.com; tjmercier at google.com
> Cc: linux-media at vger.kernel.org; dri-devel at lists.freedesktop.org; linaro-
> mm-sig at lists.linaro.org; linux-kernel at vger.kernel.org;
> wangbintian(BintianWang) <bintian.wang at honor.com>; yipengxiang
> <yipengxiang at honor.com>; <liulu.liu at honor.com>; <feng.han at honor.com>
> Subject: Re: [PATCH 2/2] dmabuf/heaps: implement
> DMA_BUF_IOCTL_RW_FILE for system_heap
> 
> On 5/13/25 14:30, wangtao wrote:
> >> -----Original Message-----
> >> From: Christian König <christian.koenig at amd.com>
> >> Sent: Tuesday, May 13, 2025 7:32 PM
> >> To: wangtao <tao.wangtao at honor.com>; sumit.semwal at linaro.org;
> >> benjamin.gaignard at collabora.com; Brian.Starkey at arm.com;
> >> jstultz at google.com; tjmercier at google.com
> >> Cc: linux-media at vger.kernel.org; dri-devel at lists.freedesktop.org;
> >> linaro- mm-sig at lists.linaro.org; linux-kernel at vger.kernel.org;
> >> wangbintian(BintianWang) <bintian.wang at honor.com>; yipengxiang
> >> <yipengxiang at honor.com>; <liulu.liu at honor.com>;
> >> <feng.han at honor.com>
> >> Subject: Re: [PATCH 2/2] dmabuf/heaps: implement
> >> DMA_BUF_IOCTL_RW_FILE for system_heap
> >>
> >> On 5/13/25 11:28, wangtao wrote:
> >>> Support direct file I/O operations for system_heap dma-buf objects.
> >>> Implementation includes:
> >>> 1. Convert sg_table to bio_vec
> >>
> >> That is usually illegal for DMA-bufs.
> > [wangtao] The term 'convert' is misleading in this context. The appropriate
> phrasing should be: Construct bio_vec from sg_table.
> 
> Well it doesn't matter what you call it. Touching the page inside an sg table of
> a DMA-buf is illegal, we even have code to actively prevent that.
[wangtao] For a driver using DMA-buf: Don't touch pages in the sg_table. But the system heap exporter (sg_table owner) should be allowed to use them.
If a driver takes ownership via dma_buf_map_attachment or similar calls, the exporter must stop using the sg_table.
User-space programs should call DMA_BUF_IOCTL_RW_FILE only when the DMA-buf is not attached.
The exporter must check ownership (e.g., ensure no map_dma_buf/vmap is active) and block new calls during operations.
I'll add these checks in patch v2.

> 
> Once more: This approach was already rejected multiple times! Please use
> udmabuf instead!
> 
> The hack you came up here is simply not necessary.
[wangtao] Many people need DMA-buf direct I/O. I tried it 2 years ago. My method is simpler, uses less CPU/power, and performs better:
  - Speed: 3418 MB/s vs. 2073 MB/s (udmabuf) at 1GHz CPU.
  - udmabuf wastes half its CPU time on __get_user_pages.
  - Creating 32x32MB DMA-bufs + reading 1GB file takes 346 ms vs. 1145 ms for udmabuf (10x slower) vs. 1503 ms for DMA-buf normal.
udmabuf is slightly faster but not enough. Switching to udmabuf is easy for small apps but hard in complex systems without major benefits.
> 
> Regards,
> Christian.
> 
> 
> > Appreciate your feedback.
> >>
> >> Regards,
> >> Christian.
> >>
> >>> 2. Set IOCB_DIRECT when O_DIRECT is supported 3. Invoke
> >>> vfs_iocb_iter_read()/vfs_iocb_iter_write() for actual I/O
> >>>
> >>> Performance metrics (UFS 4.0 device @4GB/s, Arm64 CPU @1GHz):
> >>>
> >>> | Metric             |    1MB |    8MB |    64MB |   1024MB |   3072MB |
> >>> |--------------------|-------:|-------:|--------:|---------:|-------
> >>> |--------------------|--
> >>> |--------------------|:|
> >>> | Buffer Read (us)   |   1658 |   9028 |   69295 |  1019783 |  2978179 |
> >>> | Direct Read (us)   |    707 |   2647 |   18689 |   299627 |   937758 |
> >>> | Buffer Rate (MB/s) |    603 |    886 |     924 |     1004 |     1032 |
> >>> | Direct Rate (MB/s) |   1414 |   3022 |    3425 |     3418 |     3276 |
> >>>
> >>> Signed-off-by: wangtao <tao.wangtao at honor.com>
> >>> ---
> >>>  drivers/dma-buf/heaps/system_heap.c | 118
> >>> ++++++++++++++++++++++++++++
> >>>  1 file changed, 118 insertions(+)
> >>>
> >>> diff --git a/drivers/dma-buf/heaps/system_heap.c
> >>> b/drivers/dma-buf/heaps/system_heap.c
> >>> index 26d5dc89ea16..f7b71b9843aa 100644
> >>> --- a/drivers/dma-buf/heaps/system_heap.c
> >>> +++ b/drivers/dma-buf/heaps/system_heap.c
> >>> @@ -20,6 +20,8 @@
> >>>  #include <linux/scatterlist.h>
> >>>  #include <linux/slab.h>
> >>>  #include <linux/vmalloc.h>
> >>> +#include <linux/bvec.h>
> >>> +#include <linux/uio.h>
> >>>
> >>>  static struct dma_heap *sys_heap;
> >>>
> >>> @@ -281,6 +283,121 @@ static void system_heap_vunmap(struct
> dma_buf
> >> *dmabuf, struct iosys_map *map)
> >>>  	iosys_map_clear(map);
> >>>  }
> >>>
> >>> +static struct bio_vec *system_heap_init_bvec(struct
> >> system_heap_buffer *buffer,
> >>> +			size_t offset, size_t len, int *nr_segs) {
> >>> +	struct sg_table *sgt = &buffer->sg_table;
> >>> +	struct scatterlist *sg;
> >>> +	size_t length = 0;
> >>> +	unsigned int i, k = 0;
> >>> +	struct bio_vec *bvec;
> >>> +	size_t sg_left;
> >>> +	size_t sg_offset;
> >>> +	size_t sg_len;
> >>> +
> >>> +	bvec = kvcalloc(sgt->nents, sizeof(*bvec), GFP_KERNEL);
> >>> +	if (!bvec)
> >>> +		return NULL;
> >>> +
> >>> +	for_each_sg(sgt->sgl, sg, sgt->nents, i) {
> >>> +		length += sg->length;
> >>> +		if (length <= offset)
> >>> +			continue;
> >>> +
> >>> +		sg_left = length - offset;
> >>> +		sg_offset = sg->offset + sg->length - sg_left;
> >>> +		sg_len = min(sg_left, len);
> >>> +
> >>> +		bvec[k].bv_page = sg_page(sg);
> >>> +		bvec[k].bv_len = sg_len;
> >>> +		bvec[k].bv_offset = sg_offset;
> >>> +		k++;
> >>> +
> >>> +		offset += sg_len;
> >>> +		len -= sg_len;
> >>> +		if (len <= 0)
> >>> +			break;
> >>> +	}
> >>> +
> >>> +	*nr_segs = k;
> >>> +	return bvec;
> >>> +}
> >>> +
> >>> +static int system_heap_rw_file(struct system_heap_buffer *buffer,
> >>> +bool
> >> is_read,
> >>> +		bool direct_io, struct file *filp, loff_t file_offset,
> >>> +		size_t buf_offset, size_t len)
> >>> +{
> >>> +	struct bio_vec *bvec;
> >>> +	int nr_segs = 0;
> >>> +	struct iov_iter iter;
> >>> +	struct kiocb kiocb;
> >>> +	ssize_t ret = 0;
> >>> +
> >>> +	if (direct_io) {
> >>> +		if (!(filp->f_mode & FMODE_CAN_ODIRECT))
> >>> +			return -EINVAL;
> >>> +	}
> >>> +
> >>> +	bvec = system_heap_init_bvec(buffer, buf_offset, len, &nr_segs);
> >>> +	if (!bvec)
> >>> +		return -ENOMEM;
> >>> +
> >>> +	iov_iter_bvec(&iter, is_read ? ITER_DEST : ITER_SOURCE, bvec,
> >> nr_segs, len);
> >>> +	init_sync_kiocb(&kiocb, filp);
> >>> +	kiocb.ki_pos = file_offset;
> >>> +	if (direct_io)
> >>> +		kiocb.ki_flags |= IOCB_DIRECT;
> >>> +
> >>> +	while (kiocb.ki_pos < file_offset + len) {
> >>> +		if (is_read)
> >>> +			ret = vfs_iocb_iter_read(filp, &kiocb, &iter);
> >>> +		else
> >>> +			ret = vfs_iocb_iter_write(filp, &kiocb, &iter);
> >>> +		if (ret <= 0)
> >>> +			break;
> >>> +	}
> >>> +
> >>> +	kvfree(bvec);
> >>> +	return ret < 0 ? ret : 0;
> >>> +}
> >>> +
> >>> +static int system_heap_dma_buf_rw_file(struct dma_buf *dmabuf,
> >>> +			struct dma_buf_rw_file *back)
> >>> +{
> >>> +	struct system_heap_buffer *buffer = dmabuf->priv;
> >>> +	int ret = 0;
> >>> +	__u32 op = back->flags & DMA_BUF_RW_FLAGS_OP_MASK;
> >>> +	bool direct_io = back->flags & DMA_BUF_RW_FLAGS_DIRECT;
> >>> +	struct file *filp;
> >>> +
> >>> +	if (op != DMA_BUF_RW_FLAGS_READ && op !=
> >> DMA_BUF_RW_FLAGS_WRITE)
> >>> +		return -EINVAL;
> >>> +	if (direct_io) {
> >>> +		if (!PAGE_ALIGNED(back->file_offset) ||
> >>> +			!PAGE_ALIGNED(back->buf_offset) ||
> >>> +			!PAGE_ALIGNED(back->buf_len))
> >>> +		return -EINVAL;
> >>> +	}
> >>> +	if (!back->buf_len || back->buf_len > dmabuf->size ||
> >>> +		back->buf_offset >= dmabuf->size ||
> >>> +		back->buf_offset + back->buf_len > dmabuf->size)
> >>> +		return -EINVAL;
> >>> +	if (back->file_offset + back->buf_len < back->file_offset)
> >>> +		return -EINVAL;
> >>> +
> >>> +	filp = fget(back->fd);
> >>> +	if (!filp)
> >>> +		return -EBADF;
> >>> +
> >>> +	mutex_lock(&buffer->lock);
> >>> +	ret = system_heap_rw_file(buffer, op ==
> >> DMA_BUF_RW_FLAGS_READ, direct_io,
> >>> +			filp, back->file_offset, back->buf_offset, back-
> >>> buf_len);
> >>> +	mutex_unlock(&buffer->lock);
> >>> +
> >>> +	fput(filp);
> >>> +	return ret;
> >>> +}
> >>> +
> >>>  static void system_heap_dma_buf_release(struct dma_buf *dmabuf)  {
> >>>  	struct system_heap_buffer *buffer = dmabuf->priv; @@ -308,6
> >> +425,7
> >>> @@ static const struct dma_buf_ops system_heap_buf_ops = {
> >>>  	.mmap = system_heap_mmap,
> >>>  	.vmap = system_heap_vmap,
> >>>  	.vunmap = system_heap_vunmap,
> >>> +	.rw_file = system_heap_dma_buf_rw_file,
> >>>  	.release = system_heap_dma_buf_release,  };
> >>>
> >

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