about mmap dma-buf and sync

Daniel Stone daniel at fooishbar.org
Mon Aug 24 08:52:13 PDT 2015

Hi Thomas,

On 24 August 2015 at 10:50, Thomas Hellstrom <thomas at shipmail.org> wrote:
> In any case, Ideally I'd want the struct dma_buf_sync look something like
> enum dma_buf_sync_flags {
>         DMA_BUF_SYNC_READ = (1 << 0),
>         DMA_BUF_SYNC_WRITE = (2 << 0),
>         DMA_BUF_SYNC_RW = (3 << 0),
>         DMA_BUF_SYNC_START = (0 << 2),
>         DMA_BUF_SYNC_END = (1 << 2),
>                 DMA_BUF_SYNC_END
> };
> /* begin/end dma-buf functions used for userspace mmap. */
> struct dma_buf_sync {
>         enum dma_buf_sync_flags flags;
>         __u64 x;
>         __u64 width;
>         /* 2D Sync parameters */
>         __u64 stride;
>         __u64 y;
>         __u64 height;
> }
> So again, why use a 2D sync in the first place when it's probably possible to batch 1D syncs in the driver?
> It all boils down to the number of syscalls required, and also the fact that it's probably not trivial to batch sync calls in the read-write case. One can, again, argue that there might be 3D cases or other obscure sync cases,
> but in that case, let's implement them in the future if the overhead becomes unbearable. If the sync mechanism gets established as
> mandatory, the problem will solve itself. And at least now we cover the most common case and we don't need to add a new multiplexing
> mechanism in the sync IOCTL since we already have one in the IOCTL mechanism. We can simply add a new sync IOCTL for new obscure sync cases if desired.

I still don't think this ameliorates the need for batching: consider
the case where you update two disjoint screen regions and want them
both flushed. Either you issue two separate sync calls (which can be
disadvantageous performance-wise on some hardware / setups), or you
accumulate the regions and only flush later. So either two ioctls (one
in the style of dirtyfb and one to perform the sync/flush; you can
shortcut to assume the full buffer was damaged if the former is
missing), or one like this:

struct dma_buf_sync_2d {
        enum dma_buf_sync_flags flags;

        __u64 stride_bytes;
        __u32 bytes_per_pixel;
        __u32 num_regions;

        struct {
                __u64 x;
                __u64 y;
                __u64 width;
                __u64 height;
        } regions[];


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