[PATCH v7 3/5] dma-buf: Add ioctls to allow userspace to flush
Daniel Vetter
daniel at ffwll.ch
Tue Feb 9 10:20:20 UTC 2016
On Tue, Feb 09, 2016 at 10:26:25AM +0100, David Herrmann wrote:
> Hi
>
> On Tue, Dec 22, 2015 at 10:36 PM, Tiago Vignatti
> <tiago.vignatti at intel.com> wrote:
> > From: Daniel Vetter <daniel.vetter at ffwll.ch>
> >
> > The userspace might need some sort of cache coherency management e.g. when CPU
> > and GPU domains are being accessed through dma-buf at the same time. To
> > circumvent this problem there are begin/end coherency markers, that forward
> > directly to existing dma-buf device drivers vfunc hooks. Userspace can make use
> > of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence would be
> > used like following:
> > - mmap dma-buf fd
> > - for each drawing/upload cycle in CPU 1. SYNC_START ioctl, 2. read/write
> > to mmap area 3. SYNC_END ioctl. This can be repeated as often as you
> > want (with the new data being consumed by the GPU or say scanout device)
> > - munmap once you don't need the buffer any more
> >
> > v2 (Tiago): Fix header file type names (u64 -> __u64)
> > v3 (Tiago): Add documentation. Use enum dma_buf_sync_flags to the begin/end
> > dma-buf functions. Check for overflows in start/length.
> > v4 (Tiago): use 2d regions for sync.
> > v5 (Tiago): forget about 2d regions (v4); use _IOW in DMA_BUF_IOCTL_SYNC and
> > remove range information from struct dma_buf_sync.
> > v6 (Tiago): use __u64 structured padded flags instead enum. Adjust
> > documentation about the recommendation on using sync ioctls.
> > v7 (Tiago): Alex' nit on flags definition and being even more wording in the
> > doc about sync usage.
> >
> > Cc: Sumit Semwal <sumit.semwal at linaro.org>
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > Signed-off-by: Tiago Vignatti <tiago.vignatti at intel.com>
> > ---
> > Documentation/dma-buf-sharing.txt | 21 ++++++++++++++++++-
> > drivers/dma-buf/dma-buf.c | 43 +++++++++++++++++++++++++++++++++++++++
> > include/uapi/linux/dma-buf.h | 38 ++++++++++++++++++++++++++++++++++
> > 3 files changed, 101 insertions(+), 1 deletion(-)
> > create mode 100644 include/uapi/linux/dma-buf.h
> >
> > diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
> > index 4f4a84b..32ac32e 100644
> > --- a/Documentation/dma-buf-sharing.txt
> > +++ b/Documentation/dma-buf-sharing.txt
> > @@ -350,7 +350,26 @@ Being able to mmap an export dma-buf buffer object has 2 main use-cases:
> > handles, too). So it's beneficial to support this in a similar fashion on
> > dma-buf to have a good transition path for existing Android userspace.
> >
> > - No special interfaces, userspace simply calls mmap on the dma-buf fd.
> > + No special interfaces, userspace simply calls mmap on the dma-buf fd, making
> > + sure that the cache synchronization ioctl (DMA_BUF_IOCTL_SYNC) is *always*
> > + used when the access happens. This is discussed next paragraphs.
> > +
> > + Some systems might need some sort of cache coherency management e.g. when
> > + CPU and GPU domains are being accessed through dma-buf at the same time. To
> > + circumvent this problem there are begin/end coherency markers, that forward
> > + directly to existing dma-buf device drivers vfunc hooks. Userspace can make
> > + use of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence
> > + would be used like following:
> > + - mmap dma-buf fd
> > + - for each drawing/upload cycle in CPU 1. SYNC_START ioctl, 2. read/write
> > + to mmap area 3. SYNC_END ioctl. This can be repeated as often as you
> > + want (with the new data being consumed by the GPU or say scanout device)
> > + - munmap once you don't need the buffer any more
> > +
> > + Therefore, for correctness and optimal performance, systems with the memory
> > + cache shared by the GPU and CPU i.e. the "coherent" and also the
> > + "incoherent" are always required to use SYNC_START and SYNC_END before and
> > + after, respectively, when accessing the mapped address.
> >
> > 2. Supporting existing mmap interfaces in importers
> >
> > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > index b2ac13b..9a298bd 100644
> > --- a/drivers/dma-buf/dma-buf.c
> > +++ b/drivers/dma-buf/dma-buf.c
> > @@ -34,6 +34,8 @@
> > #include <linux/poll.h>
> > #include <linux/reservation.h>
> >
> > +#include <uapi/linux/dma-buf.h>
> > +
> > static inline int is_dma_buf_file(struct file *);
> >
> > struct dma_buf_list {
> > @@ -251,11 +253,52 @@ out:
> > return events;
> > }
> >
> > +static long dma_buf_ioctl(struct file *file,
> > + unsigned int cmd, unsigned long arg)
> > +{
> > + struct dma_buf *dmabuf;
> > + struct dma_buf_sync sync;
> > + enum dma_data_direction direction;
> > +
> > + dmabuf = file->private_data;
> > +
> > + if (!is_dma_buf_file(file))
> > + return -EINVAL;
>
> Why? This can never happen, and you better not use dma_buf_ioctl()
> outside of dma_buf_fops..
> I guess it's simply copied from the other fop callbacks, but I don't
> see why. dma_buf_poll() doesn't do it, neither should this, or one of
> the other 3 callbacks.
>
> > +
> > + switch (cmd) {
> > + case DMA_BUF_IOCTL_SYNC:
> > + if (copy_from_user(&sync, (void __user *) arg, sizeof(sync)))
> > + return -EFAULT;
> > +
> > + if (sync.flags & DMA_BUF_SYNC_RW)
> > + direction = DMA_BIDIRECTIONAL;
> > + else if (sync.flags & DMA_BUF_SYNC_READ)
> > + direction = DMA_FROM_DEVICE;
> > + else if (sync.flags & DMA_BUF_SYNC_WRITE)
> > + direction = DMA_TO_DEVICE;
> > + else
> > + return -EINVAL;
>
> This looks bogus. It always ends up being "DMA_BIDIRECTIONAL" or
> EINVAL. I recommend changing it to:
>
> switch (sync.flags & DMA_BUF_SYNC_RW) {
> case DMA_BUF_SYNC_READ:
> direction = DMA_FROM_DEVICE;
> break;
> case DMA_BUF_SYNC_WRITE:
> direction = DMA_TO_DEVICE;
> break;
> case DMA_BUF_SYNC_READ:
> direction = DMA_BIDIRECTIONAL;
> break;
> default:
> return -EINVAL;
> }
>
> > +
> > + if (sync.flags & ~DMA_BUF_SYNC_VALID_FLAGS_MASK)
> > + return -EINVAL;
>
> Why isn't this done immediately after copy_from_user()?
>
> > +
> > + if (sync.flags & DMA_BUF_SYNC_END)
> > + dma_buf_end_cpu_access(dmabuf, direction);
> > + else
> > + dma_buf_begin_cpu_access(dmabuf, direction);
>
> Why are SYNC_START and SYNC_END exclusive? It feels very natural to me
> to invoke both at the same time (especially if two objects are stored
> in the same dma-buf).
>
> > +
> > + return 0;
> > + default:
> > + return -ENOTTY;
> > + }
> > +}
> > +
> > static const struct file_operations dma_buf_fops = {
> > .release = dma_buf_release,
> > .mmap = dma_buf_mmap_internal,
> > .llseek = dma_buf_llseek,
> > .poll = dma_buf_poll,
> > + .unlocked_ioctl = dma_buf_ioctl,
> > };
> >
> > /*
> > diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-buf.h
> > new file mode 100644
> > index 0000000..4a9b36b
> > --- /dev/null
> > +++ b/include/uapi/linux/dma-buf.h
> > @@ -0,0 +1,38 @@
> > +/*
> > + * Framework for buffer objects that can be shared across devices/subsystems.
> > + *
> > + * Copyright(C) 2015 Intel Ltd
> > + *
> > + * This program is free software; you can redistribute it and/or modify it
> > + * under the terms of the GNU General Public License version 2 as published by
> > + * the Free Software Foundation.
> > + *
> > + * This program is distributed in the hope that it will be useful, but WITHOUT
> > + * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
> > + * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
> > + * more details.
> > + *
> > + * You should have received a copy of the GNU General Public License along with
> > + * this program. If not, see <http://www.gnu.org/licenses/>.
> > + */
> > +
> > +#ifndef _DMA_BUF_UAPI_H_
> > +#define _DMA_BUF_UAPI_H_
> > +
> > +/* begin/end dma-buf functions used for userspace mmap. */
> > +struct dma_buf_sync {
> > + __u64 flags;
> > +};
>
> Please add '#include <linux/types.h>', otherwise this header cannot be
> compiled on its own (missing __u64).
>
> > +
> > +#define DMA_BUF_SYNC_READ (1 << 0)
> > +#define DMA_BUF_SYNC_WRITE (2 << 0)
> > +#define DMA_BUF_SYNC_RW (DMA_BUF_SYNC_READ | DMA_BUF_SYNC_WRITE)
> > +#define DMA_BUF_SYNC_START (0 << 2)
> > +#define DMA_BUF_SYNC_END (1 << 2)
> > +#define DMA_BUF_SYNC_VALID_FLAGS_MASK \
> > + (DMA_BUF_SYNC_RW | DMA_BUF_SYNC_END)
> > +
> > +#define DMA_BUF_BASE 'b'
> > +#define DMA_BUF_IOCTL_SYNC _IOW(DMA_BUF_BASE, 0, struct dma_buf_sync)
>
> Why _IOW? A read-only ioctl should be able to call DMA_BUF_SYNC_READ, right?
Oops, looks like more work is needed. Thanks for your review.
I dropped patches 3-5 again meanwhile from drm-misc.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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