v4 looks good to me as well.
--Jason
On Wed, 2022-06-01 at 16:13 +0000, Simon Ser wrote:
> To discover support for new DMA-BUF IOCTLs, user-space has no
> choice but to try to perform the IOCTL on an existing DMA-BUF.
> However, user-space may want to figure out whether or not the
> IOCTL is available before it has a DMA-BUF at hand, e.g. at
> initialization time in a Wayland compositor.
>
> Add a /sys/kernel/dmabuf/caps directory which allows the DMA-BUF
> subsystem to advertise supported features. Add a
> sync_file_import_export entry which indicates that importing and
> exporting sync_files from/to DMA-BUFs is supported.
>
> v2: Add missing files lost in a rebase
>
> v3:
> - Create separate file in Documentation/ABI/testing/, add it to
> MAINTAINERS
> - Fix kernel version (Daniel)
> - Remove unnecessary brackets (Jason)
> - Fix SPDX comment style
>
> v4: improve sysfs code (Greg)
>
> Signed-off-by: Simon Ser <contact@xxxxxxxxxxx>
> Reviewed-by: Jason Ekstrand <jason.ekstrand@xxxxxxxxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> Cc: Bas Nieuwenhuizen <bas@xxxxxxxxxxxxxxxxxxx>
> Cc: Christian König <christian.koenig@xxxxxxx>
> Cc: Greg KH <greg@xxxxxxxxx>
> ---
> .../ABI/testing/sysfs-kernel-dmabuf-caps | 13 ++++++
> MAINTAINERS | 1 +
> drivers/dma-buf/Makefile | 2 +-
> drivers/dma-buf/dma-buf-sysfs-caps.c | 31 +++++++++++++
> drivers/dma-buf/dma-buf-sysfs-caps.h | 15 +++++++
> drivers/dma-buf/dma-buf-sysfs-stats.c | 16 ++-----
> drivers/dma-buf/dma-buf-sysfs-stats.h | 6 ++-
> drivers/dma-buf/dma-buf.c | 45
> +++++++++++++++++--
> include/uapi/linux/dma-buf.h | 6 +++
> 9 files changed, 115 insertions(+), 20 deletions(-)
> create mode 100644 Documentation/ABI/testing/sysfs-kernel-dmabuf-
> caps
> create mode 100644 drivers/dma-buf/dma-buf-sysfs-caps.c
> create mode 100644 drivers/dma-buf/dma-buf-sysfs-caps.h
>
> diff --git a/Documentation/ABI/testing/sysfs-kernel-dmabuf-caps
> b/Documentation/ABI/testing/sysfs-kernel-dmabuf-caps
> new file mode 100644
> index 000000000000..f83af422fd18
> --- /dev/null
> +++ b/Documentation/ABI/testing/sysfs-kernel-dmabuf-caps
> @@ -0,0 +1,13 @@
> +What: /sys/kernel/dmabuf/caps
> +Date: May 2022
> +KernelVersion: v5.20
> +Contact: Simon Ser <contact@xxxxxxxxxxx>
> +Description: This directory advertises DMA-BUF capabilities
> supported by the
> + kernel.
> +
> +What: /sys/kernel/dmabuf/caps/sync_file_import_export
> +Date: May 2022
> +KernelVersion: v5.20
> +Contact: Simon Ser <contact@xxxxxxxxxxx>
> +Description: This file is read-only and advertises support for
> importing and
> + exporting sync_files from/to DMA-BUFs.
> diff --git a/MAINTAINERS b/MAINTAINERS
> index 11da16bfa123..8966686f7231 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -5871,6 +5871,7 @@ L: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> L: linaro-mm-sig@xxxxxxxxxxxxxxxx (moderated for non-
> subscribers)
> S: Maintained
> T: git git://anongit.freedesktop.org/drm/drm-misc
> +F: Documentation/ABI/testing/sysfs-kernel-dmabuf-caps
> F: Documentation/driver-api/dma-buf.rst
> F: drivers/dma-buf/
> F: include/linux/*fence.h
> diff --git a/drivers/dma-buf/Makefile b/drivers/dma-buf/Makefile
> index 4c9eb53ba3f8..afc874272710 100644
> --- a/drivers/dma-buf/Makefile
> +++ b/drivers/dma-buf/Makefile
> @@ -1,6 +1,6 @@
> # SPDX-License-Identifier: GPL-2.0-only
> obj-y := dma-buf.o dma-fence.o dma-fence-array.o dma-fence-chain.o \
> - dma-resv.o
> + dma-resv.o dma-buf-sysfs-caps.o
> obj-$(CONFIG_DMABUF_HEAPS) += dma-heap.o
> obj-$(CONFIG_DMABUF_HEAPS) += heaps/
> obj-$(CONFIG_SYNC_FILE) += sync_file.o
> diff --git a/drivers/dma-buf/dma-buf-sysfs-caps.c b/drivers/dma-
> buf/dma-buf-sysfs-caps.c
> new file mode 100644
> index 000000000000..6eb27b81469f
> --- /dev/null
> +++ b/drivers/dma-buf/dma-buf-sysfs-caps.c
> @@ -0,0 +1,31 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * DMA-BUF sysfs capabilities.
> + *
> + * Copyright (C) 2022 Simon Ser
> + */
> +
> +#include <linux/kobject.h>
> +#include <linux/sysfs.h>
> +
> +#include "dma-buf-sysfs-caps.h"
> +
> +static ssize_t sync_file_import_export_show(struct kobject *kobj,
> + struct kobj_attribute
> *attr,
> + char *buf)
> +{
> + return sysfs_emit(buf, "1\n");
> +}
> +
> +static struct kobj_attribute dma_buf_sync_file_import_export_attr =
> + __ATTR_RO(sync_file_import_export);
> +
> +static struct attribute *dma_buf_caps_attrs[] = {
> + &dma_buf_sync_file_import_export_attr.attr,
> + NULL,
> +};
> +
> +const struct attribute_group dma_buf_caps_group = {
> + .name = "caps",
> + .attrs = dma_buf_caps_attrs,
> +};
> diff --git a/drivers/dma-buf/dma-buf-sysfs-caps.h b/drivers/dma-
> buf/dma-buf-sysfs-caps.h
> new file mode 100644
> index 000000000000..e40a93513f06
> --- /dev/null
> +++ b/drivers/dma-buf/dma-buf-sysfs-caps.h
> @@ -0,0 +1,15 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * DMA-BUF sysfs capabilities.
> + *
> + * Copyright (C) 2022 Simon Ser
> + */
> +
> +#ifndef _DMA_BUF_SYSFS_CAPS_H
> +#define _DMA_BUF_SYSFS_CAPS_H
> +
> +struct attribute_group;
> +
> +extern const struct attribute_group dma_buf_caps_group;
> +
> +#endif // _DMA_BUF_SYSFS_CAPS_H
> diff --git a/drivers/dma-buf/dma-buf-sysfs-stats.c b/drivers/dma-
> buf/dma-buf-sysfs-stats.c
> index 2bba0babcb62..e2e62f83ce18 100644
> --- a/drivers/dma-buf/dma-buf-sysfs-stats.c
> +++ b/drivers/dma-buf/dma-buf-sysfs-stats.c
> @@ -141,23 +141,14 @@ static const struct kset_uevent_ops
> dmabuf_sysfs_no_uevent_ops = {
> .filter = dmabuf_sysfs_uevent_filter,
> };
>
> -static struct kset *dma_buf_stats_kset;
> static struct kset *dma_buf_per_buffer_stats_kset;
> -int dma_buf_init_sysfs_statistics(void)
> +int dma_buf_init_sysfs_statistics(struct kset *kset)
> {
> - dma_buf_stats_kset = kset_create_and_add("dmabuf",
> -
> &dmabuf_sysfs_no_uevent_ops,
> - kernel_kobj);
> - if (!dma_buf_stats_kset)
> - return -ENOMEM;
> -
> dma_buf_per_buffer_stats_kset =
> kset_create_and_add("buffers",
>
> &dmabuf_sysfs_no_uevent_ops,
> -
> &dma_buf_stats_kset->kobj);
> - if (!dma_buf_per_buffer_stats_kset) {
> - kset_unregister(dma_buf_stats_kset);
> + &kset-
> >kobj);
> + if (!dma_buf_per_buffer_stats_kset)
> return -ENOMEM;
> - }
>
> return 0;
> }
> @@ -165,7 +156,6 @@ int dma_buf_init_sysfs_statistics(void)
> void dma_buf_uninit_sysfs_statistics(void)
> {
> kset_unregister(dma_buf_per_buffer_stats_kset);
> - kset_unregister(dma_buf_stats_kset);
> }
>
> int dma_buf_stats_setup(struct dma_buf *dmabuf)
> diff --git a/drivers/dma-buf/dma-buf-sysfs-stats.h b/drivers/dma-
> buf/dma-buf-sysfs-stats.h
> index a49c6e2650cc..798c54fb8ee3 100644
> --- a/drivers/dma-buf/dma-buf-sysfs-stats.h
> +++ b/drivers/dma-buf/dma-buf-sysfs-stats.h
> @@ -8,9 +8,11 @@
> #ifndef _DMA_BUF_SYSFS_STATS_H
> #define _DMA_BUF_SYSFS_STATS_H
>
> +struct kset;
> +
> #ifdef CONFIG_DMABUF_SYSFS_STATS
>
> -int dma_buf_init_sysfs_statistics(void);
> +int dma_buf_init_sysfs_statistics(struct kset *kset);
> void dma_buf_uninit_sysfs_statistics(void);
>
> int dma_buf_stats_setup(struct dma_buf *dmabuf);
> @@ -18,7 +20,7 @@ int dma_buf_stats_setup(struct dma_buf *dmabuf);
> void dma_buf_stats_teardown(struct dma_buf *dmabuf);
> #else
>
> -static inline int dma_buf_init_sysfs_statistics(void)
> +static inline int dma_buf_init_sysfs_statistics(struct kset *kset)
> {
> return 0;
> }
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 5e1b0534b3ce..61c5be57558e 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -30,6 +30,7 @@
> #include <uapi/linux/dma-buf.h>
> #include <uapi/linux/magic.h>
>
> +#include "dma-buf-sysfs-caps.h"
> #include "dma-buf-sysfs-stats.h"
>
> static inline int is_dma_buf_file(struct file *);
> @@ -1546,22 +1547,57 @@ static inline void
> dma_buf_uninit_debugfs(void)
> }
> #endif
>
> +/* Capabilities and statistics files do not need to send uevents. */
> +static int dmabuf_sysfs_uevent_filter(struct kobject *kobj)
> +{
> + return 0;
> +}
> +
> +static const struct kset_uevent_ops dmabuf_sysfs_no_uevent_ops = {
> + .filter = dmabuf_sysfs_uevent_filter,
> +};
> +
> +static const struct attribute_group *dma_buf_sysfs_groups[] = {
> + &dma_buf_caps_group,
> + NULL,
> +};
> +
> +static struct kset *dma_buf_kset;
> +
> static int __init dma_buf_init(void)
> {
> int ret;
>
> - ret = dma_buf_init_sysfs_statistics();
> + dma_buf_kset = kset_create_and_add("dmabuf",
> +
> &dmabuf_sysfs_no_uevent_ops,
> + kernel_kobj);
> + if (!dma_buf_kset)
> + return -ENOMEM;
> +
> + ret = sysfs_create_groups(&dma_buf_kset->kobj,
> dma_buf_sysfs_groups);
> if (ret)
> - return ret;
> + goto err_kset;
> +
> + ret = dma_buf_init_sysfs_statistics(dma_buf_kset);
> + if (ret)
> + goto err_kset;
>
> dma_buf_mnt = kern_mount(&dma_buf_fs_type);
> - if (IS_ERR(dma_buf_mnt))
> - return PTR_ERR(dma_buf_mnt);
> + if (IS_ERR(dma_buf_mnt)) {
> + ret = PTR_ERR(dma_buf_mnt);
> + goto err_sysfs_stats;
> + }
>
> mutex_init(&db_list.lock);
> INIT_LIST_HEAD(&db_list.head);
> dma_buf_init_debugfs();
> return 0;
> +
> +err_sysfs_stats:
> + dma_buf_uninit_sysfs_statistics();
> +err_kset:
> + kset_unregister(dma_buf_kset);
> + return ret;
> }
> subsys_initcall(dma_buf_init);
>
> @@ -1570,5 +1606,6 @@ static void __exit dma_buf_deinit(void)
> dma_buf_uninit_debugfs();
> kern_unmount(dma_buf_mnt);
> dma_buf_uninit_sysfs_statistics();
> + kset_unregister(dma_buf_kset);
> }
> __exitcall(dma_buf_deinit);
> diff --git a/include/uapi/linux/dma-buf.h b/include/uapi/linux/dma-
> buf.h
> index 70e213a0d7d9..ab3afd5da75a 100644
> --- a/include/uapi/linux/dma-buf.h
> +++ b/include/uapi/linux/dma-buf.h
> @@ -114,6 +114,9 @@ struct dma_buf_sync {
> * ordering via these fences, it is the respnosibility of userspace
> to use
> * locks or other mechanisms to ensure that no other context adds
> fences or
> * submits work between steps 1 and 3 above.
> + *
> + * Userspace can check the availability of this API via
> + * /sys/kernel/dmabuf/caps/sync_file_import_export.
> */
> struct dma_buf_export_sync_file {
> /**
> @@ -146,6 +149,9 @@ struct dma_buf_export_sync_file {
> * synchronized APIs such as Vulkan to inter-op with dma-buf
> consumers
> * which expect implicit synchronization such as OpenGL or most
> media
> * drivers/video.
> + *
> + * Userspace can check the availability of this API via
> + * /sys/kernel/dmabuf/caps/sync_file_import_export.
> */
> struct dma_buf_import_sync_file {
> /**
