Re: [PATCH v3] dma-buf: Add a capabilities directory

["Christian König" <christian.koenig@amd.com>]

Am 27.05.22 um 09:34 schrieb Simon Ser:
> 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.

I find a separate directory rather unusual, but can't come up with any 
better idea either.

IIRC the security module had a mask file with names for the supported 
capabilities.

>
> 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
>
> Signed-off-by: Simon Ser <contact@emersion.fr>
> Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Greg KH <greg@kroah.com>
> ---
>   .../ABI/testing/sysfs-kernel-dmabuf-caps      | 13 +++++
>   MAINTAINERS                                   |  1 +
>   drivers/dma-buf/Makefile                      |  2 +-
>   drivers/dma-buf/dma-buf-sysfs-caps.c          | 51 +++++++++++++++++++
>   drivers/dma-buf/dma-buf-sysfs-caps.h          | 16 ++++++
>   drivers/dma-buf/dma-buf-sysfs-stats.c         | 16 ++----
>   drivers/dma-buf/dma-buf-sysfs-stats.h         |  6 ++-
>   drivers/dma-buf/dma-buf.c                     | 43 ++++++++++++++--
>   include/uapi/linux/dma-buf.h                  |  6 +++
>   9 files changed, 134 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@emersion.fr>
> +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@emersion.fr>
> +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@lists.freedesktop.org
>   L:	linaro-mm-sig@lists.linaro.org (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..82b91eb874a9
> --- /dev/null
> +++ b/drivers/dma-buf/dma-buf-sysfs-caps.c
> @@ -0,0 +1,51 @@
> +// 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,
> +};
> +
> +static const struct attribute_group dma_buf_caps_attr_group = {
> +	.attrs = dma_buf_caps_attrs,
> +};

Didn't we had macros for those? I think I have seen something for that.

Apart from those two random thoughts looks good to me as well.

Regards,
Christian.

> +
> +static struct kobject *dma_buf_caps_kobj;
> +
> +int dma_buf_init_sysfs_capabilities(struct kset *kset)
> +{
> +	int ret;
> +
> +	dma_buf_caps_kobj = kobject_create_and_add("caps", &kset->kobj);
> +	if (!dma_buf_caps_kobj)
> +		return -ENOMEM;
> +
> +	ret = sysfs_create_group(dma_buf_caps_kobj, &dma_buf_caps_attr_group);
> +	if (ret)
> +		kobject_put(dma_buf_caps_kobj);
> +	return ret;
> +}
> +
> +void dma_buf_uninit_sysfs_capabilities(void)
> +{
> +	kobject_put(dma_buf_caps_kobj);
> +}
> 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..d7bcef490b31
> --- /dev/null
> +++ b/drivers/dma-buf/dma-buf-sysfs-caps.h
> @@ -0,0 +1,16 @@
> +/* 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 kset;
> +
> +int dma_buf_init_sysfs_capabilities(struct kset *kset);
> +void dma_buf_uninit_sysfs_capabilities(void);
> +
> +#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..b5c5a5050508 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 +1547,54 @@ 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 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 = dma_buf_init_sysfs_capabilities(dma_buf_kset);
>   	if (ret)
> -		return ret;
> +		goto err_kset;
> +
> +	ret = dma_buf_init_sysfs_statistics(dma_buf_kset);
> +	if (ret)
> +		goto err_sysfs_caps;
>   
>   	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_sysfs_caps:
> +	dma_buf_uninit_sysfs_capabilities();
> +err_kset:
> +	kset_unregister(dma_buf_kset);
> +	return ret;
>   }
>   subsys_initcall(dma_buf_init);
>   
> @@ -1570,5 +1603,7 @@ static void __exit dma_buf_deinit(void)
>   	dma_buf_uninit_debugfs();
>   	kern_unmount(dma_buf_mnt);
>   	dma_buf_uninit_sysfs_statistics();
> +	dma_buf_uninit_sysfs_capabilities();
> +	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 {
>   	/**