[RFC 0/4] Introduce drmfs pseudo filesystem for drm subsystem

[Laurent Pinchart]

Hi Swati,

On Monday 19 Dec 2016 16:12:22 swati.dhingra at intel.com wrote:
> From: Swati Dhingra <swati.dhingra at intel.com>
> 
> Currently, we don't have a stable ABI which can be used for the purpose of
> providing output debug/loggging/crc and other such data from DRM.
> The ABI in current use (filesystems, ioctls, et al.) have their own
> constraints and are intended to output a particular type of data.
> Few cases in point:
> sysfs	- stable ABI, but constrained to one textual value per file
> debugfs	- unstable ABI, free-for-all
> ioctls	- not as suitable to many single purpose continuous data
> 	  dumping, we would very quickly run out ioctl space; requires more
> 	  userspace support than "cat"
> device nodes -  a real possibilty, kernel instantiation is more tricky,
> 		requires udev (+udev.rules) or userspace discovery of the
> 		dynamic major:minor (via sysfs) [mounting a registered
> 		filesystem is easy in comparison]
> netlink	- stream based, therefore involves numerous copies.
> 
> Debugfs is the lesser among the evils here, thereby we have grown used to
> the convenience and flexibility in presentation that debugfs gives us
> (including relayfs inodes) that we want some of that hierachy in stable user
> ABI form.

Seriously, why ? A subsystem growing its own file system sounds so wrong. It 
seems that you want to have all the benefits of a stable ABI without going 
through the standardization effort that this requires. I can see so many ways 
that drmfs could be abused, with drivers throwing in new data with little or 
no review. You'll need very compelling arguments to convince me.

> Due to these limitations, there is a need for a new pseudo filesytem, that
> would act as a stable 'free-for-all' ABI, with the heirarchial structure and
> thus convenience of debugfs. This will be managed by drm, thus named
> 'drmfs'. DRM would register this filesystem to manage a canonical
> mountpoint, but this wouldn't limit everyone to only using that pseudofs
> underneath.
> 
> This can serve to hold various kinds of output data from Linux DRM
> subsystems, for the files which can't truely fit anywhere else with
> existing ABI's but present so, for the lack of a better place.
> 
> In this patch series, we have introduced a pseudo filesystem named as
> 'drmfs' for now. The filesystem is introduced in the first patch, and the
> subsequent patches make use of the filesystem interfaces, in drm driver,
> and making them available for use by the drm subsystem components, one of
> which is i915. We've moved the location of i915 GuC logs from debugfs to
> drmfs in the last patch. Subsequently, more such files such as pipe_crc,
> error states, memory stats, etc. can be move to this filesystem, if the
> idea introduced here is acceptable per se. The filesystem introduced is
> being used to house the data generated by i915 driver in this patch series,
> but will hopefully be generic enough to provide scope for usage by any
> other drm subsystem component.
> 
> The patch series is being floated as RFC to gather feedback on the idea and
> infrastructure proposed here and it's suitability to address the specific
> problem statement/use case.
> 
> v2: fix the bat failures caused due to missing config check
> 
> v3: Changes made:
>     - Move the location of drmfs from fs/ to drivers/gpu/drm/ (Chris)
>     - Moving config checks to header (Chris,Daniel)
> 
> v4: Added the kernel Documentaion (using Sphinx).
> 
> Sourab Gupta (4):
>   drm: Introduce drmfs pseudo filesystem interfaces
>   drm: Register drmfs filesystem from drm init
>   drm: Create driver specific root directory inside drmfs
>   drm/i915: Creating guc log file in drmfs instead of debugfs
> 
>  Documentation/gpu/drm-uapi.rst             |  76 ++++
>  drivers/gpu/drm/Kconfig                    |   9 +
>  drivers/gpu/drm/Makefile                   |   1 +
>  drivers/gpu/drm/drm_drv.c                  |  26 ++
>  drivers/gpu/drm/drmfs.c                    | 566 ++++++++++++++++++++++++++
>  drivers/gpu/drm/i915/i915_guc_submission.c |  33 +-
>  include/drm/drm_drv.h                      |   3 +
>  include/drm/drmfs.h                        |  77 ++++
>  include/uapi/linux/magic.h                 |   3 +
>  9 files changed, 773 insertions(+), 21 deletions(-)
>  create mode 100644 drivers/gpu/drm/drmfs.c
>  create mode 100644 include/drm/drmfs.h

-- 
Regards,

Laurent Pinchart