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

[Alex Deucher]

On Mon, Dec 12, 2016 at 1:14 AM, sourab gupta <sourab.gupta at intel.com> wrote:
> On Mon, 2016-12-05 at 03:06 -0800, Dhingra, Swati 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.
>>
>> 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.
>>
>> TODO: Create documentation. Will do so in next version.
>>
>> 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)
>>
>>
>> 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
>>
>>  drivers/gpu/drm/Kconfig                    |   9 +
>>  drivers/gpu/drm/Makefile                   |   1 +
>>  drivers/gpu/drm/drm_drv.c                  |  12 +
>>  drivers/gpu/drm/drmfs.c                    | 555 +++++++++++++++++++++++++++++
>>  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 +
>>  8 files changed, 672 insertions(+), 21 deletions(-)
>>  create mode 100644 drivers/gpu/drm/drmfs.c
>>  create mode 100644 include/drm/drmfs.h
>>
>> --
>> 2.7.4
>>
>
> Hi dri-devel folks,
>
> Any feedback on the proposed drmfs infrastructure being proposed here?

I haven't looked too closely, but so far we haven't run into anything
that we haven't been able to support via some combination of sysfs and
debugfs.

Alex