Date:   Fri, 12 Jul 2019 00:07:57 -0300
From:   Rodrigo Siqueira <rodrigosiqueiramelo@...il.com>
To:     Haneen Mohammed <hamohammed.sa@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Simon Ser <contact@...rsion.fr>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] drm/vkms: Introduce basic support for configfs

On 07/10, Daniel Vetter wrote:
> On Mon, Jul 01, 2019 at 12:23:39AM -0300, Rodrigo Siqueira wrote:
> > This patchset introduces the support for configfs in vkms by adding a
> > primary structure for handling the vkms subsystem and exposing
> > connectors as a use case.  This series allows enabling/disabling virtual
> > and writeback connectors on the fly. The first patch of this series
> > reworks the initialization and cleanup code of each type of connector,
> > with this change, the second patch adds the configfs support for vkms.
> > It is important to highlight that this patchset depends on
> > https://patchwork.freedesktop.org/series/61738/.
> > 
> > After applying this series, the user can utilize these features with the
> > following steps:
> > 
> > 1. Load vkms without parameter
> > 
> >   modprobe vkms
> > 
> > 2. Mount a configfs filesystem
> > 
> >   mount -t configfs none /mnt/
> > 
> > After that, the vkms subsystem will look like this:
> > 
> > vkms/
> >  |__connectors
> >     |__Virtual
> >         |__ enable
> > 
> > The connectors directories have information related to connectors, and
> > as can be seen, the virtual connector is enabled by default. Inside a
> > connector directory (e.g., Virtual) has an attribute named ‘enable’
> > which is used to enable and disable the target connector. For example,
> > the Virtual connector has the enable attribute set to 1. If the user
> > wants to enable the writeback connector it is required to use the mkdir
> > command, as follows:
> > 
> >   cd /mnt/vkms/connectors
> >   mkdir Writeback
> > 
> > After the above command, the writeback connector will be enabled, and
> > the user could see the following tree:
> > 
> > vkms/
> >  |__connectors
> >     |__Virtual
> >     |   |__ enable
> >     |__Writeback
> >         |__ enable
> > 
> > If the user wants to remove the writeback connector, it is required to
> > use the command rmdir, for example
> > 
> >   rmdir Writeback
> > 
> > Another way to enable and disable a connector it is by using the enable
> > attribute, for example, we can disable the Virtual connector with:
> > 
> >   echo 0 > /mnt/vkms/connectors/Virtual/enable
> > 
> > And enable it again with:
> > 
> >   echo 1 > /mnt/vkms/connectors/Virtual/enable
> > 
> > It is important to highlight that configfs 'obey' the parameters used
> > during the vkms load and does not allow users to remove a connector
> > directory if it was load via module parameter. For example:
> > 
> >   modprobe vkms enable_writeback=1
> > 
> > vkms/
> >  |__connectors
> >     |__Virtual
> >     |   |__ enable
> >     |__Writeback
> >         |__ enable
> > 
> > If the user tries to remove the Writeback connector with “rmdir
> > Writeback”, the operation will be not permitted because the Writeback
> > connector was loaded with the modules. However, the user may disable the
> > writeback connector with:
> > 
> >   echo 0 > /mnt/vkms/connectors/Writeback/enable

Thanks for detail this issue, I just have some few questions inline.
 
> I guess I should have put a warning into that task that step one is
> designing the interface. Here's the fundamental thoughts:
> 
> - The _only_ thing we can hotplug after drm_dev_register() is a
>   drm_connector. That's an interesting use-case, but atm not really
>   supported by the vkms codebase. So we can't just enable/disable
>   writeback like this. We also can't change _anything_ else in the drm
>   driver like this.

In the first patch of this series, I tried to decouple enable/disable
for virtual and writeback connectors; I tried to take advantage of
drm_connector_[register/unregister] in each connector. Can we use the
first patch or it doesn't make sense?

I did not understand why writeback connectors should not be registered
and unregister by calling drm_connector_[register/unregister], is it a
writeback or vkms limitation? Could you detail why we cannot change
connectors as I did?

Additionally, below you said "enable going from 1 -> 0, needs to be
treated like a physical hotunplug", do you mean that we first have to
add support for drm_dev_plug and drm_dev_unplug in vkms?
 
> - The other bit we want is support multiple vkms instances, to simulate
>   multi-gpus and fun stuff like that.

Do you mean something like this:

configfs/vkms/instance1
|_enable_device 
|_more_stuff
configfs/vkms/instance2
|_enable_device
|_more_stuff
configfs/vkms/instanceN
|_enable_device
|_more_stuff

Will each instance work like a totally independent device? What is the
main benefit of this? I can think about some use case for testing
prime, but I cannot see other things.
 
> - Therefore vkms configs should be at the drm_device level, so a
>   directory under configfs/vkms/ represents an entire device.
> 
> - We need a special config item to control
>   drm_dev_register/drm_dev_unregister. While a drm_device is registers,
>   all other config items need to fail if userspace tries to change them.
>   Maybe this should be a top-level "enable" property.
> 
> - Every time enable goes from 0 -> 1 we need to create a completely new
>   vkms instance. The old one might still be around, waiting for the last
>   few references to disappear.
> 
> - enable going from 1 -> 0 needs to be treated like a physical hotunplug,
>   i.e. not drm_dev_unregister but drm_dev_unplug. We also need to annotate
>   all the vkms code with drm_dev_enter/exit() as the kerneldoc of
>   drm_dev_unplug explains.
> 
> - rmdir should be treated like enable going from 1 -> 0. Or maybe we
>   should disable enable every going from 1 -> 0, would propably simplify
>   everything.
> 
> - The initial instance created at module load also neeeds to be removable
>   like this.
> 
> Once we have all this, then can we start to convert driver module options
> over to configs and add cool features. But lots of infrastructure needed
> first.
> 
> Also, we probably want some nasty testcases which races an rmdir in
> configfs against userspace still doing ioctl calls against vkms. This is
> ideal for validation the hotunplug infrastructure we have in drm.
> 
> An alternative is adding connector hotplugging. But I think before we do
> that we need to have support for more crtc and more connectors as static
> module options. So maybe not a good starting point for configfs.

So, probably the first set of tasks should be:

1. Enable multiple CRTC via module parameters. For example:
  modprobe vkms crtcs=13
2. Enable multiple connectors via module parameters. For example:
  modprobe vkms crtcs=3 connector=3 // 3 virtual connectors per crtc

Thanks again,
 
> The above text should probably be added to the vkms.rst todo item ...
> -Daniel
> 
> > 
> > 
> > Rodrigo Siqueira (2):
> >   drm/vkms: Add enable/disable functions per connector
> >   drm/vkms: Introduce configfs for enabling/disabling connectors
> > 
> >  drivers/gpu/drm/vkms/Makefile         |   3 +-
> >  drivers/gpu/drm/vkms/vkms_configfs.c  | 229 ++++++++++++++++++++++++++
> >  drivers/gpu/drm/vkms/vkms_drv.c       |   6 +
> >  drivers/gpu/drm/vkms/vkms_drv.h       |  17 ++
> >  drivers/gpu/drm/vkms/vkms_output.c    |  84 ++++++----
> >  drivers/gpu/drm/vkms/vkms_writeback.c |  31 +++-
> >  6 files changed, 332 insertions(+), 38 deletions(-)
> >  create mode 100644 drivers/gpu/drm/vkms/vkms_configfs.c
> > 
> > -- 
> > 2.21.0
> > 
> > 
> > -- 
> > Rodrigo Siqueira
> > https://siqueira.tech
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Rodrigo Siqueira
https://siqueira.tech

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