Re: [PATCH RESEND v2 1/2] drm/xen-front: Add support for Xen PV display frontend

[Oleksandr Andrushchenko <andr2000@gmail.com>]

On 03/20/2018 03:47 PM, Daniel Vetter wrote:
> On Tue, Mar 20, 2018 at 01:58:01PM +0200, Oleksandr Andrushchenko wrote:
>> On 03/19/2018 05:28 PM, Daniel Vetter wrote:
>>> There should be no difference between immediate removal and delayed
>>> removal of the drm_device from the xenbus pov. The lifetimes of the
>>> front-end (drm_device) and backend (the xen bus thing) are entirely
>>> decoupled:
>> Well, they are not decoupled for simplicity of handling,
>> please see below
>>> So for case 2 you only have 1 case:
>>>
>>> - drm_dev_unplug
>>> - tear down the entire xenbus backend completely
>>> - all xenbus access will be caught with drm_dev_entre/exit (well right
>>> now drm_dev_is_unplugged) checks, including any access to your private
>>> drm_device data
>>> - once drm_device->open_count == 0 the core will tear down the
>>> drm_device instance and call your optional drm_driver->release
>>> callback.
>>>
>>> So past drm_dev_unplug the drm_device is in zombie state and the only
>>> thing that will happen is a) it rejects all ioctls and anything else
>>> userspace might ask it to do and b) gets releases once the last
>>> userspace reference is gone.
>> I have re-worked the driver with this in mind [1]
>> So, I now use drm_dev_unplug and destroy the DRM device
>> on drm_driver.release.
>> In context of unplug work I also merged xen_drm_front_drv.c and
>> xen_drm_front.c as these are too coupled together now.
>>
>> Could you please take a look and tell me if this is what you mean?
>>> If the backend comes up again, you create a _new_ drm_device instance
>>> (while the other one is still in the process of eventually getting
>>> released).
>> We only have a single xenbus instance, so this way I'll need
>> to handle list of such zombies. For that reason I prefer to
>> wait until the DRM device is destroyed, telling the backend
>> to hold on until then (via going into XenbusStateReconfiguring state).
> Why exactly do you need to keep track of your drm_devices from the xenbus?
> Once unplugged, there should be no connection with the "hw" for your
> device, in neither direction. Maybe I need to look again, but this still
> smells funny and not like something you should ever do.
Ok, probably new reworked code will make things cleaner and answer
your concerns. I also removed some obsolete stuff, e.g. platform device,
so this path became even cleaner now ;)
>> Another drawback of such approach is that I'll have different
>> minors at run-time, e.g. card0, card1, etc.
>> For software which has /dev/dri/card0 hardcoded it may be a problem.
>> But this is minor, IMO
> Fix userspace :-)
>
> But yeah unlikely this is a problem, hotplugging is fairly old thing.
>
>>> In short, your driver code should never have a need to look at
>>> drm_device->open_count. I hope this explains it a bit better.
>>> -Daniel
>>>
>> Yes, you are correct: at [1] I am not touching drm_device->open_count
>> anymore and everything just happens synchronously
>> [1] https://github.com/andr2000/linux/commits/drm_tip_pv_drm_v3
> Please just resend, makes it easier to comment inline.
I need to wait for Xen community reviewers before resending, so this
is why I hoped you can take a look before that, so I have a chance to
address more of your comments in v4
> -Daniel
Thank you,
Oleksandr