Re: [PATCH v3] drm/xen-front: Add support for Xen PV display frontend

[Daniel Vetter]

On Fri, Mar 23, 2018 at 05:54:49PM +0200, Oleksandr Andrushchenko wrote:
> 
> > My apologies, but I found a few more things that look strange and should
> > be cleaned up. Sorry for this iterative review approach, but I think we're
> > slowly getting there.
> Thank you for reviewing!
> > Cheers, Daniel
> > 
> > > ---
> > > +static int xen_drm_drv_dumb_create(struct drm_file *filp,
> > > +		struct drm_device *dev, struct drm_mode_create_dumb *args)
> > > +{
> > > +	struct xen_drm_front_drm_info *drm_info = dev->dev_private;
> > > +	struct drm_gem_object *obj;
> > > +	int ret;
> > > +
> > > +	ret = xen_drm_front_gem_dumb_create(filp, dev, args);
> > > +	if (ret)
> > > +		goto fail;
> > > +
> > > +	obj = drm_gem_object_lookup(filp, args->handle);
> > > +	if (!obj) {
> > > +		ret = -ENOENT;
> > > +		goto fail_destroy;
> > > +	}
> > > +
> > > +	drm_gem_object_unreference_unlocked(obj);
> > You can't drop the reference while you keep using the object, someone else
> > might sneak in and destroy your object. The unreference always must be
> > last.
> Will fix, thank you
> > > +
> > > +	/*
> > > +	 * In case of CONFIG_DRM_XEN_FRONTEND_CMA gem_obj is constructed
> > > +	 * via DRM CMA helpers and doesn't have ->pages allocated
> > > +	 * (xendrm_gem_get_pages will return NULL), but instead can provide
> > > +	 * sg table
> > > +	 */
> > > +	if (xen_drm_front_gem_get_pages(obj))
> > > +		ret = xen_drm_front_dbuf_create_from_pages(
> > > +				drm_info->front_info,
> > > +				xen_drm_front_dbuf_to_cookie(obj),
> > > +				args->width, args->height, args->bpp,
> > > +				args->size,
> > > +				xen_drm_front_gem_get_pages(obj));
> > > +	else
> > > +		ret = xen_drm_front_dbuf_create_from_sgt(
> > > +				drm_info->front_info,
> > > +				xen_drm_front_dbuf_to_cookie(obj),
> > > +				args->width, args->height, args->bpp,
> > > +				args->size,
> > > +				xen_drm_front_gem_get_sg_table(obj));
> > > +	if (ret)
> > > +		goto fail_destroy;
> > > +
> > The above also has another race: If you construct an object, then it must
> > be fully constructed by the time you publish it to the wider world. In gem
> > this is done by calling drm_gem_handle_create() - after that userspace can
> > get at your object and do nasty things with it in a separate thread,
> > forcing your driver to Oops if the object isn't fully constructed yet.
> > 
> > That means you need to redo this code here to make sure that the gem
> > object is fully set up (including pages and sg tables) _before_ anything
> > calls drm_gem_handle_create().
> You are correct, I need to rework this code
> > 
> > This probably means you also need to open-code the cma side, by first
> > calling drm_gem_cma_create(), then doing any additional setup, and finally
> > doing the registration to userspace with drm_gem_handle_create as the very
> > last thing.
> Although I tend to avoid open-coding, but this seems the necessary measure
> here
> > 
> > Alternativet is to do the pages/sg setup only when you create an fb (and
> > drop the pages again when the fb is destroyed), but that requires some
> > refcounting/locking in the driver.
> Not sure this will work: nothing prevents you from attaching multiple FBs to
> a single dumb handle
> So, not only ref-counting should be done here, but I also need to check if
> the dumb buffer,
> we are attaching to, has been created already

No, you must make sure that no dumb buffer can be seen by anyone else
before it's fully created. If you don't register it in the file_priv idr
using drm_gem_handle_create, no one else can get at your buffer. Trying to
paper over this race from all the other places breaks the gem core code
design, and is also much more fragile.

> So, I will rework with open-coding some stuff from CMA helpers
> 
> > 
> > Aside: There's still a lot of indirection and jumping around which makes
> > the code a bit hard to follow.
> Probably I am not sure of which indirection we are talking about, could you
> please
> specifically mark those annoying you?

I think it's the same indirection we talked about last time, it still
annoys me. But it's still ok if you prefer this way I think :-)

> 
> > 
> > > +
> > > +static void xen_drm_drv_release(struct drm_device *dev)
> > > +{
> > > +	struct xen_drm_front_drm_info *drm_info = dev->dev_private;
> > > +	struct xen_drm_front_info *front_info = drm_info->front_info;
> > > +
> > > +	drm_atomic_helper_shutdown(dev);
> > > +	drm_mode_config_cleanup(dev);
> > > +
> > > +	xen_drm_front_evtchnl_free_all(front_info);
> > > +	dbuf_free_all(&front_info->dbuf_list);
> > > +
> > > +	drm_dev_fini(dev);
> > > +	kfree(dev);
> > > +
> > > +	/*
> > > +	 * Free now, as this release could be not due to rmmod, but
> > > +	 * due to the backend disconnect, making drm_info hang in
> > > +	 * memory until rmmod
> > > +	 */
> > > +	devm_kfree(&front_info->xb_dev->dev, front_info->drm_info);
> > > +	front_info->drm_info = NULL;
> > > +
> > > +	/* Tell the backend we are ready to (re)initialize */
> > > +	xenbus_switch_state(front_info->xb_dev, XenbusStateInitialising);
> > This needs to be in the unplug code. Yes that means you'll have multiple
> > drm_devices floating around, but that's how hotplug works. That would also
> > mean that you need to drop the front_info pointer from the backend at
> > unplug time.
> > 
> > If you don't like those semantics then the only other option is to never
> > destroy the drm_device, but only mark the drm_connector as disconnected
> > when the xenbus backend is gone. But this half-half solution here where
> > you hotunplug the drm_device but want to keep it around still doesn't work
> > from a livetime pov.
> I'll try to play with this:
> 
> on backend disconnect I will do the following:
>     drm_dev_unplug(dev)
>     xen_drm_front_evtchnl_free_all(front_info);
>     dbuf_free_all(&front_info->dbuf_list);
>     devm_kfree(&front_info->xb_dev->dev, front_info->drm_info);
>     front_info->drm_info = NULL;
>     xenbus_switch_state(front_info->xb_dev, XenbusStateInitialising);
> 
> on drm_driver.release callback:
> 
>     drm_atomic_helper_shutdown(dev);
>     drm_mode_config_cleanup(dev);
> 
>     drm_dev_fini(dev);
>     kfree(dev);
> 
> Does the above make sense?

I think so, yes. One nit: Since you need to call devm_kfree either pick a
different struct device that has the correct lifetime, or switch to the
normal kmalloc/kfree versions.
> 
> > > +static struct xenbus_driver xen_driver = {
> > > +	.ids = xen_driver_ids,
> > > +	.probe = xen_drv_probe,
> > > +	.remove = xen_drv_remove,
> > I still don't understand why you have both the remove and fini versions of
> > this. See other comments, I think the xenbus vs. drm_device lifetime stuff
> > still needs to be cleaned up some more. This shouldn't be that hard
> > really.
> > 
> > Or maybe I'm just totally misunderstanding this frontend vs. backend split
> > in xen, so if you have a nice gentle intro text for why that exists, it
> > might help.
> Probably misunderstanding comes from the fact that it is possible if backend
> dies it may still have its XenBus state set to connected, thus
> displback_disconnect callback will never be called. For that reason on rmmod
> I call fini for the DRM driver to destroy it.
> 
> > > +	/*
> > > +	 * pflip_timeout is set to current jiffies once we send a page flip and
> > > +	 * reset to 0 when we receive frame done event from the backed.
> > > +	 * It is checked during drm_connector_helper_funcs.detect_ctx to detect
> > > +	 * time-outs for frame done event, e.g. due to backend errors.
> > > +	 *
> > > +	 * This must be protected with front_info->io_lock, so races between
> > > +	 * interrupt handler and rest of the code are properly handled.
> > > +	 */
> > > +	unsigned long pflip_timeout;
> > > +
> > > +	bool conn_connected;
> > I'm pretty sure this doesn't work. Especially the check in display_check
> > confuses me, if there's ever an error then you'll never ever be able to
> > display anything again, except when someone disables the display.
> That was the idea to allow dummy user-space to get an error in
> display_check and close, going through display_disable.
> Yes, compositors will die in this case.
> 
> > If you want to signal errors with the output then this must be done
> > through the new link-status property and
> > drm_mode_connector_set_link_status_property. Rejecting kms updates in
> > display_check with -EINVAL because the hw has a temporary issue is kinda
> > not cool (because many compositors just die when this happens). I thought
> > we agreed already to remove that, sorry for not spotting that in the
> > previous version.
> Unfortunatelly, there is little software available which will benefit
> from this out of the box. I am specifically interested in embedded
> use-cases, e.g. Android (DRM HWC2 - doesn't support hotplug, HWC1.4 doesn't
> support link status), Weston (no device hotplug, but connectors and
> outputs).
> Other software, like kmscube, modetest will not handle that as well.
> So, such software will hang forever until killed.

Then you need to fix your userspace. You can't invent new uapi which will
break existing compositors like this. Also I thought you've fixed the
"hangs forever" by sending out the uevent in case the backend disappears
or has an error. That's definitely something that should be fixed, current
userspace doesn't expect that events never get delivered.

> > Some of the conn_connected checks also look a bit like they should be
> > replaced by drm_dev_is_unplugged instead, but I'm not sure.
> I believe you are talking about drm_simple_display_pipe_funcs?
> Do you mean I have to put drm_dev_is_unplugged in display_enable,
> display_disable and display_update callbacks?

Yes. Well, as soon as Noralf's work has landed they'll switch to a
drm_dev_enter/exit pair, but same idea.

> > > +static int connector_detect(struct drm_connector *connector,
> > > +		struct drm_modeset_acquire_ctx *ctx,
> > > +		bool force)
> > > +{
> > > +	struct xen_drm_front_drm_pipeline *pipeline =
> > > +			to_xen_drm_pipeline(connector);
> > > +	struct xen_drm_front_info *front_info = pipeline->drm_info->front_info;
> > > +	unsigned long flags;
> > > +
> > > +	/* check if there is a frame done event time-out */
> > > +	spin_lock_irqsave(&front_info->io_lock, flags);
> > > +	if (pipeline->pflip_timeout &&
> > > +			time_after_eq(jiffies, pipeline->pflip_timeout)) {
> > > +		DRM_ERROR("Frame done event timed-out\n");
> > > +
> > > +		pipeline->pflip_timeout = 0;
> > > +		pipeline->conn_connected = false;
> > > +		xen_drm_front_kms_send_pending_event(pipeline);
> > > +	}
> > > +	spin_unlock_irqrestore(&front_info->io_lock, flags);
> > If you want to check for timeouts please use a worker, don't piggy-pack on
> > top of the detect callback.
> Ok, will have a dedicated work for that. The reasons why I put this into the
> detect callback were:
> - the periodic worker is already there, and I do nothing heavy
>   in this callback
> - if frame done has timed out it most probably means that
>   backend has gone, so 10 sec period of detect timeout is also ok: thus I
> don't
>   need to schedule a work each page flip which could be a bit costly
> So, probably I will also need a periodic work (or kthread/timer) for frame
> done time-outs

Yes, please create your own timer/worker for this, stuffing random other
things into existing workers makes the locking hierarchy more complicated
for everyone. And it's confusing for core devs trying to understand what
your driver does :-)

Most drivers have piles of timers/workers doing various stuff, they're
real cheap.

> > > +static int connector_mode_valid(struct drm_connector *connector,
> > > +		struct drm_display_mode *mode)
> > > +{
> > > +	struct xen_drm_front_drm_pipeline *pipeline =
> > > +			to_xen_drm_pipeline(connector);
> > > +
> > > +	if (mode->hdisplay != pipeline->width)
> > > +		return MODE_ERROR;
> > > +
> > > +	if (mode->vdisplay != pipeline->height)
> > > +		return MODE_ERROR;
> > > +
> > > +	return MODE_OK;
> > > +}
> > mode_valid on the connector only checks probe modes. Since that is
> > hardcoded this doesn't do much, which means userspace can give you a wrong
> > mode, and you fall over.
> Agree, I will remove this callback completely: I have
> drm_connector_funcs.fill_modes == drm_helper_probe_single_connector_modes,
> so it will only pick my single hardcoded mode from
> drm_connector_helper_funcs.get_modes
> callback (connector_get_modes).

No, you still need your mode_valid check. Userspace can ignore your mode
list and give you something totally different. But it needs to be moved to
the drm_simple_display_pipe_funcs vtable.

> > You need to use one of the other mode_valid callbacks instead,
> > drm_simple_display_pipe_funcs has the one you should use.
> > 
> Not sure I understand why do I need to provide a callback here?
> For simple KMS the drm_simple_kms_crtc_mode_valid callback is used,
> which always returns MODE_OK if there is no .mode_valid set for the pipe.
> As per my understanding drm_simple_kms_crtc_mode_valid is only called for
> modes, which were collected by drm_helper_probe_single_connector_modes,
> so I assume each time .validate_mode is called it can only have my hardcoded
> mode to validate?

Please read the kerneldoc again, userspace can give you modes that are not
coming from drm_helper_probe_single_connector_modes. If the kerneldoc
isn't clear, then please submit a patch to make it clearer.

> > > +
> > > +static int display_check(struct drm_simple_display_pipe *pipe,
> > > +		struct drm_plane_state *plane_state,
> > > +		struct drm_crtc_state *crtc_state)
> > > +{
> > > +	struct xen_drm_front_drm_pipeline *pipeline =
> > > +			to_xen_drm_pipeline(pipe);
> > > +
> > > +	return pipeline->conn_connected ? 0 : -EINVAL;
> > As mentioned, this -EINVAL here needs to go. Since you already have a
> > mode_valid callback you can (should) drop this one here entirely.
> Not sure how mode_valid is relevant to this code [1]: This function is
> called
> in the check phase of an atomic update, specifically when the underlying
> plane is checked. But, anyways: the reason for this callback and it
> returning
> -EINVAL is primarialy for a dumb user-space which cannot handle hotplug
> events.

Fix your userspace. Again, you can't invent new uapi like this which ends
up being inconsistent with other existing userspace.

> But, as you mentioned before, it will make most compositors die, so I will
> remove this

Yup, sounds good.

Cheers, Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch