[Piglit] [PATCH 1/2] egl: Add sanity test for EGL_EXT_device_query (v3)

Mathias Fröhlich Mathias.Froehlich at gmx.net
Mon Sep 5 07:48:50 UTC 2016




On Friday, 2 September 2016 14:02:07 CEST Emil Velikov wrote:

> On 2 September 2016 at 07:15, Mathias Fr�hlich

> <Mathias.Froehlich at gmx.net> wrote:


> >

> > Great!

> >

> > One question that I cannot forsee from your branch:

> >

> > The EGL_EXT_device_enumeration spec says

> >

> >

> >

> > [...] All implementations must support

> >

> > at least one device.

> >

> > [...]

> >

> >

> >

> > Which means to me that once an application sucsessfully asked for

> > EGL_EXT_device_query, this calling application can rely on recieving at

> > least one usable(?) EGL device. As a last resort, that single guaranteed

> > device can be a software renderer, but the application gets at least

> > something that lets it render pictures in some way.

> >

> Yes we do need at least one device, which (modulo a few small changes)

> is applicable with the above branch. There is no need for the single

> guaranteed device to be software renderer.

Well, how are you getting this single (drm) device when you are on a board with a pure framebuffer console on some simple VGA hardware just sufficient to bring up the boot screen?


This situation is very common on some sort of modern systems. See below.


> > Sure, the intent of the extension is to privide access to hw backed

> > implementations.

> >

> Fully agree.


> >

> >

> > For us it means that we need to provide a software rendering context for the

> > case that there is either no drm capable graphics driver.

> I'm missing something here - barring the vendor neutrial EGL

> requirement for EGL_EXT_device_base how is the presence or absence of

> the device extensions going to affect any of your work.

> Afaict all of them are simply not applicable in the software renderer case.

Now I am confused, what do you mean with 'your (my) work'?


What I mean here - putting together what I read in the branch:

On compile time of mesa, libdrm is there and usable, so lib EGL announces EGL_EXT_device_enumeration so eglQueryDevicesEXT shall be there and return at least one device. Now put that mesa libraries onto a fresh installed cluster system (just by installing a linux distribution that contains the mentioned precompield mesa package). That cluster node I mean has nothing drm capable as its head never faces a console user appart from the operator seeing the boot screen at most once, if not even that is automated away with a kickstart install via network.

How are you going to handle this situation?


Of course a typical installation out there has selected nodes installed with a/several GPU(s) each. This gpu is supposed to be used for producing visualization results of your simulation. No monitors attached, just to reduce the usually huge amount of simulation data (up to several terrabytes or even more) to something that you can actually download to your computer which is several thousand pictures (well more similar use cases but all share the property that you do not want to copy the simulatoin data but you can copy picture data in some sense). Sure on this node you expect EGL_EXT_device_enumeration to deliver a gpu and I would hope that we (mesa/oss graphics stack) also want to deliver EGL_EXT_platform_device where you can make use of that single EGLDevice to grab an EGLDisplay.


In reality today, such cluster nodes are equipped exclusively with nvidia cards and the binary blob. VirtualGL is installed running a totally open X server that delivers the application local gl contexts via the binary blob through virtualgl. So having EGL_EXT_platform_device together with EGL_EXT_device_query is what those software vendors that have understood the security implications with VirtualGL will use in the future. Those vendors who do not (want to) think about security implications will probably continue to use the above virtualgl setup as this does not require any invest for changes in their software.


> > Or an even more

> > nasty case, when the device node is just not accessible by the user. I have

> > seen distros that restrict the permissions of the render node devices to the

> > user loged in the running X server. So, even if there is hardware that you

> > could potentially use, you may not be able to access it.

> >

> The libdrm helper provides a list of devices which have at least one

> node available - be that card, control or render. For the purposes of

> EGL_EXT_device_drm we could consider the card or render, although the

> card one is exposed in pretty much all the open-source drivers and is

> independent of the kernel age.


> That said if distributions restricts permissions to all of those then

> ... I'm inclined to go with Distribution/User Error. Then again please

> poke us if you see such cases.

Fedora 24 that I use to write this mail on is such an example. And yes this is kind of a different topic, but one that we (mesa) has to cope with I think. And yes I know where redhats bug tracker is.


> > ... remember, the major intent of this set of extensions is to provide

> > applications with an off screen rendering context for the case where you do

> > not have a local X/wayland/whatnotdisplayserver running.

> >

> Note: it's not display server but platform ;-) One could use

> EGL_KHR_platform_gbm if their graphics card vendor implements the

> extension.

Ok, I call it platform then.


> > Alternatively to providing a cpu rasterizer as a fallback, we could supress

> > announcing the EXT_device_enumeration extension if there is no hw backed

> > driver available. And in turn EGL_EXT_device_base which depends on

> > EGL_EXT_device_query.

> >

> As above - modulo a few small changes this is what the current branch does.


> > That would at least require some infrastructure to dynamically enable client

> > extensions. It would still be unclear what to do then when the render nodes

> > seem accessible when initializing/enumerating but the permissions change

> > until the application wants to create the display using

> > eglGetPlatformDisplayEXT later.

> >

> I'm not currently sold whether the card or render node should be

> exposed via the EGL_EXT_device_drm extension. In the case of the

> latter we could easily not advertise EGL_EXT_device_base if there's no

> such devices available.

> That will be as a workaround the eglQueryDeviceStringEXT should return

> EGL_BAD_DEVICE_EXT or EGL_BAD_PARAMETER in case of error text.


> Unless we add another extension to elaborate/handle things better/differently.


Render nodes please. The point is to gain a hw context *without* a platform that is bound to any kind of monitor/console/virtual machine monitor....

The really usefull appication is *non* *interactive* usage.


> > What are your plans?

> >

> So in general I'm leaning towards:

> - parties interested in using EGL device without hardware device -

> we don't expose it, unless we have an extension how it should work

> - distributions explicitly restricting access to drm devices - we

> don't expose it, they get what they are asking.

> - the vendor neutral EGL requirement of EGL_EXT_device_base - I'm

> leaning that we should rework that so that vendor implementations

> supporting software rendering will continue to work.


That's sounds plausible to me.

I just do not see that the device query extension is suppressed when no usable render node or equivalent is available.


> Thanks for bringing this up. I (un)fortunately forgot that software

> rendering and EGL is a thing :-)

Well, I was also picking up this work last autumn until I realised that

'returns at least one device' part. And I never got around to either introduce

runtime disabled client extensions or a software rasterizer backed device.


Shall we move this to mesa-devel?




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