[Mesa-dev] [PATCH] egl/dri2: implement platform_null.

[Emil Velikov]

On 23/01/15 02:00, Haixia Shi wrote:
> Hi Emil,
> 
> On Thu, Jan 22, 2015 at 4:38 PM, Emil Velikov <emil.l.velikov at gmail.com> wrote:
>> On 22/01/15 22:23, Haixia Shi wrote:
>>> Hi Emil
>>>
>>> On Thu, Jan 22, 2015 at 1:36 PM, Emil Velikov <emil.l.velikov at gmail.com> wrote:
>>>> Hi Haixia Shi,
>>>>
>>>> On 22/01/15 17:35, Haixia Shi wrote:
>>>>> Try the render node first and use it if available. Otherwise fall back to
>>>>> normal nodes.
>>>>>
>>>> What is the use-case for such a platform - I assume it's worth
>>>> mentioning in the commit message ?
>>>>
>>>> No other platform picks the device at random as seen below. Why did you
>>>> choose such an approach ? It seems like one can easily shoot themselves
>>>> by using it.
>>>
>>> CC Stephane. The goal here is just to pick the first available node
>>> for off-screen rendering only.
>>>
>> Hmm I'm guessing that using the drm/gbm platform is out of the question
>> ? Iirc there has been a bit of love on the gbm topic, and afaiu this
>> solution is to be used with minigbm ?
> 
> Yes this solutions is to be used with minigbm.
> 
>>
>> What I'm thinking here is:
>> If you're testing a device with provides two or more nodes (be that the
>> classic card or the render ones), one cannot guarantee that the kernel
>> module for hw#1 will be loaded first. Thus even if one presumes that
>> they are working on (testing) hw#1 that may or may not be the case.
>>
>> Not 100% sure on the module order part, so I could be wrong.
> 
> I don't have a good answer for that... any suggestion on how best to
> pick the right one?
> 
Might be worth having a look at how platform_drm does it. But we warned
there be dragons :)

>>>>> +         char *card_path;
>>>>> +         if (asprintf(&card_path, node_path_fmt, base + i) < 0)
>>>>> +            continue;
>>>>> +
>>>>> +         dri2_dpy->fd = open(card_path, O_RDWR);
>>>> If you open a normal node (card%d) I believe that you'll need an
>>>> authenticate hook in dri2_egl_display_vtbl. Does things work without it
>>>> on your system/platform ?
>>>
>>> You're correct; normal node would require the legacy auth hook, and it
>>> would only work without auth if the process is run as root, which is
>>> why we're trying render nodes first.
>>>
>> So you're saying that people without render nodes should run egl(mesa)
>> as root ? That does not sound like a wise suggestion imho.
>>
>> Basically what I'm trying to say is - if you have a fall-back to normal
>> nodes, some form of auth ought to be in place.
> 
> I see your point. Would it be cleaner if we simply require render node
> to be present? The normal node (card%d) and the auth hook is more
> trouble than its worth.
> 
It's up-to you if you want to keep it.
I'm just pointing out that having a fall-back that (a) mostly fails,
without giving a clear indication as to why, or (b) forces you to run
the app as root is counter-intuitive (not the best security practise)
for most people.


-Emil