Re: [PATCH 2/2] drm: make unplugged flag specific to udl driver

[David Herrmann <dh.herrmann@gmail.com>]

Hi

On Wed, Feb 10, 2016 at 11:02 PM, Stéphane Marchesin
<stephane.marchesin@gmail.com> wrote:
> On Wed, Feb 10, 2016 at 1:54 PM, David Herrmann <dh.herrmann@gmail.com> wrote:
>>>> If a device is unplugged, you should consider all your resources to be
>>>> destroyed. There is no reason to release them manually. User-space
>>>> *must* be able to deal with asynchronous unplugs.
>>>
>>> So the problem if you do that is that things like a buffer's memory
>>> pages can disappear from under you. How would you deal with that case?
>>> User space certainly can't have a segfault handler catch that just in
>>> case :)
>>
>> If you rip out hardware resources, then you better be able to deal
>> with it. Sure, UDL is an exception as it doesn't have memory resources
>> on the chip. But it sounds fishy to me, if you base your API on it. On
>> a lot of other devices, the memory will simply not be there. So you
>> cannot keep it around.
>
> The thing is, you are not unplugging a device here; you are unplugging
> a USB monitor. As a proof that this is just a monitor, I can plug
> another USB monitor with the same driver and pick up where I left off.
> I guess I am saying that the concept of unplugging a device is not
> applicable here (or to any driver that I know, for that matter).
>
> Other drivers already handle all this by, for example, failing page
> flips if the monitor is gone. We basically want to do the same for
> UDL; I don't think we need to invent a new level of unplug here.

No, you rip out a display controller. You don't just unplug a monitor,
you remove the whole hardware. If UDL were treated on the same level
as connector-hotplugging, then you should implement it that way. But
currently it is not.

Btw., even if you do this, you still have to deal with devices going
away, and new devices being plugged. DRM does not support
CRTC/connector hotplugging, yet. Hence, making UDL a singleton driver
will not work either.

>>
>> There are many ways to invalidate memory mappings. You either unmap
>> the entire range (and user-space must deal with SIGBUS, which is
>> completely feasible and a lot of code already does it), or you replace
>> all with a zero page, or you duplicate all pages, ... IMO, user-space
>> has to start dealing with hardware unplug properly and stop pretending
>> it cannot happen.
>
> What you are suggesting is much more complicated than you claim, for
> example if you destroy the dmabuf which is shared with another driver,
> what happens? User space definitely can't deal with that.
>
> I think we should wait until we have unpluggable display hardware
> before inventing really complex support for it.

PCI Hotplugging. Has been around since a decade. Intel just got lucky
that they cannot be ripped out. The other drivers don't care (and
break horribly if you do).

Feel free to come up with a better way to deal with UDL hotplugging.
However, I for myself recommend doing it properly, on the bus level.
Like all other subsystems do.

Thanks
David
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