KMS cursor BO semantics

[Maarten Maathuis]

On Fri, Nov 4, 2011 at 11:30 PM, Thomas Hellstrom <thellstrom at vmware.com> wrote:
> On 11/04/2011 04:34 PM, Daniel Vetter wrote:
>>
>> On Fri, Nov 04, 2011 at 12:59:59PM +0100, Thomas Hellstrom wrote:
>>
>>>
>>> Hi.
>>>
>>> I have a question about the semantics of the DRM_IOCTL_MODE_CURSOR iotcl:
>>>
>>> Some hardware (vmware's virtual in particular) may not be able to
>>> pick up the changes from a bo directly, since the cursor data is
>>> sent though the command stream. Hence we need a notification when
>>> the cursor image has changed.
>>>
>>> Could we *require* that a cursor image change needs to be followed
>>> by an ioctl call with the flag
>>> DRM_MODE_CURSOR_BO?
>>>
>>
>> On i915 we need the cursor in physical memory for some (old) platforms,
>> which is seperate storage from the bo backing storage. So we have the same
>> problem. We've solved it by intercepting pwrite ioctl calls and demanding
>> that userspace only uses these for cursor updates. Is there a special
>> reason you can't use such a driver-specific trick?
>> -Daniel
>>
>
> We have something similar in use today: We snoop DMAs to hardware
> cursor surfaces, but this gets a bit nasty when apps start to do hardware
> render to cursor surfaces, and
> we simply ignore that today.
>
> Furthermore,  maps rather than pwrites are the common usage-pattern for
> buffer-backed cursors on vmwgfx, and while it's possible to dirty those
> buffers based on page-faults, like we do with fb surfaces, we'd rather avoid
> having to implement and maintain that.
>
> I'm not sure whether / how you handle the case of hardware render to cursor
> surfaces on i915, but it seems to me like if a lot of drivers need to
> implement driver specific "tricks" to meet the semantics of a generic
> interface, we should perhaps consider specifying those semantics in a way
> that helps avoid driver-specific workarounds?
>
> /Thomas
>
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As far as i know nouveau keeps an internal buffer object for the
cursor and relies purely on the ioctl to copy the cursor into the
actual cursor memory area. So why do you need tricks?

-- 
Far away from the primal instinct, the song seems to fade away, the
river get wider between your thoughts and the things we do and say.