Re: [PATCH 1/2] drm: Avoid calling the cursor_set2 callback from the drm_mode_cursor ioctl

[Thomas Hellstrom <thellstrom@vmware.com>]

On 11/27/2015 11:11 AM, Daniel Vetter wrote:
> On Thu, Nov 26, 2015 at 10:52:14AM -0800, Thomas Hellstrom wrote:
>> If the drm_mode_cursor_ioctl is called and the cursor_set2 callback is
>> implemented, the cursor hotspot is set to (0,0) which is incompatible
>> with vmwgfx where the hotspot should instead remain unchanged.
>>
>> So if the driver implements both cursor_set2 and cursor_set, prefer calling
>> the latter from the drm_mode_cursor ioctl.
>>
>> Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
> That looks like papering over a bug in the client - it simply shouldn't
> mix these two if it expects the hotspot to stick around. There was also
> recently a big discussion about hotspot behaviour, and the conclusion was
> that qxl got it all wrong. Since that was specifically added for qxl I'm
> not sure how well this was ever tested ...

No, this is not the case, This is for old Xorg userspace that first sets
the hotspot using our ancient
driver-private ioctl and then calls drm_mode_cursor() to update the cursor.

Now if we were to implement cursor_set2, which is apparently needed to
get gnome-shell/Wayland cursors in the right place, without this fix, it
would break old Xorg, so we don't have much choice in this case from
what I can tell.

The root problem here is that the drm_mode_cursor() behaviour in the
presence of cursor_set2 didn't take the existing vmware hotspot
semantics into account.

> The other problem seems to be that X is racy with updating cursors (since
> it does a setPos and setCursor separately, despite that this ioctl can do
> it in one go) and so if you move the hotspot you get a jumpy cursor.
> Radeon tried to paper over that but fundamentally you need to fix X and
> use atomic (or at least universal plane cursor support) to fix this.
>
> Given all that I'm leaning towards "the future is atomic", let's please
> not add hacks to legacy code. Aside: Atomic doesn't have the hotspot-x/y
> properties yet, but really the only reason was that we don't yet have an
> atomic driver needing them. Adding those props will be a tiny patch of a
> few lines.

Sinclair has started working on atomic support for vmwgfx, but we really
need to fix this also in
the legacy code, ugly or not.

Thanks,
Thomas


>
> Cheers, Daniel
>> ---
>>  drivers/gpu/drm/drm_crtc.c | 10 ++++++----
>>  1 file changed, 6 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
>> index 24c5434..93f80a5 100644
>> --- a/drivers/gpu/drm/drm_crtc.c
>> +++ b/drivers/gpu/drm/drm_crtc.c
>> @@ -2874,7 +2874,8 @@ static int drm_mode_cursor_universal(struct drm_crtc *crtc,
>>  
>>  static int drm_mode_cursor_common(struct drm_device *dev,
>>  				  struct drm_mode_cursor2 *req,
>> -				  struct drm_file *file_priv)
>> +				  struct drm_file *file_priv,
>> +				  bool from_2)
>>  {
>>  	struct drm_crtc *crtc;
>>  	int ret = 0;
>> @@ -2907,7 +2908,8 @@ static int drm_mode_cursor_common(struct drm_device *dev,
>>  			goto out;
>>  		}
>>  		/* Turns off the cursor if handle is 0 */
>> -		if (crtc->funcs->cursor_set2)
>> +		if (crtc->funcs->cursor_set2 &&
>> +		    (from_2 || !crtc->funcs->cursor_set))
>>  			ret = crtc->funcs->cursor_set2(crtc, file_priv, req->handle,
>>  						      req->width, req->height, req->hot_x, req->hot_y);
>>  		else
>> @@ -2953,7 +2955,7 @@ int drm_mode_cursor_ioctl(struct drm_device *dev,
>>  	memcpy(&new_req, req, sizeof(struct drm_mode_cursor));
>>  	new_req.hot_x = new_req.hot_y = 0;
>>  
>> -	return drm_mode_cursor_common(dev, &new_req, file_priv);
>> +	return drm_mode_cursor_common(dev, &new_req, file_priv, false);
>>  }
>>  
>>  /**
>> @@ -2976,7 +2978,7 @@ int drm_mode_cursor2_ioctl(struct drm_device *dev,
>>  {
>>  	struct drm_mode_cursor2 *req = data;
>>  
>> -	return drm_mode_cursor_common(dev, req, file_priv);
>> +	return drm_mode_cursor_common(dev, req, file_priv, true);
>>  }
>>  
>>  /**
>> -- 
>> 2.4.3
>>
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