RandR bugs

Thomas Winischhofer thomas at winischhofer.net
Thu Sep 29 04:16:02 PDT 2005

Hash: SHA1

Alan Hourihane wrote:
> On Thu, 2005-09-29 at 12:40 +0200, Thomas Winischhofer wrote:
>>>>3) This is a more conceptual issue:
>>>>The sis driver allows switching output devices during server-runtime.
>>>>However, not all modes are supported for all available output devices.
>>>>Under normal circumstances, the driver's mode validation takes care of this.
>>>>RandR does not allow setting a configuration, be it display size, be it
>>>>rotation/reflection, without (re-)setting the display mode. This is
>>>>For example, if I have a 1280x800 virtual screen and a current display
>>>>mode of 1024x768 (eg because I switched from LCD to TV, and TV does not
>>>>support 1280x800), setting the RandR config just in order to change the
>>>>rotation will cause RandR to try to switch the display mode to 1280x800
>>>>(because that is the desired screen size). This will fail (due to the
>>>>driver validating the display mode).
>>>>At this point everything goes havoc due to the bugs mentioned in 1) and
>>>>2) above.
>>>>However, even if the failures of xf86SwitchMode() are handled properly,
>>>>the whole RandR request will fail.
>>>>So, I propose the following to solve this: RandR should only touch the
>>>>display mode if it is too large for the desired screen size. Otherwise
>>>>the display mode should be left untouched.
>>>Sounds reasonable. Have you got a patch ? That's much easier to comment
>>>on. It might be reasonable to open a bug on bugzilla for this so
>>>reasonable patches can be made and reviewed.
>>>Also, I'm not sure RRFunc is needed at all. It seems to me that
>>>xf86SwitchMode() is good enough to let the driver handle the necessary
>>>The only thing that is missing is a call that the driver can make to
>>>obtain the current rotation mode (i.e. randrp->rotation). Removing the
>>>need completely for RRFunc.
>>I somewhat like the idea of the DriverFunc (as it is called in HEAD).
>>And it's already used for Egbert's method to eventually run the server
>>without being root.
> Mmm, I've not heard about that.

See the fbdev and sisusb drivers and xf86Init.c for this. Both drivers
allow running the server as non-root.

>>And using the driver's switchmode function does not work around the
>>issue neither. Basically, I don't see what stuff like screen size or
>>rotation or reflection have to do with the display mode. It is not
>>RandR's business to mess with the display mode, IMHO.
> RandR doesn't mess with the display mode at all. It just validates the
> calling data against the current modepool.
> We already use xf86SwitchMode(), as RandR allows us to switch to
> different resolutions. 

What is "resolution" in that case? This sounds very much like "display
resolution", ie "display mode".

But I question just that: Is it really the "*Resize* and *Rotate*"
extension's job to set the display mode/rate?

It completely ignores that eventually the user wants a different display
mode than screen size...

> Semantically, it's no different to handle
> rotation in that call as well. Given that the driver doesn't have to
> implement DriverFunc at all, and it could conceivably just deal with
> things in it's own SwitchMode, I've got no problems with DriverFunc
> staying around.

If the rotation parameter is accessible somewhere from switchmode, I'll
be find with that as well. I, by no means, insist of having this
"DriverFunc" (although it could come handy for binary compatibility,
because eventual further driver hooks don't need an entry in pScrn or
anywhere else. Just a new "op".)

I tried to think of a way to work around the issue mention above. Now,
what is the intended way to handle the user-provided "rate" in case
randr would not touch the display mode according to my proposal...?

That is a question that touches the whole idea of RandR.



- --
Thomas Winischhofer
thomas AT winischhofer DOT net	       *** http://www.winischhofer.net

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