Re: [Xen-devel] [PATCH v3] xen, input: add xen-kbdfront module parameter for setting resolution

[Juergen Gross <jgross@xxxxxxxx>]

On 12/04/17 18:24, Dmitry Torokhov wrote:
> On Wed, Apr 12, 2017 at 06:04:30PM +0200, Juergen Gross wrote:
>> On 12/04/17 17:16, Dmitry Torokhov wrote:
>>> Hi Juergen,
>>>
>>> On Tue, Apr 11, 2017 at 02:30:37PM +0200, Juergen Gross wrote:
>>>> Add a parameter for setting the resolution of xen-kbdfront in order to
>>>> be able to cope with a (virtual) frame buffer of arbitrary resolution.
>>>>
>>>> While at it remove the pointless second reading of parameters from
>>>> Xenstore in the device connection phase: all parameters are available
>>>> during device probing already and that is where they should be read.
>>>>
>>>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>>>> ---
>>>> V3: - merged the two patches
>>>>     - read Xenstore parameters during probing of the device only
>>>
>>> I guess 2 module parameters are not big deal, but could you tell me why
>>> you can't always have them specified in Xenstore?
>>
>> This will depend on Xen version then. I'd prefer to have a way to
>> specify the resolution in case a new kernel is running as a guest
>> of an old Xen version.
> 
> Who would be seeting up the kernel parameters in this case? User
> manually in guest bootloader config?

Either the guest administrator through guest boot loader or the Xen
administrator in the guest config file.

In the case where the problem has been reported (automatic tests of
graphical tools) this would be in the guest config file.

>>> Also, I still think you are going in the wrong direction here. Can your
>>> framebuffer size change after booting the guest? If it can, you have to
>>> reconcile the new size and the coordinates reported by the pointing
>>> device, and I think guest should be doing it. If you look, for example,
>>> at vmmouse driver, they do not try to match coordinates it reports to
>>> the screen.
>>
>> I'm no expert in input driver interface. Can you tell me how the mouse
>> pointer is positioned in case of the vmmouse driver? Are the real limits
>> used just the minimum of pointing device and screen size limits? So
>> specifying a size of 0xffff * 0xffff like the vmmouse driver does will
>> work with any screen size being smaller than that?
> 
> I think 0xffff is just the limits for coordinates reported through this
> interface; the expectation is that guest window size is not larger than
> that. I do not recall if the backend reports real screen pointer
> position, of offset from the (0,0) of guest's window, Thomas might tel
> you. IIRC code in vmware tools package (that runs in guest) gets
> notifications about screen changes and reacts accordingly.

So a small test revealed that only with a resolution matching that of
the virtual console the virtual mouse pointer will be at the same
position as the real mouse pointer. Working with different resolutions
will require some sort of scaling available only if _both_ resolutions
are known. And as you don't like the input driver knowing about the
console I'm limited to fixed values via module parameters (taking into
account the embedded scenario without udev).


Juergen


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