Re: [PATCH] Do set var even if no fb_check_var() provided.

[Helge Deller <deller@gmx.de>]

Ville Syrjälä wrote:
> On Wed, Aug 27, 2008 at 11:49:15PM +0200, Helge Deller wrote:
>> Geert Uytterhoeven wrote:
>>> On Wed, 27 Aug 2008, Takashi Yoshii wrote:
>>>> The behavior of FBIOSET_VSCREENINFO seems to be weired on FB which does
>>>> not
>>>>  provides fbops->fb_check_var.
>>>> It doesn't set anything, but gets info->var instead, and returns no
>>>> error. It is just same as FBIOGET_VSCREENINFO does.
>>>>
>>>> IMHO, this should be one of the following candidates,
>>>>  1. set var without check.
>>>>  2. do nothing but return error (setting var is not supported).
>>>> or
>>>>  3. it's a bug (fb_check_var should always be provided).
>>>>
>>>> The patch at the bottom implements "1".
>>>>
>>>> Because I don't know API specification, nor the history of the code,
>>>> I would like people who knows well to discuss this.
>>> If the driver doesn't provide a fb_check_var(), it means it cannot
>>> change video mode. Hence this rules out #1.
>>>
>>> #2 is not acceptable, as it will break existing applications. It's also
>>> incorrect, as FBIOPUT_VSCREENINFO should succeed for supported modes.
>>> For unsupported modes, the mode should be rounded up to a supported mode,
>>> if possible. In the case of drivers that support one fixed mode only, this
>>> rounding up is relaxed to `rounding' to the sole supported mode.
>>>
>>> #3 is also wrong, as fb_check_var() has been deliberately made optional to
>>> simplify drivers that support one fixed mode only.
>>>
>>> Conclusion: nothing should be changed?
>> I'm not sure.
>> On parisc we just stumbled over exactly this problem where I think Takashi's
>> proposal "1." is probably the right solution.
> 
> What about this?
> 
> 4. Provide a generic check_var() that does some basic sanity checking
> against info->var (eg. check xres, yres and bits_per_pixel).

Yes, possible.
But that wouldn't solve issues with Xorg's current implementation, as 
Xorg is currently checking the monitor sync values as well (see 
https://bugs.freedesktop.org/show_bug.cgi?id=17153)

Helge

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