Re: Re: [PATCH] fbdev: colormap fixes

On Thu, 28 Jul 2005, Jon Smirl wrote:

I can't see a way to query how long of cmap the device supports using
the current fbdev ioctls.

Look at the lengths of the color bitfields?

I wouldn't even be messing with cmap except for the true/direct color
support and gamma ramps. Don't I need to know how long the cmap is in
order to set the right gamma ramp?

Of course.

If I set a 256 entry gamma ramp on a card with 1024 entries, will it
still work right? Or do I need to set all 1024 entries?

No, you have to set all 1024 entries. And you will notice, because the lengths
of the color bitfields will be 10, not 8.

On 7/28/05, Jon Smirl [off-list ref] wrote:

quoted

On 7/28/05, Geert Uytterhoeven [off-list ref] wrote:

quoted

On Thu, 28 Jul 2005, Antonino A. Daplas wrote:

quoted

Jon Smirl wrote:

quoted

On 7/28/05, Geert Uytterhoeven [off-list ref] wrote:

quoted

On Wed, 27 Jul 2005, Linux Kernel Mailing List wrote:

There are a couple of ways to fix this.
1) Add a check to limit use of the sysfs attributes to 256 entries. If
you want more you have to use /dev/fb0 and the ioctl. More is an
uncommon case.
2) Switch this to a binary parameter. Now you have to use tools like
hexdump instead of cat to work with the data. It was nice to be able
to use cat to see the current map.

Does anyone have preferences for which way to fix it?

Or...

 3) Add another file in sysfs which specifies at what index and how many
entries will be read or written from or to the cmap. With this additional
sysfs file, it should be able to handle any reasonable cmap length, but
it will take more than one reading of the color_map file. Another
advantage is that the entire color map need not be read or written if
only one field needs to be changed.

I've attached a test patch.  Let me know what you think.

I like it! ... But, a disadvantages is that it needs to store state between two
non-atomic operations. E.g. imagine two processes doing this at the same time.

Two attributes is a big problem with atomicity. If you want access to
the full cmap I would switch to a binary attribute or use the existing
IOCTL.

Note that you can set the entire map with the new patch, you just
can't read it. You can store 1-N entries at any base you want.

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds


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