Re: RFC: bitmap line alignment

["Antonino A. Daplas" <adaplas@gmail.com>]

Knut Petersen wrote:
> Hi everybody
> 
> I learned the hard way that image->width*image->height is not equal to the
> number of bits consumed by the image bitmap. Every single line of bits is
> aligned to a byte boundary.
> 
> Wouldn´t it be much more efficient to align every line to an u32 boundary?
> Despite the obvious advantages of aligned cpu access to large arrays it
> would also help to simplify the code for at least the cyberblade
> acceleration
> engine.

If you create your own info->pixmap and an info->pixmap.scan_align = 4, the
driver will be fed a bitmap that have a scanline that is 32-bit aligned.
The default if you don't specify anything, of course, will be an 8-bit
aligned bitmap which, for the sake of code efficiency, is the alignment
assumed by the cfb_* drawing functions.

(An example usage is in the patch I attached just a few days ago when
I commented how the cyberblade should be intelligent enough to handle
bitmaps with variable widths.)
> 
> The cyberblade always requires 32 bit quantities, so if the width of the
> image
> is eg. 33, I have to feed _two_ u32 to it for every pixel line. Is the
> same true
> for other acceleration engines?

Yes, nvidiafb, rivafb and savagefb all require a scan_align of 4, the
i810fb needs u16 (scan_align = 2).

Tony


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