[PATCH v2] fb: fix fast-path detection

[Stefan Ring]

Daniel Kurtz <djkurtz at ...> writes:

> Unfortunately, the calculation to determine if source and destination
> lines overlapped was incorrect:
>   (1) it converts width from pixels to bytes, but width is actually in
>       bits, not pixels.
>   (2) it adds this byte offset to dst/srcLine, which implicitly converts
>       the offset from bytes to sizeof(FbBits).
> 
> So, "careful" was true if the destination line was within 699392 bytes,
> instead of just within its 1366 * 4 = 5464 byte row.
> 
> This bug causes us to take the slow path for large non-overlapping rows
> that are "close" in memory.  As a data point, XGetImage(1366x768) on my
> ARM chromebook was taking ~140 ms, but with this fixed, it now takes
> about 60 ms.

I'm glad this got fixed now. It was noticed back in 2011 [1], but maybe a bit
more persistence on my part would have saved countless Watt hours.

[1] http://permalink.gmane.org/gmane.comp.freedesktop.xorg.devel/23346