sm750_hw_imageblit() advances its monochrome source pointer by
src_delta per scanline, and computes the correct rounded-up stride
internally as:
bytes_per_scan = (width + start_bit + 7) / 8;
Its only caller, lynxfb_ops_imageblit(), instead passed src_delta as
image->width >> 3. For widths not a multiple of 8 this under-counted
the stride, so the source pointer fell further behind the real
per-scanline layout on every line, corrupting the rendered image.
Rather than just fixing the caller's calculation, remove src_delta
as a parameter entirely and have sm750_hw_imageblit() advance by the
bytes_per_scan it already computes for itself. There has only ever
been one caller, and that caller was passing an out-of-sync
derivative of the same width/start_bit values sm750_hw_imageblit()
already has, so keeping stride as a separate parameter served no
purpose beyond letting the two calculations drift apart, which is
exactly what happened here.
Rounding up, rather than down, is the direction consistent with the
rest of the fbdev core: struct fb_image mono bitmap data (the same
image->data this driver receives) is walked elsewhere with byte
strides derived from a ceiling division of width by 8. The generic
mono bit iterator in drivers/video/fbdev/core/fb_imageblit.h advances
scanlines with "iter->data += BITS_TO_BYTES(iter->width)", and
BITS_TO_BYTES() (include/linux/bitops.h) is a ceiling division.
sm750_hw_imageblit()'s own "(width + start_bit + 7) / 8" is that same
ceiling division with an added start_bit offset, so the caller's
">> 3" (floor) was the one calculation out of step with how this data
layout is handled everywhere else.
Found by code review of sm750_hw_imageblit()'s internal stride
calculation against what its only caller was passing in, and
confirmed with a clean -Werror build. I do not have this hardware,
so this has not been exercised at runtime on real sm750 silicon.
Fixes: 81dee67e21 ("staging: sm750fb: add sm750 to staging")
Cc: stable@vger.kernel.org
Reviewed-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Muhammad Bilal <meatuni001@gmail.com>
Link: https://patch.msgid.link/20260901113031.161610-1-meatuni001@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
cc7cd2a922