Hi Laurent,
On Tue, Jan 12, 2021 at 5:38 AM Laurent Pinchart laurent.pinchart@ideasonboard.com wrote:
On Mon, Jan 11, 2021 at 01:57:02PM +0100, Geert Uytterhoeven wrote:
As nwl_dsi.lanes is u32, and NSEC_PER_SEC is 1000000000L, the second multiplication in
dsi->lanes * 8 * NSEC_PER_SECwill overflow on a 32-bit platform. Fix this by making the constant unsigned long long, forcing 64-bit arithmetic.
While iMX8 is arm64, this driver is currently used on 64-bit platforms only, where long is 64-bit, so this cannot happen. But the issue may start to happen when the driver is reused for a 32-bit SoC, or when code is copied for a new driver.
Signed-off-by: Geert Uytterhoeven geert+renesas@glider.be
--- a/drivers/gpu/drm/bridge/nwl-dsi.c +++ b/drivers/gpu/drm/bridge/nwl-dsi.c @@ -195,7 +195,7 @@ static u32 ps2bc(struct nwl_dsi *dsi, unsigned long long ps) u32 bpp = mipi_dsi_pixel_format_to_bpp(dsi->format);
return DIV64_U64_ROUND_UP(ps * dsi->mode.clock * bpp,
dsi->lanes * 8 * NSEC_PER_SEC);
dsi->lanes * 8ULL * NSEC_PER_SEC);I wonder if we could get rid of a whole class of bugs by turning NSEC_PER_SEC into a ULL, but I suppose there are valid cases where a 32-bit integer is enough.
Indeed, and 64-bit arithmetic is more expensive on 32-bit platforms. I considered that change, but doing so would require updates all over the place (e.g. printing a value derived from NSEC_PER_SEC, divisions that need to be changed to do_div or div_u64(), ...)
Note that the selftests already use such a definition.
Reviewed-by: Laurent Pinchart laurent.pinchart@ideasonboard.com
Thanks!
How did you come across this by the way ?
https://lore.kernel.org/linux-renesas-soc/CAMuHMdXQvPY_mYicjPKjDSCwdO_rP-9PJ... and of course I grepped for similar use patterns...
Gr{oetje,eeting}s,
Geert