As nwl_dsi.lanes is u32, and NSEC_PER_SEC is 1000000000L, the second
multiplication in
dsi->lanes * 8 * NSEC_PER_SEC
will 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 <[email protected]>
---
Compile-tested only.
---
drivers/gpu/drm/bridge/nwl-dsi.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/bridge/nwl-dsi.c b/drivers/gpu/drm/bridge/nwl-dsi.c
index 66b67402f1acd57d..a8da3081efdcc84e 100644
--- 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);
}
/*
--
2.25.1
Hi Geert,
Thank you for the patch.
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_SEC
>
> will 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 <[email protected]>
> ---
> Compile-tested only.
> ---
> drivers/gpu/drm/bridge/nwl-dsi.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/bridge/nwl-dsi.c b/drivers/gpu/drm/bridge/nwl-dsi.c
> index 66b67402f1acd57d..a8da3081efdcc84e 100644
> --- 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.
Reviewed-by: Laurent Pinchart <[email protected]>
How did you come across this by the way ?
> }
>
> /*
--
Regards,
Laurent Pinchart
Hi Laurent,
On Tue, Jan 12, 2021 at 5:38 AM Laurent Pinchart
<[email protected]> 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_SEC
> >
> > will 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 <[email protected]>
> > --- 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 <[email protected]>
Thanks!
> How did you come across this by the way ?
https://lore.kernel.org/linux-renesas-soc/CAMuHMdXQvPY_mYicjPKjDSCwdO_rP-9PJOvqD0J6=S3Opr1ycg@mail.gmail.com/
and of course I grepped for similar use patterns...
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- [email protected]
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