Re: [PATCH RFC 09/15] arm_mpam: Fix MSC MMIO window size to use resource_size() instead of end - start

[Yin Li]

On 9/3/2026 9:23 PM, Ben Horgan wrote:
> Hi Yin,
>
> On 03/09/2026 11:20, Ben Horgan wrote:
> > Hi Yin,
> >
> > On 11/08/2026 14:30, Yin Li wrote:
> > > struct resource uses an inclusive end address, so the correct size is
> > > end - start + 1. The previous calculation of end - start was off by one,
> > > resulting in a mapped window one byte smaller than the actual resource.
> > > Use resource_size() which correctly computes end - start + 1.
> > >
> > > Signed-off-by: Yin Li <yin.li@xxxxxxxxxxxxxxxx>
> >
> > I just got a kernel ci report for this one which asks for tags:
> >
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > Closes: https://lore.kernel.org/oe-kbuild-all/202609030809.ObirDhR3-lkp@xxxxxxxxx/
> >
> > It doesn't look to give a useful fixes tag though. I'd go with this as that's where the error was
> > introduced.
> >
> > Fixes: f04046f2577a ("arm_mpam: Add probe/remove for mpam msc driver and kbuild boiler plate")
> >
> > Looks good to me.
> >
> > Reviewed-by: Ben Horgan <ben.horgan@xxxxxxx>
>
> Scratch that. As Ilpo points out,[1], there is no functional bug but just some misleading naming
> which never the less would be good to fix. This does require > in the warnings becoming >= though
> and there would be no need for fixes tag. Do you agree with this analysis?
>
> Thanks,
>
> Ben
>
> [1]
> https://lore.kernel.org/lkml/03055fbc-281f-4ed9-9282-4853d560e17f@xxxxxxx/T/#mdb57d40c888ff4ce656a9d9a00ecf5d99466530c

Hi Ben,

Thanks, and thanks to Ilpo for the detailed analysis.

Agreed — the current code is functionally correct because 
mapped_hwpage_sz holds the last mapped byte (end - start) rather than 
the real size, and the ">" check matches that.
My patch switched it to resource_size() but left the check as ">", which 
actually breaks that balance and introduces a real off-by-one — the ">" 
needs to become ">=" once the size is accurate. So there's no functional 
bug in the original code, and no Fixes tag is needed.

I'll update the patch to switch to resource_size() and change the
corresponding ">" checks to ">=" together, so the naming becomes
accurate while keeping the behaviour unchanged. I'll also reword the
commit message to describe this as a naming/readability cleanup rather
than a bugfix.


> > Thanks,
> >
> > Ben
> >
> > > ---
> > >   drivers/resctrl/mpam_devices.c | 2 +-
> > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
> > > index 1e082fb60e30..5d1854d97371 100644
> > > --- a/drivers/resctrl/mpam_devices.c
> > > +++ b/drivers/resctrl/mpam_devices.c
> > > @@ -2296,7 +2296,7 @@ static struct mpam_msc *do_mpam_msc_drv_probe(struct platform_device *pdev)
> > >   			dev_err_once(dev, "Failed to map MSC base address\n");
> > >   			return ERR_CAST(io);
> > >   		}
> > > -		msc->mapped_hwpage_sz = msc_res->end - msc_res->start;
> > > +		msc->mapped_hwpage_sz = resource_size(msc_res);
> > >   		msc->mapped_hwpage = io;
> > >   	} else {
> > >   		return ERR_PTR(-EINVAL);

--
Thx and BRs,
Yin