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

[Yin Li <yin.li@oss.qualcomm.com>]

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@oss.qualcomm.com>
>>
>> I just got a kernel ci report for this one which asks for tags:
>>
>> Reported-by: kernel test robot <lkp@intel.com>
>> Closes: https://lore.kernel.org/oe-kbuild-all/202609030809.ObirDhR3-lkp@intel.com/
>>
>> 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@arm.com>
> 
> 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@arm.com/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