Re: [PATCH v4 06/10] arm_mpam: propagate MSC access errors for state saving function

[Andre Przywara <andre.przywara@arm.com>]

Hi,

On 7/24/26 12:07, Sudeep Holla wrote:
> On Thu, Jul 23, 2026 at 05:54:50PM +0200, Andre Przywara wrote:
>> Allow the mpam_save_mbwu_state() function to return an error, and
>> propagate read and write errors from the lower level up.
>>
>> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
>> ---
>>   drivers/resctrl/mpam_devices.c | 29 ++++++++++++++++++++++-------
>>   1 file changed, 22 insertions(+), 7 deletions(-)
>>
>> diff --git a/drivers/resctrl/mpam_devices.c b/drivers/resctrl/mpam_devices.c
>> index bcff53477133..6329443c451f 100644
>> --- a/drivers/resctrl/mpam_devices.c
>> +++ b/drivers/resctrl/mpam_devices.c
>> @@ -1833,22 +1833,37 @@ static int mpam_save_mbwu_state(void *arg)
>>   
>>   		mon_sel = FIELD_PREP(MSMON_CFG_MON_SEL_MON_SEL, i) |
>>   			  FIELD_PREP(MSMON_CFG_MON_SEL_RIS, ris->ris_idx);
>> -		mpam_write_monsel_reg(msc, CFG_MON_SEL, mon_sel);
>> -		mpam_read_monsel_reg(msc, CFG_MBWU_FLT, &cur_flt);
>> -		mpam_read_monsel_reg(msc, CFG_MBWU_CTL, &cur_ctl);
>> -		mpam_write_monsel_reg(msc, CFG_MBWU_CTL, 0);
>> +		ret = mpam_write_monsel_reg(msc, CFG_MON_SEL, mon_sel);
>> +		if (ret)
>> +			return ret;
>> +		ret = mpam_read_monsel_reg(msc, CFG_MBWU_FLT, &cur_flt);
>> +		if (ret)
>> +			return ret;
>> +		ret = mpam_read_monsel_reg(msc, CFG_MBWU_CTL, &cur_ctl);
>> +		if (ret)
>> +			return ret;
> 
> How does it work in general with PCC. Now that you can fail at any point,
> what happens to the write that occurs before a failed read like above one.
> Who will take care of erasing those new writes or it doesn't matter ?
> Just checking as I don't have much knowledge on MPAM intrinsics.

TBH I don't know, but I think we consider MPAM botched at this point, 
and just stop the driver, similar to an error IRQ? But I am not sure 
this is properly implemented at this point. The focus of these first six 
patches was merely to lay the dirty groundwork for *being able* to 
handle errors, and do this now rather than in the future.

Cheers,
Andre