Re: [PATCH v3 02/19] hwmon: (mr75203) fix VM sensor allocation when "intel, vm-map" not defined

["Farber, Eliav" <farbere@amazon.com>]

On 9/1/2022 5:44 PM, Guenter Roeck wrote:
> On Thu, Sep 01, 2022 at 11:39:58AM +0300, Farber, Eliav wrote:
>> On 8/31/2022 2:48 PM, Guenter Roeck wrote:
>> > On 8/30/22 22:49, Farber, Eliav wrote:
>> > > On 8/31/2022 8:36 AM, Guenter Roeck wrote:
>> > > > On 8/30/22 12:21, Eliav Farber wrote:
>> > > > > Bug fix - in case "intel,vm-map" is missing in device-tree
>> > > > > ,'num' is set
>> > > > > to 0, and no voltage channel infos are allocated.
>> > > > >
>> > > > > Signed-off-by: Eliav Farber <farbere@amazon.com>
>> > > > > ---
>> > > > >   drivers/hwmon/mr75203.c | 28 ++++++++++++----------------
>> > > > >   1 file changed, 12 insertions(+), 16 deletions(-)
>> > > > >
>> > > > > diff --git a/drivers/hwmon/mr75203.c b/drivers/hwmon/mr75203.c
>> > > > > index 046523d47c29..0e29877a1a9c 100644
>> > > > > --- a/drivers/hwmon/mr75203.c
>> > > > > +++ b/drivers/hwmon/mr75203.c
>> > > > > @@ -580,8 +580,6 @@ static int mr75203_probe(struct
>> > > > > platform_device *pdev)
>> > > > >       }
>> > > > >
>> > > > >       if (vm_num) {
>> > > > > -             u32 num = vm_num;
>> > > > > -
>> > > > >               ret = pvt_get_regmap(pdev, "vm", pvt);
>> > > > >               if (ret)
>> > > > >                       return ret;
>> > > > > @@ -594,30 +592,28 @@ static int mr75203_probe(struct
>> > > > > platform_device *pdev)
>> > > > >               ret = device_property_read_u8_array(dev, 
>> "intel,vm-map",
>> > > > > pvt->vm_idx, vm_num);
>> > > > >               if (ret) {
>> > > > > -                     num = 0;
>> > > > > +                     /*
>> > > > > +                      * Incase intel,vm-map property is not
>> > > > > defined, we
>> > > > > +                      * assume incremental channel numbers.
>> > > > > +                      */
>> > > > > +                     for (i = 0; i < vm_num; i++)
>> > > > > + pvt->vm_idx[i] = i;
>> > > > >               } else {
>> > > > >                       for (i = 0; i < vm_num; i++)
>> > > > >                               if (pvt->vm_idx[i] >= vm_num ||
>> > > > > - pvt->vm_idx[i] == 0xff) {
>> > > > > -                                     num = i;
>> > > > > + pvt->vm_idx[i] == 0xff)
>> > > > >                                       break;
>> > > >
>> > > > So all vm_idx values from 0x00 to 0xfe would be acceptable ?
>> > > > Does the chip really have that many registers (0x200 + 0x40 +
>> > > > 0x200 * 0xfe) ?
>> > > > Is that documented somewhere ?
>> > > According to the code vm_num is limited to 32 because the mask is
>> > > only 5 bits:
>> > >
>> > > #define VM_NUM_MSK    GENMASK(20, 16)
>> > > #define VM_NUM_SFT    16
>> > > vm_num = (val & VM_NUM_MSK) >> VM_NUM_SFT;
>> > >
>> > > In practice according to the data sheet I have:
>> > > 0 <= VM instances <= 8
>> > >
>> > Sorry, my bad. I misread the patch and thought the first part of
>> > the if statement was removed.
>> >
>> > Anyway, what is the difference between specifying an vm_idx value of
>> > 0xff and not specifying anything ? Or, in other words, taking the dt
>> > example, the difference between
>> >        intel,vm-map = [03 01 04 ff ff];
>> > and
>> >        intel,vm-map = [03 01 04];
>>
>> The actual number of VMs is read from a HW register:
>>     ret = regmap_read(pvt->c_map, PVT_IP_CONFIG, &val);
>>     ...
>>     vm_num = (val & VM_NUM_MSK) >> VM_NUM_SFT;
>>
>> Also, using:
>>     ret = device_property_read_u8_array(dev, "intel,vm-map", vm_idx,
>>                         vm_num);
>> in the driver will fail if vm_num > sizeof array in device-tree.
>>
>> So, if for example vm_num = 5, but you will want to map only 3 of them
>> you most set property to be:
>>     intel,vm-map = [03 01 04 ff ff];
>> otherwise if you set:
>>     intel,vm-map = [03 01 04];
>> it will assume the property doesn't, and will continue the flow in code
>> as if it doesn’t exist (which is not what the user wanted, and before my
>> fix also has a bug).
>
> There should be some error handling to catch this case (ie if the number
> of entries does not match the expected count), or if a value in the array
> is larger or equal to vm_num. Today the latter is silently handled as end
> of entries (similar to 0xff), but that should result in an error.
> This would avoid situations like
>        intel,vm-map = [01 02 03 04 05];
> ie where the person writing the devicetree file accidentally entered
> index values starting with 1 instead of 0. A mismatch between vm_num
> and the number of entries in the array is silently handled as if there
> was no property at all, which is at the very least misleading and
> most definitely unexpected and should also result in an error.


I assume it is possible to tell according to the return value, if property
doesn’t exist at all, or if it does exists and size of array in
device-tree is smaller than vm_num.
In [PATCH v3 17/19] Andy wrote that “code shouldn't be a YAML validator.
Drop this and make sure you have correct DT schema” so I’m a bit confused
if code should validate “intel,bm-map” or if it is the user responsibility.
As this property was not added by me, I prefer not to fix it as part of
this series of patches.


> Also, what happens if the devicetree content is something like the
> following ? Would that be valid ?
>        intel,vm-map = [00 01 01 01 01 01];

If device-tree content would be:
     intel,vm-map = [00 01 01 01 01 01];
and assuming 16 channels for each VM, the hwmon sub-system will expose 90
sysfs to read voltage values.
In practice 16 – 31, 32 – 47, 48 – 63, 64 – 89 will all report the same
input signals for VM1.

--
Regards, Eliav