Date:   Thu, 1 Sep 2022 10:11:45 -0700
From:   Guenter Roeck <linux@...ck-us.net>
To:     "Farber, Eliav" <farbere@...zon.com>
Cc:     jdelvare@...e.com, robh+dt@...nel.org, p.zabel@...gutronix.de,
        rtanwar@...linear.com, linux-hwmon@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        talel@...zon.com, hhhawa@...zon.com, jonnyc@...zon.com,
        hanochu@...zon.com, ronenk@...zon.com, itamark@...zon.com,
        shellykz@...zon.com, shorer@...zon.com, amitlavi@...zon.com,
        almogbs@...zon.com, dkl@...zon.com, andriy.shevchenko@...el.com
Subject: Re: [PATCH v3 02/19] hwmon: (mr75203) fix VM sensor allocation when
 "intel, vm-map" not defined

On 9/1/22 08:24, Farber, Eliav wrote:
> 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@...zon.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.
> 

You are changing the driver all over the place with 19 patches, including
this code, but you don't want to add code that validates the devicetree
data ? That seems odd.

> 
>> 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.
> 

Does that make any sense, and is there a valid reason to have a mapping
table like the one in this example ?

Thanks,
Guenter

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