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Subject: Re: [PATCH v3 02/19] hwmon: (mr75203) fix VM sensor allocation when "intel,
 vm-map" not defined
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Date:   Thu, 1 Sep 2022 21:36:08 +0300
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To:     Guenter Roeck <linux@roeck-us.net>
CC:     <jdelvare@suse.com>, <robh+dt@kernel.org>,
        <p.zabel@pengutronix.de>, <rtanwar@maxlinear.com>,
        <linux-hwmon@vger.kernel.org>, <devicetree@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <talel@amazon.com>,
        <hhhawa@amazon.com>, <jonnyc@amazon.com>, <hanochu@amazon.com>,
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        <shorer@amazon.com>, <amitlavi@amazon.com>, <almogbs@amazon.com>,
        <dkl@amazon.com>, <andriy.shevchenko@intel.com>,
        "Farber, Eliav" <farbere@amazon.com>
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From:   "Farber, Eliav" <farbere@amazon.com>
In-Reply-To: <a48f6c26-232a-f3ae-01d1-277e5c9800ee@roeck-us.net>
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On 9/1/2022 8:11 PM, Guenter Roeck wrote:
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>
>
> 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@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.
>>
>
> 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.
>
OK. I have added patch #20 to validate that same VM index doesn't appear
more than once in intel,vm-map.

u32 vm_mask = 0;

for (i = 0; i < vm_num; i++) {
     if (vm_idx[i] >= vm_num || vm_idx[i] == 0xff)
         break;

     if (vm_mask & BIT(vm_idx[i])) {
         dev_err(dev, "Same VM appears more than once in intel,vm-map\n",
             vm_idx[i]);
         return EINVAL;
     }

     vm_mask |= BIT(vm_idx[i]);
}


>>
>>> 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 ?

I can't find any sense in having such a mapping.
Anyway the new patch will not allow it to happen.

--
Regards, Eliav