Re: [PATCH v3 07/11] bus: mhi: host: Add support to read MHI capabilities

[Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>]

On 5/21/2025 8:22 PM, Jeffrey Hugo wrote:
> On 5/19/2025 3:42 AM, Krishna Chaitanya Chundru wrote:
>> From: Vivek Pernamitta <quic_vpernami@quicinc.com>
>>
>> As per MHI spec v1.2,sec 6.6, MHI has capability registers which are
>> located after the ERDB array. The location of this group of registers is
>> indicated by the MISCOFF register. Each capability has a capability ID to
>> determine which functionality is supported and each capability will point
>> to the next capability supported.
>>
>> Add a basic function to read those capabilities offsets.
>>
>> Signed-off-by: Vivek Pernamitta <quic_vpernami@quicinc.com>
>> Signed-off-by: Krishna Chaitanya Chundru 
>> <krishna.chundru@oss.qualcomm.com>
>> ---
>>   drivers/bus/mhi/common.h    |  4 ++++
>>   drivers/bus/mhi/host/init.c | 29 +++++++++++++++++++++++++++++
>>   2 files changed, 33 insertions(+)
>>
>> diff --git a/drivers/bus/mhi/common.h b/drivers/bus/mhi/common.h
>> index 
>> dda340aaed95a5573a2ec776ca712e11a1ed0b52..eedac801b80021e44f7c65d33cd50760e06c02f2 100644
>> --- a/drivers/bus/mhi/common.h
>> +++ b/drivers/bus/mhi/common.h
>> @@ -16,6 +16,7 @@
>>   #define MHICFG                0x10
>>   #define CHDBOFF                0x18
>>   #define ERDBOFF                0x20
>> +#define MISCOFF                0x24
>>   #define BHIOFF                0x28
>>   #define BHIEOFF                0x2c
>>   #define DEBUGOFF            0x30
>> @@ -113,6 +114,9 @@
>>   #define MHISTATUS_MHISTATE_MASK        GENMASK(15, 8)
>>   #define MHISTATUS_SYSERR_MASK        BIT(2)
>>   #define MHISTATUS_READY_MASK        BIT(0)
>> +#define MISC_CAP_MASK            GENMASK(31, 0)
>> +#define CAP_CAPID_MASK            GENMASK(31, 24)
>> +#define CAP_NEXT_CAP_MASK        GENMASK(23, 12)
>>   /* Command Ring Element macros */
>>   /* No operation command */
>> diff --git a/drivers/bus/mhi/host/init.c b/drivers/bus/mhi/host/init.c
>> index 
>> 13e7a55f54ff45b83b3f18b97e2cdd83d4836fe3..a7137a040bdce1c58c98fe9c2340aae4cc4387d1 100644
>> --- a/drivers/bus/mhi/host/init.c
>> +++ b/drivers/bus/mhi/host/init.c
>> @@ -467,6 +467,35 @@ int mhi_init_dev_ctxt(struct mhi_controller 
>> *mhi_cntrl)
>>       return ret;
>>   }
>> +static int mhi_find_capability(struct mhi_controller *mhi_cntrl, u32 
>> capability, u32 *offset)
>> +{
>> +    u32 val, cur_cap, next_offset;
>> +    int ret;
>> +
>> +    /* Get the 1st supported capability offset */
> 
> "first"?  Does not seem like you are short on space here.
> 
Misc register will have the offest of the 1st capability register
from there capabilities will have linked list format.
>> +    ret = mhi_read_reg_field(mhi_cntrl, mhi_cntrl->regs, MISCOFF,
>> +                 MISC_CAP_MASK, offset);
> 
> This fits on one line.
> 
>> +    if (ret)
>> +        return ret;
> 
> Blank line here would be nice.
> 
>> +    do {
>> +        if (*offset >= mhi_cntrl->reg_len)
>> +            return -ENXIO;
>> +
>> +        ret = mhi_read_reg(mhi_cntrl, mhi_cntrl->regs, *offset, &val);
>> +        if (ret)
>> +            return ret;
> 
> 
> There is no sanity checking we can do on val?  We've had plenty of 
> issues blindly trusting the device.  I would like to avoid having more.
> 
we can check if val is not all F's as sanity other than that we can't
check any other things as we don't know if the value is valid or not.
Let me know if you have any taught on this.
> Also looks like if we find the capability we are looking for, we return 
> the offset without validating it.
> 
For offset I can have a check to make sure the offset is not crossing
mhi reg len like above.

- Krishna Chaitanya.
>> +
>> +        cur_cap = FIELD_GET(CAP_CAPID_MASK, val);
>> +        next_offset = FIELD_GET(CAP_NEXT_CAP_MASK, val);
>> +        if (cur_cap == capability)
>> +            return 0;
>> +
>> +        *offset = next_offset;
>> +    } while (next_offset);
>> +
>> +    return -ENXIO;
>> +}
>> +
>>   int mhi_init_mmio(struct mhi_controller *mhi_cntrl)
>>   {
>>       u32 val;
>>
>