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

[Jeffrey Hugo <quic_jhugo@quicinc.com>]

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.

> +	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.

Also looks like if we find the capability we are looking for, we return 
the offset without validating it.

> +
> +		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;
>