Re: [QUESTION] Public availability of the MHI specification

On 4/28/26 10:12, Manivannan Sadhasivam wrote:
> Hi,
> 
> On Tue, Apr 28, 2026 at 06:32:08AM +0000, [email protected] wrote:
>> Hi,
>>
>> I am looking into the MHI bus subsystem (drivers/bus/mhi/) to better
>> understand the protocol and the semantics behind the MMIO register
>> layout defined in drivers/bus/mhi/common.h. My motivation is that I
>> would like to use MHI to communicate with a PCIe device rather than
>> reinventing a similar protocol from scratch.
>>
>> The in-tree documentation (Documentation/mhi/) gives a good
>> architectural overview, and several comments in the code refer to the
>> MHI specification by section number (e.g. drivers/bus/mhi/ep/main.c:57
>> mentions "section 4.3, Interrupt moderation") and by version (commit
>> 48f98496b1de mentions "MHI spec v1.2"). However, I could not find a
>> pointer to the specification document itself.
>>
>> Could you tell me:
>> 1. Is the MHI specification publicly available? If so, where can it be
>> downloaded (Qualcomm developer portal, elsewhere)?
>> 2. Is MHI intended/suitable for use with non-Qualcomm devices, or would
>> you recommend against going down that path?
>>
> 
> MHI is a Qualcomm's proprietary, patented [1] protocol. So you cannot use it in
> non-Qcom products without getting prior approval from Qualcomm. Also, the spec
> is not open as it is not meant to be developed outside of Qualcomm products.

Understood. It would be nice to have a generic solution for this 
recurring communication need, as the doorbell and ring buffer mechanism 
is a pattern shared by many drivers, especially PCIe ones.

> 
> If you just want a protocol that works on top of physical transport like PCIe on
> all platforms, I'd suggest using an opensource transport like VirtIO.
> 

Thanks for the pointer — not sure it fits my needs, I will investigate 
in more detail.

Regards,
Ludovic

> - Mani
> 
> [1] https://patents.google.com/patent/US20140207991A1/en
> 
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