linux-kernel - Re: [PATCH] bus: mhi: host: Add sysfs entry to force device to enter EDL

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Date: Thu, 11 Jan 2024 12:08:20 -0700
From: Jeffrey Hugo <quic_jhugo@...cinc.com>
To: Qiang Yu <quic_qianyu@...cinc.com>,
        Manivannan Sadhasivam
	<mani@...nel.org>
CC: <mhi@...ts.linux.dev>, <linux-arm-msm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <quic_cang@...cinc.com>,
        <quic_mrana@...cinc.com>, Bhaumik Bhatt <quic_bbhatt@...cinc.com>
Subject: Re: [PATCH] bus: mhi: host: Add sysfs entry to force device to enter
 EDL

On 1/9/2024 2:20 AM, Qiang Yu wrote:
> 
> On 1/3/2024 12:52 AM, Manivannan Sadhasivam wrote:
>> On Tue, Jan 02, 2024 at 08:31:15AM -0700, Jeffrey Hugo wrote:
>>> On 12/25/2023 12:47 AM, Qiang Yu wrote:
>>>> From: Bhaumik Bhatt <quic_bbhatt@...cinc.com>
>>>>
>>>> Forcing the device (eg. SDX75) to enter Emergency Download Mode 
>>>> involves
>>>> writing the 0xEDEDEDED cookie to the channel 91 doorbell register and
>>>> forcing an SOC reset afterwards. Allow users of the MHI bus to 
>>>> exercise the
>>>> sequence using a sysfs entry.
>>> I don't see this documented in the spec anywhere.  Is this standard 
>>> behavior
>>> for all MHI devices?
>>>
>>> What about devices that don't support EDL mode?
>>>
>>> How should the host avoid using this special cookie when EDL mode is not
>>> desired?
>>>
>> All points raised by Jeff are valid. I had discussions with Hemant and 
>> Bhaumik
>> previously on allowing the devices to enter EDL mode in a generic 
>> manner and we
>> didn't conclude on one final approach.
>>
>> Whatever way we come up with, it should be properly described in the 
>> MHI spec
>> and _should_ be backwards compatible.
> 
> Hi Mani, Jeff. The method of entering EDL mode is documented in MHI spec 
> v1.2, Chapter 13.2.
> 
> Could you please check once?

I do see it listed there.  However that was a FR for SDX55, so devices 
prior to that would not support this.  AIC100 predates this change and 
would not support the functionality.  I verified the AIC100 
implementation is not aware of this cookie.

Also, that functionality depends on channel 91 being reserved per the 
table 9-2, however that table only applies to modem class devices as it 
is under chapter 9 "Modem protocols over PCIe".  Looking at the ath11k 
and ath12k implementations in upstream, it looks like they partially 
comply.  Other devices have different MHI channel definitions.

Chapter 9 doesn't appear to be in older versions of the spec that I 
have, so it is unclear if this functionality is backwards compatible 
(was channel 91 used for another purpose in pre-SDX55 modems).

I'm not convinced this belongs in the MHI core.  At a minimum, the MHI 
controller(s) for the applicable devices needs to opt-in to this.

-Jeff