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

[Jeffrey Hugo]

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@xxxxxxxxxxx>
> > > >
> > > > 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