linux-kernel - Re: [PATCH] bus: mhi: host: Update session id for each suspend and resume

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Message-ID: <d6gvgkzmrgixm3gtvfepfwh2vm7mgm75ksjd2xzy2jrkmmixks@i54qn2467g2b>
Date: Tue, 14 Oct 2025 18:21:18 +0530
From: Manivannan Sadhasivam <mani@...nel.org>
To: Slark Xiao <slark_xiao@....com>
Cc: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>, 
	mhi@...ts.linux.dev, linux-arm-msm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] bus: mhi: host: Update session id for each suspend and
 resume

On Tue, Oct 14, 2025 at 03:50:03PM +0800, Slark Xiao wrote:
> At 2025-09-16 11:26:16, "Slark Xiao" <slark_xiao@....com> wrote:
> >
> >
> >At 2025-09-15 15:26:33, "Krishna Chaitanya Chundru" <krishna.chundru@....qualcomm.com> wrote:
> >>
> >>
> >>On 9/12/2025 3:38 PM, Slark Xiao wrote:
> >>> On Qualcomm module side, there is a UART print as below:
> >>> session id: 0x355fe689 state:2
> >>> session id: 0x1f478e42 state:5
> >>> The session id reads from register BHI_IMGTXDB(0x218), and the
> >>> state indicates the D3 or D0 state.
> >>> 
> >>> In Windows side, MHI driver would update this session id for each
> >>> suspend/resume progress. We benefit from this mechanism since it
> >>> could help sync each suspend and resume progress between host and
> >>> device, especially for some suspend issue which needs to take
> >>> hundreds or thousands cycle. We can easy to figure out which
> >>> suspend cycle get a problem and what's happened at that time for
> >>> both host and device because they have same id.
> >>> 
> >>> But in Linux side, this session id value would always be 0x0.
> >>> So we add it for potential debug usage.
> >>> 
> >>> Signed-off-by: Slark Xiao <slark_xiao@....com>
> >>> ---
> >>>   drivers/bus/mhi/host/pm.c | 15 +++++++++++++++
> >>>   1 file changed, 15 insertions(+)
> >>> 
> >>> diff --git a/drivers/bus/mhi/host/pm.c b/drivers/bus/mhi/host/pm.c
> >>> index 33d92bf2fc3e..a6573f687363 100644
> >>> --- a/drivers/bus/mhi/host/pm.c
> >>> +++ b/drivers/bus/mhi/host/pm.c
> >>> @@ -864,6 +864,13 @@ int mhi_pm_suspend(struct mhi_controller *mhi_cntrl)
> >>>   	struct device *dev = &mhi_cntrl->mhi_dev->dev;
> >>>   	enum mhi_pm_state new_state;
> >>>   	int ret;
> >>> +	void __iomem *base = mhi_cntrl->bhi;
> >>> +	u32 session_id;
> >>> +
> >>> +	session_id = MHI_RANDOM_U32_NONZERO(BHI_TXDB_SEQNUM_BMSK);
> >>> +	dev_dbg(dev, "Starting enter suspend, session id: 0x%x\n",
> >>> +		sessin_id);
> >>> +	mhi_write_reg(mhi_cntrl, base, BHI_IMGTXDB, session_id);
> >>I agree this will help in debugging, but unless it is documented
> >>in the MHI spec we can't have this. Since in future if there is
> >>some other purpose for this register we end up facing issues.
> >>
> >>if it already part of spec point it in the commit text.
> >>
> >>- Krishna Chaitanya.
> >Hi Krishna,
> >I am not a member of Qualcomm so I don't have a detailed MHI
> >spec to describe this register usage.
> >But I get some information from their Windows MHI driver:
> >
> >1. There is release note which describe it:
> >UDE/QMI/win/qcude/installer/ReadMe.txt 
> >UDE/QCUDE.Standalone.Source.1.00.44.Windows-AnyCPU_ReadMe_1.txt
> >......
> >MHI driver 1.1.0.2
> >  b. Add write to BHI_IMGTXDB for debug purposes.
> >......
> >
> >2.From the code side,  there is a same operation for MHI driver
> >enter into M0:
> >HostDriver/win/NTAD/MhiHost/Mhi/src/Mhi.c
> >......
> >void MhiRequestM0(PMHI_DEV_CTXT MhiCtxt)
> >{
> >.....
> >   /* Set BHI_IMGTXDB */
> >   KeQuerySystemTime(&randSeed);
> >   MhiCtxt->SessionID = RtlRandomEx(&randSeed.LowPart);
> >   MhiTrace(TRACE_LEVEL_ERROR, TRACE_FLAG_MHICONFIG, "NEW SessionID: 0x%x\n", MhiCtxt->SessionID);
> >   MHI_WRITE_REG(deviceContext->BHIContext.BhiBase, BHI_IMGTXDB, MhiCtxt->SessionID);
> >
> >   MhiTrace(TRACE_LEVEL_ERROR, TRACE_FLAG_MHICONFIG, "Req -> M0\n");
> >......
> >
> >Not sure if above information is enough for this commit.
> >
> >Thanks
> 
> Any updates about this commit?
> 

Since this session ID behavior is not documented in the MHI/BHI spec, we can't
have this patch.

- Mani

-- 
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