linux-kernel - Re: [PATCH v5] bus: mhi: host: Avoid ringing EV DB if there is no elements to process

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Date:   Tue, 7 Mar 2023 22:11:41 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Vivek Pernamitta <quic_vpernami@...cinc.com>
Cc:     mhi@...ts.linux.dev, quic_qianyu@...cinc.com,
        quic_vbadigan@...cinc.com, quic_krichai@...cinc.com,
        quic_skananth@...cinc.com, mrana@...cinc.com,
        Alex Elder <elder@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jeffrey Hugo <quic_jhugo@...cinc.com>,
        Hemant Kumar <hemantk@...eaurora.org>,
        Bhaumik Bhatt <bbhatt@...eaurora.org>,
        "open list:MHI BUS" <linux-arm-msm@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] bus: mhi: host: Avoid ringing EV DB if there is no
 elements to process

On Thu, Feb 23, 2023 at 09:18:36PM +0530, Manivannan Sadhasivam wrote:
> On Wed, Feb 22, 2023 at 11:07:48PM +0530, Vivek Pernamitta wrote:
> > As mhi_poll function can be called by mhi client drivers
> > which will call process_event, which will ring DB even if
> > there no ring elements to process. This could cause
> > doorbell event to be processed by MHI device to check for
> > any ring elements even it is none and also it will occupy
> > lot of bandwidth on peripheral when mhi_poll() is called in
> > aggressive loop.
> > 
> 
> The change looks good to me but who is the actual in-kernel user of
> mhi_poll() API? It is being exported and if there is no upstream client
> driver making use of it, then it shouldn't be.
> 
> I'm gonna submit a patch to remove this API altogether.
> 

The patch removing mhi_poll() API is now merged. Can you please respin this
patch removing the note to the API?

I still think this patch is relevant without that API.

Thanks,
Mani

> > Signed-off-by: Vivek Pernamitta <quic_vpernami@...cinc.com>
> > Reviewed-by: Jeffrey Hugo <quic_jhugo@...cinc.com>
> 
> Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> 
> Thanks,
> Mani
> 
> > 
> > ---
> > changes since v4:
> > 	updating the commit text with more information.
> > changes since v3:
> > 	- Updating commit text for multiple versions of patches.
> > changes since v2:
> > 	- Updated comments in code.
> > changes since v1:
> > 	- Add an check to avoid ringing EV DB in mhi_process_ctrl_ev_ring().
> > ---
> >  drivers/bus/mhi/host/main.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/bus/mhi/host/main.c b/drivers/bus/mhi/host/main.c
> > index df0fbfe..1bbdb75 100644
> > --- a/drivers/bus/mhi/host/main.c
> > +++ b/drivers/bus/mhi/host/main.c
> > @@ -961,7 +961,9 @@ int mhi_process_ctrl_ev_ring(struct mhi_controller *mhi_cntrl,
> >  	}
> >  
> >  	read_lock_bh(&mhi_cntrl->pm_lock);
> > -	if (likely(MHI_DB_ACCESS_VALID(mhi_cntrl)))
> > +
> > +	/* Ring EV DB only if there is any pending element to process */
> > +	if (likely(MHI_DB_ACCESS_VALID(mhi_cntrl)) && count)
> >  		mhi_ring_er_db(mhi_event);
> >  	read_unlock_bh(&mhi_cntrl->pm_lock);
> >  
> > @@ -1031,7 +1033,9 @@ int mhi_process_data_event_ring(struct mhi_controller *mhi_cntrl,
> >  		count++;
> >  	}
> >  	read_lock_bh(&mhi_cntrl->pm_lock);
> > -	if (likely(MHI_DB_ACCESS_VALID(mhi_cntrl)))
> > +
> > +	/* Ring EV DB only if there is any pending element to process */
> > +	if (likely(MHI_DB_ACCESS_VALID(mhi_cntrl)) && count)
> >  		mhi_ring_er_db(mhi_event);
> >  	read_unlock_bh(&mhi_cntrl->pm_lock);
> >  
> > -- 
> > 2.7.4
> > 
> 

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