linux-kernel - Re: [PATCH v4] bus: mhi: host: Avoid ringing EV DB if there is no elements to process

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Date:   Wed, 22 Feb 2023 08:43:27 -0700
From:   Jeffrey Hugo <quic_jhugo@...cinc.com>
To:     Vivek Pernamitta <quic_vpernami@...cinc.com>, <mhi@...ts.linux.dev>
CC:     <quic_qianyu@...cinc.com>, <manivannan.sadhasivam@...aro.org>,
        <quic_vbadigan@...cinc.com>, <quic_krichai@...cinc.com>,
        <quic_skananth@...cinc.com>,
        Manivannan Sadhasivam <mani@...nel.org>,
        "Alex Elder" <elder@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Paul Davey <paul.davey@...iedtelesis.co.nz>,
        "open list:MHI BUS" <linux-arm-msm@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4] bus: mhi: host: Avoid ringing EV DB if there is no
 elements to process

On 2/22/2023 4:43 AM, Vivek Pernamitta wrote:
> Avoid ringing Event DB if there is no elements to process.

This is almost exactly identical to $SUBJECT and therefore redundant.

> 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.

I think that you could be more clear on why this is a problem that 
should be addressed.  Perhaps add a sentence like "These doorbell events 
needlessly interrupt the MHI device to checked for ring elements when 
there are none."

With the commit text updated, you can add
Reviewed-by: Jeffrey Hugo <quic_jhugo@...cinc.com>

> 
> Signed-off-by: Vivek Pernamitta <quic_vpernami@...cinc.com>
> 
> ---
> 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);
>