linux-kernel - Re: [PATCH 3/3] bus: mhi: core: Remove link

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Date:   Tue, 7 Apr 2020 11:28:09 +0530
From:   Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To:     Jeffrey Hugo <jhugo@...eaurora.org>
Cc:     hemantk@...eaurora.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] bus: mhi: core: Remove link_status() callback

Hi Jeff,

On Mon, Apr 06, 2020 at 03:04:37PM -0600, Jeffrey Hugo wrote:
> If the MHI core detects invalid data due to a PCI read, it calls into
> the controller via link_status() to double check that the link is infact
> down.  All in all, this is pretty pointless, and racy.  There are no good
> reasons for this, and only drawbacks.
> 
> Its pointless because chances are, the controller is going to do the same
> thing to determine if the link is down - attempt a PCI access and compare
> the result.  This does not make the link status decision any smarter.
> 
> Its racy because its possible that the link was down at the time of the
> MHI core access, but then recovered before the controller access.  In this
> case, the controller will indicate the link is not down, and the MHI core
> will precede to use a bad value as the MHI core does not attempt to retry
> the access.
> 
> Retrying the access in the MHI core is a bad idea because again, it is
> racy - what if the link is down again?  Furthermore, there may be some
> higher level state associated with the link status, that is now invalid
> because the link went down.
> 
> The only reason why the MHI core could see "invalid" data when doing a PCI
> access, that is actually valid, is if the register actually contained the
> PCI spec defined sentinel for an invalid access.  In this case, it is
> arguable that the MHI implementation broken, and should be fixed, not
> worked around.
> 
> Therefore, remove the link_status() callback before anyone attempts to
> implement it.
> 
> Signed-off-by: Jeffrey Hugo <jhugo@...eaurora.org>

LGTM. But as per the IRC discussion I'd like the mhi_reg_read() to be
implemented as a callback in mhi_controller struct inorder to truly make MHI
a PCI agnostic bus.

Since we don't have any controller driver in mainline, I think it is the
good time to do this change.

For this,

Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>

Thanks,
Mani

> ---
>  drivers/bus/mhi/core/init.c | 6 ++----
>  drivers/bus/mhi/core/main.c | 5 ++---
>  include/linux/mhi.h         | 2 --
>  3 files changed, 4 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/bus/mhi/core/init.c b/drivers/bus/mhi/core/init.c
> index b38359c..2af08d57 100644
> --- a/drivers/bus/mhi/core/init.c
> +++ b/drivers/bus/mhi/core/init.c
> @@ -812,10 +812,8 @@ int mhi_register_controller(struct mhi_controller *mhi_cntrl,
>  	if (!mhi_cntrl)
>  		return -EINVAL;
>  
> -	if (!mhi_cntrl->runtime_get || !mhi_cntrl->runtime_put)
> -		return -EINVAL;
> -
> -	if (!mhi_cntrl->status_cb || !mhi_cntrl->link_status)
> +	if (!mhi_cntrl->runtime_get || !mhi_cntrl->runtime_put ||
> +	    !mhi_cntrl->status_cb)
>  		return -EINVAL;
>  
>  	ret = parse_config(mhi_cntrl, config);
> diff --git a/drivers/bus/mhi/core/main.c b/drivers/bus/mhi/core/main.c
> index eb4256b..473278b8 100644
> --- a/drivers/bus/mhi/core/main.c
> +++ b/drivers/bus/mhi/core/main.c
> @@ -20,9 +20,8 @@ int __must_check mhi_read_reg(struct mhi_controller *mhi_cntrl,
>  {
>  	u32 tmp = readl(base + offset);
>  
> -	/* If there is any unexpected value, query the link status */
> -	if (PCI_INVALID_READ(tmp) &&
> -	    mhi_cntrl->link_status(mhi_cntrl))
> +	/* If the value is invalid, the link is down */
> +	if (PCI_INVALID_READ(tmp))
>  		return -EIO;
>  
>  	*out = tmp;
> diff --git a/include/linux/mhi.h b/include/linux/mhi.h
> index ad19960..be704a4 100644
> --- a/include/linux/mhi.h
> +++ b/include/linux/mhi.h
> @@ -335,7 +335,6 @@ struct mhi_controller_config {
>   * @syserr_worker: System error worker
>   * @state_event: State change event
>   * @status_cb: CB function to notify power states of the device (required)
> - * @link_status: CB function to query link status of the device (required)
>   * @wake_get: CB function to assert device wake (optional)
>   * @wake_put: CB function to de-assert device wake (optional)
>   * @wake_toggle: CB function to assert and de-assert device wake (optional)
> @@ -417,7 +416,6 @@ struct mhi_controller {
>  
>  	void (*status_cb)(struct mhi_controller *mhi_cntrl,
>  			  enum mhi_callback cb);
> -	int (*link_status)(struct mhi_controller *mhi_cntrl);
>  	void (*wake_get)(struct mhi_controller *mhi_cntrl, bool override);
>  	void (*wake_put)(struct mhi_controller *mhi_cntrl, bool override);
>  	void (*wake_toggle)(struct mhi_controller *mhi_cntrl);
> -- 
> Qualcomm Technologies, Inc. is a member of the
> Code Aurora Forum, a Linux Foundation Collaborative Project.