linux-kernel - Re: [PATCH v3 2/6] bus: mhi: core: Remove link

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Date:   Tue, 28 Apr 2020 11:54:03 -0700
From:   Hemant Kumar <hemantk@...eaurora.org>
To:     Jeffrey Hugo <jhugo@...eaurora.org>,
        manivannan.sadhasivam@...aro.org
Cc:     bbhatt@...eaurora.org, linux-arm-msm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/6] bus: mhi: core: Remove link_status() callback



On 4/27/20 8:59 AM, 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>
> Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> ---
Reviewed-by: Hemant Kumar <hemantk@...eaurora.org>
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