linux-kernel - Re: [RFC] mmc: cqhci: commit descriptors before setting the doorbell

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Message-ID: <2275bfdc-ff30-719e-ae78-8c8bd20b8c80@ti.com>
Date:   Wed, 16 Oct 2019 17:46:32 +0530
From:   Faiz Abbas <faiz_abbas@...com>
To:     Adrian Hunter <adrian.hunter@...el.com>,
        <linux-kernel@...r.kernel.org>, <linux-mmc@...r.kernel.org>
CC:     <ulf.hansson@...aro.org>, <asutoshd@...eaurora.org>,
        <riteshh@...eaurora.org>, <venkatg@...eaurora.org>
Subject: Re: [RFC] mmc: cqhci: commit descriptors before setting the doorbell

Adrian,

On 15/10/19 7:15 PM, Adrian Hunter wrote:
> On 15/10/19 10:55 AM, Faiz Abbas wrote:
>> Hi,
>>
>> On 15/10/19 12:08 AM, Faiz Abbas wrote:
>>> Add a write memory barrier to make sure that descriptors are actually
>>> written to memory before ringing the doorbell.
>>>
>>> Signed-off-by: Faiz Abbas <faiz_abbas@...com>
>>> ---
>>>
>>> This patch fixes a very infrequent ADMA error (1 out of 100 times) that
>>> I have been seeing after enabling command queuing for J721e.
>>> Also looking at memory-barriers.txt and this commit[1],
>>> it looks like we should be doing this before any descriptor write
>>> followed by a doorbell ring operation. It'll be nice if someone with more
>>> expertise in memory barriers can comment.
>>>
>>> [1] ad1a1b9cd67a ("scsi: ufs: commit descriptors before setting the
>>>     doorbell")
>>
>> So I see that cqhci_readl/writel() use readl/writel_relaxed() which
>> seems to be causing this issue. Should I just fix this by converting
>> those to readl/writel with memory barriers instead?
> 
> Perhaps we could do both changes i.e. add wmb() and convert to non-relaxed
> readl/writel
> 

readl is implemented as  readl_relaxed(); __rmb();
and
writel is implemented as wmb(); writel_relaxed();

I think another wmb() before writel will be redundant.

Maybe this patch is good enough in itself.

Thanks,
Faiz