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

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Date:   Fri, 18 Oct 2019 12:52:55 +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 16/10/19 5:46 PM, Faiz Abbas wrote:
> 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.
> 

Do you agree?

Thanks,
Faiz