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Subject: Re: [PATCH 05/16] bus: mhi: core: Add support for ringing
 channel/event ring doorbells
To:     Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>,
        Arnd Bergmann <arnd@arndb.de>
Cc:     gregkh <gregkh@linuxfoundation.org>, smohanad@codeaurora.org,
        Kalle Valo <kvalo@codeaurora.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        hemantk@codeaurora.org,
        linux-arm-msm <linux-arm-msm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <20200123111836.7414-1-manivannan.sadhasivam@linaro.org>
 <20200123111836.7414-6-manivannan.sadhasivam@linaro.org>
 <CAK8P3a2pZEdsAi6YQ5z3YD=zD1iZLu+WPirhwmxeZ33k7sjkeg@mail.gmail.com>
 <20200123120050.GB8937@mani>
From:   Jeffrey Hugo <jhugo@codeaurora.org>
Message-ID: <e56387b3-6588-1153-48bd-ba92b5609769@codeaurora.org>
Date:   Thu, 23 Jan 2020 07:44:14 -0700
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On 1/23/2020 5:00 AM, Manivannan Sadhasivam wrote:
> Hi Arnd,
> 
> On Thu, Jan 23, 2020 at 12:39:06PM +0100, Arnd Bergmann wrote:
>> On Thu, Jan 23, 2020 at 12:19 PM Manivannan Sadhasivam
>> <manivannan.sadhasivam@linaro.org> wrote:
>>
>>> +int __must_check mhi_read_reg(struct mhi_controller *mhi_cntrl,
>>> +                             void __iomem *base, u32 offset, u32 *out)
>>> +{
>>> +       u32 tmp = readl_relaxed(base + offset);
>> ....
>>> +void mhi_write_reg(struct mhi_controller *mhi_cntrl, void __iomem *base,
>>> +                  u32 offset, u32 val)
>>> +{
>>> +       writel_relaxed(val, base + offset);
>>
>> Please avoid using _relaxed accessors by default, and use the regular
>> ones instead. There are a number of things that can go wrong with
>> the relaxed version, so ideally each caller should have a comment
>> explaining why this instance is safe without the barriers and why it
>> matters to not have it.
>>
>> If there are performance critical callers of mhi_read_reg/mhi_write_reg,
>> you could add mhi_read_reg_relaxed/mhi_write_reg_relaxed for those
>> and apply the same rules there.
>>
>> Usually most mmio accesses are only needed for reconfiguration or
>> other slow paths.
>>
> 
> Fair point. I'll defer to readl/writel APIs and I also need to add
> le32_to_cpu/cpu_to_le32 to them.

I would expect we would be using these in the "hot" path.

I'm a bit confused, I thought the convention was to put a comment why a 
barrier was necessary, now we should be putting a comment why a barrier 
is not necessary?


-- 
Jeffrey Hugo
Qualcomm Technologies, Inc. is a member of the
Code Aurora Forum, a Linux Foundation Collaborative Project.