linux-kernel - Re: [PATCH 05/16] bus: mhi: core: Add support for ringing channel/event ring doorbells

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Message-ID: <e56387b3-6588-1153-48bd-ba92b5609769@codeaurora.org>
Date:   Thu, 23 Jan 2020 07:44:14 -0700
From:   Jeffrey Hugo <jhugo@...eaurora.org>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Arnd Bergmann <arnd@...db.de>
Cc:     gregkh <gregkh@...uxfoundation.org>, smohanad@...eaurora.org,
        Kalle Valo <kvalo@...eaurora.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        hemantk@...eaurora.org,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 05/16] bus: mhi: core: Add support for ringing
 channel/event ring doorbells

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@...aro.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.