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

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Message-ID: <CAK8P3a1ACFdUdEywEQRAJ=HRYOp+0Y051ffNgAgqvSgjOEGyMA@mail.gmail.com>
Date:   Thu, 23 Jan 2020 13:44:32 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
Cc:     gregkh <gregkh@...uxfoundation.org>, smohanad@...eaurora.org,
        Jeffrey Hugo <jhugo@...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 Thu, Jan 23, 2020 at 1:01 PM Manivannan Sadhasivam
<manivannan.sadhasivam@...aro.org> wrote:
> 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.

What do you need the byteswap for? All of the above already
assume that the registers are little-endian.

       Arnd