linux-kernel - Re: [RFC 09/13] iommu/arm-smmu: Make use of dev_64bit_mmio

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Date:   Tue, 02 Mar 2021 13:42:16 +0100
From:   Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     DTML <devicetree@...r.kernel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Will Deacon <will@...nel.org>,
        Neil Armstrong <narmstrong@...libre.com>,
        David Woodhouse <dwmw2@...radead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Rob Herring <robh+dt@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Robin Murphy <robin.murphy@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [RFC 09/13] iommu/arm-smmu: Make use of
 dev_64bit_mmio_supported()

Hi Arnd, thanks for the reviews!

On Tue, 2021-03-02 at 10:32 +0100, Arnd Bergmann wrote:
> On Fri, Feb 26, 2021 at 3:03 PM Nicolas Saenz Julienne
> <nsaenzjulienne@...e.de> wrote:
> 
> >         if (smmu->impl && unlikely(smmu->impl->write_reg))
> >                 smmu->impl->write_reg(smmu, page, offset, val);
> > -       else
> > +       else if (dev_64bit_mmio_supported(smmu->dev))
> >                 writel_relaxed(val, arm_smmu_page(smmu, page) + offset);
> > +       else
> > +               hi_lo_writeq_relaxed(val, arm_smmu_page(smmu, page) + offset);
> >  }
> 
> This is a writel_relaxed(), not a writeq_relaxed(), so I suppose you don't
> have to change it at all.

Yes, that was silly of me. I was worrying about the semantics of the whole
thing, and missed basic stuff like this.

> > +       else if (dev_64bit_mmio_supported(smmu->dev))
> > +               return readq_relaxed(arm_smmu_page(smmu, page) + offset);
> > +       else
> > +               return hi_lo_readq_relaxed(arm_smmu_page(smmu, page) + offset);
> > }
> 
> 
> I see this pattern repeat across multiple drivers. I think Christoph
> had originally
> suggested folding the if/else logic into the writel_relaxed() that is defined in
> include/linux/io-64-nonatomic-hi-lo.h, but of course that doesn't work if you
> need to pass a device pointer.
> 
> It might still make sense to have another wrapper in that same file though,
> something like
> 
> static inline hi_lo_writeq_relaxed_if_possible(struct device *dev, __u64 val,
>                     volatile void __iomem *addr)
> {
>        if (dev_64bit_mmio_supported(smmu->dev)) {
>               readq_relaxed(arm_smmu_page(smmu, page) + offset);
>        } else {
>                writel_relaxed(val >> 32, addr + 4);
>                writel_relaxed(val, addr);
>        }
> }

I like the idea. I'll try to integrate it into the next revision.

Regards,
Nicolas


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