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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Mon, 20 May 2019 11:25:44 +0200
Message-ID: <CAK8P3a0FfSvTF8kkQ8pyKFNX9-fSXvtEyMBYTjtM+VOPxMPkWg@mail.gmail.com>
Subject: Re: [PATCH 09/18] soc: qcom: ipa: GSI transactions
To:     Alex Elder <elder@linaro.org>
Cc:     David Miller <davem@davemloft.net>,
        Bjorn Andersson <bjorn.andersson@linaro.org>,
        Ilias Apalodimas <ilias.apalodimas@linaro.org>,
        syadagir@codeaurora.org, mjavid@codeaurora.org,
        evgreen@chromium.org, Ben Chan <benchan@google.com>,
        Eric Caruso <ejcaruso@google.com>, abhishek.esse@gmail.com,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Sun, May 19, 2019 at 7:11 PM Alex Elder <elder@linaro.org> wrote:
> On 5/17/19 1:44 PM, Alex Elder wrote:
> > On 5/17/19 1:33 PM, Arnd Bergmann wrote:
> >> On Fri, May 17, 2019 at 8:08 PM Alex Elder <elder@linaro.org>
>
> So it seems that I must *not* apply a volatile qualifier,
> because doing so restricts the compiler from making the
> single instruction optimization.

Right, I guess that makes sense.

> If I've missed something and you have another suggestion for
> me to try let me know and I'll try it.

A memcpy() might do the right thing as well. Another idea would
be a cast to __int128 like

#ifdef CONFIG_ARCH_SUPPORTS_INT128
typedef __int128 tre128_t;
#else
typedef struct { __u64 a; __u64 b; } tre128_t;
#else

static inline void set_tre(struct gsi_tre *dest_tre, struct gs_tre *src_tre)
{
     *(volatile tre128_t *)dest_tre = *(tre128_t *)src_tre;
}

      Arnd