linux-kernel - Re: [PATCH 09/18] soc: qcom: ipa: GSI transactions

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Date:   Mon, 20 May 2019 11:25:44 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Alex Elder <elder@...aro.org>
Cc:     David Miller <davem@...emloft.net>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        syadagir@...eaurora.org, mjavid@...eaurora.org,
        evgreen@...omium.org, Ben Chan <benchan@...gle.com>,
        Eric Caruso <ejcaruso@...gle.com>, abhishek.esse@...il.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/18] soc: qcom: ipa: GSI transactions

On Sun, May 19, 2019 at 7:11 PM Alex Elder <elder@...aro.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@...aro.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