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

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Date:   Mon, 20 May 2019 10:36:29 -0700
From:   Evan Green <evgreen@...omium.org>
To:     Alex Elder <elder@...aro.org>
Cc:     Arnd Bergmann <arnd@...db.de>, David Miller <davem@...emloft.net>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        syadagir@...eaurora.org, mjavid@...eaurora.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 Mon, May 20, 2019 at 9:50 AM Alex Elder <elder@...aro.org> wrote:
>
> On 5/20/19 11:34 AM, Evan Green wrote:
> > On Mon, May 20, 2019 at 7:44 AM Alex Elder <elder@...aro.org> wrote:
> >>
> >> On 5/20/19 9:43 AM, Arnd Bergmann wrote:
> >>> I have no idea how two 8-bit assignments could do that,
> >>> it sounds like a serious gcc bug, unless you mean two
> >>> 8-byte assignments, which would be within the range
> >>> of expected behavior. If it's actually 8-bit stores, please
> >>> open a bug against gcc with a minimized test case.
> >>
> >> Sorry, it's 8 *byte* assignments, not 8 bit.    -Alex
> >
> > Is it important to the hardware that you're writing all 128 bits of
>
> No, it is not important in the ways you are describing.
>
> We're just geeking out over how to get optimal performance.
> A single 128-bit write is nicer than two 64-bit writes,
> or more smaller writes.
>
> The hardware won't touch the TRE until the doorbell gets
> rung telling it that it is permitted to do so.  The doorbell
> is an I/O write, which implies a memory barrier, so the entire
> TRE will be up-to-date in memory regardless of whether we
> write it 128 bits or 8 bits at a time.
>

Ah, understood. Carry on!