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SubjectRe: [PATCH 09/18] soc: qcom: ipa: GSI transactions
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On 5/20/19 4:25 AM, Arnd Bergmann wrote:
> 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

I find memcpy() does the right thing.

> be a cast to __int128 like

I find that my environment supports 128 bit integers. But...

> #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;
> }
...this produces two 8-bit assignments. Could it be because
it's implemented as two 64-bit values? I think so. Dropping
the volatile qualifier produces a single "stp" instruction.

The only other thing I thought I could do to encourage
the compiler to do the right thing is define the type (or
variables) to have 128-bit alignment. And doing that for
the original simple assignment didn't change the (desirable)
outcome, but I don't think it's really necessary in this
case, considering the single instruction uses two 64-bit
registers.

I'm going to leave it as it was originally; it's the simplest:
*dest_tre = tre;

I added a comment about structuring the code this way with
the intention of getting the single instruction. If a different
compiler produces different result

-Alex

>
> Arnd
>

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