linux-kernel - Re: [RESEND PATCH] arm64: v8.4: Support for new floating point multiplication variant

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Message-ID: <d39500ed-0d17-a8b2-b7e2-9e48b5d8d43a@huawei.com>
Date:   Tue, 12 Dec 2017 09:44:06 +0800
From:   gengdongjiu <gengdongjiu@...wei.com>
To:     Dave Martin <Dave.Martin@....com>
CC:     Mark Rutland <Mark.Rutland@....com>,
        "guohanjun@...wei.com" <guohanjun@...wei.com>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        Suzuki Poulose <Suzuki.Poulose@....com>,
        "Catalin Marinas" <Catalin.Marinas@....com>,
        "corbet@....net" <corbet@....net>,
        "Will Deacon" <Will.Deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxarm@...wei.com" <linuxarm@...wei.com>,
        "zhihui.gao@...wei.com" <zhihui.gao@...wei.com>,
        "huangshaoyu@...wei.com" <huangshaoyu@...wei.com>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "arvind.yadav.cs@...il.com" <arvind.yadav.cs@...il.com>,
        Robin Murphy <Robin.Murphy@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "zhanghaibin7@...wei.com" <zhanghaibin7@...wei.com>
Subject: Re: [RESEND PATCH] arm64: v8.4: Support for new floating point
 multiplication variant


On 2017/12/11 21:29, Dave Martin wrote:
>> Thanks for the point out.
>> In fact, this feature only adds two instructions:
>> FP16 * FP16 + FP32
>> FP16 * FP16 - FP32
>>
>> The spec call this bit to ID_AA64ISAR0_EL1.FHM, I do not know why it
>> will call "FHM", I  think call it "FMLXL" may be better, which can
>> stand for FMLAL/FMLSL instructions.
> Although "FHM" is cryptic, I think it makes sense to keep this as "FHM"
> to match the ISAR0 field name -- we've tended to follow this policy
> for other extension names unless there's a much better or more obvious
> name available
Agree with you, I also think the "FHM" is better.

> 
> For "FMLXL", new instructions might be added in the future that match
> the same pattern, and then "FMLXL" could become ambiguous.  So maybe
> this is not the best choice.
Ok.

> 
>>> Maybe something like "widening half-precision floating-point multiply
>>> accumulate" is acceptable wording consistent with the existing
>>> architecture, but I just made that up, so it's not official ;)
>> how about something like "performing a multiplication of each FP16
>> element of one vector with the corresponding FP16 element of a second
>> vector, and to add or subtract this without an intermediate rounding
>> to the corresponding FP32 element in a third vector."?
> We could have that, I guess.
Ok, thanks!

> 
>>>> instructions set. Let the userspace know about it via a
>>>> HWCAP bit and MRS emulation.