linux-kernel - Re: [PATCH 2/2] media: verisilicon: change confusingly named relaxed register access

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Message-ID: <707b241b-c361-afde-c264-b3cd54f81f44@collabora.com>
Date:   Tue, 20 Jun 2023 10:00:56 +0200
From:   Benjamin Gaignard <benjamin.gaignard@...labora.com>
To:     Arnd Bergmann <arnd@...db.de>,
        Nicolas Dufresne <nicolas.dufresne@...labora.com>,
        Arnd Bergmann <arnd@...nel.org>,
        Ezequiel Garcia <ezequiel@...guardiasur.com.ar>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Mauro Carvalho Chehab <mchehab@...nel.org>
Cc:     Hans Verkuil <hverkuil-cisco@...all.nl>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        linux-media@...r.kernel.org, linux-rockchip@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] media: verisilicon: change confusingly named relaxed
 register access


Le 19/06/2023 à 21:26, Arnd Bergmann a écrit :
> On Mon, Jun 19, 2023, at 20:29, Nicolas Dufresne wrote:
>> Le lundi 19 juin 2023 à 16:49 +0200, Arnd Bergmann a écrit :
>>>> In this text you spoke about potential performance side effects of existing code
>>>> and your changes, but its left all very vague and theoretical. Have you done any
>>>> measurement ? Do you need help with the manner ?
>>> I don't have this hardware and have not done any measurements.
>>> Obviously the only point of using relaxed accessors is to
>>> improve performance in critical code paths, but from the way they
>>> are used here it seems that this was instead just an accident
>>> and nobody else did any comparisons either.
>>>
>>> My guess would be that if one wanted to speed up the register
>>> access, a better way would be to use a regmap cache to avoid
>>> reading registers when the contents are already known.
>> All I know is that for the majority of registers when programming stateless
>> codecs, each 32bit word of registers are fully written too, the read value is
>> not always meaningful (its a value from last time the HW has been triggered) and
>> should be ignored, so better to not do that. As for regmap, there is folks that
>> have reported regmap to be completely overkill for this type of hardware.
> Right, most likely neither the cache nor avoiding the readl() is necessary,
> and that was exactly my point to start with: don't add potentially dangerous
> microoptimizations like relaxed accessors unless the obvious optimizations
> are also needed and used.
>
> Obviously, testing my patch would still be a good idea before applying it.

I have test the patches on IMX8M (HEVC decoder) and RK3588 (AV1 decoder).
I notice not regression or problems, conformance tests scores remain identical.

For the both patches:
Tested-by: Benjamin Gaignard <benjamin.gaignard@...labora.com>

Thanks for the patches,
Benjamin

>
>         Arnd