linux-kernel - Re: [PATCH v4 08/35] iommu/mediatek: Use kmalloc for protect buffer

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Date:   Wed, 16 Feb 2022 13:54:59 +0800
From:   Yong Wu <yong.wu@...iatek.com>
To:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
CC:     Robin Murphy <robin.murphy@....com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        Tomasz Figa <tfiga@...omium.org>,
        <linux-mediatek@...ts.infradead.org>,
        <srv_heupstream@...iatek.com>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <iommu@...ts.linux-foundation.org>,
        Hsin-Yi Wang <hsinyi@...omium.org>, <youlin.pei@...iatek.com>,
        <anan.sun@...iatek.com>, <xueqi.zhang@...iatek.com>,
        <yen-chang.chen@...iatek.com>, <mingyuan.ma@...iatek.com>,
        <yf.wang@...iatek.com>, <libo.kang@...iatek.com>,
        <chengci.xu@...iatek.com>, Joerg Roedel <joro@...tes.org>,
        Rob Herring <robh+dt@...nel.org>,
        "Matthias Brugger" <matthias.bgg@...il.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v4 08/35] iommu/mediatek: Use kmalloc for protect buffer

On Thu, 2022-01-27 at 12:08 +0100, AngeloGioacchino Del Regno wrote:
> Il 25/01/22 09:56, Yong Wu ha scritto:
> > No need zero for the protect buffer that is only accessed by the
> > IOMMU HW
> > translation fault happened.
> > 
> > Signed-off-by: Yong Wu <yong.wu@...iatek.com>
> 
> I would rather keep this a devm_kzalloc instead... the cost is very
> minimal and
> this will be handy when new hardware will be introduced, as it may
> require a bigger
> buffer: in that case, "older" platforms will use only part of it and
> we may get
> garbage data at the end.

Currently this is to avoid zero 512 bytes for all the platforms.

Sorry, I don't understand why it is unnecessary when the new hardware
requires a bigger buffer. If the buffer becomes bigger, then clearing
it to 0 need more cost. then this patch is more helpful?

The content in this buffer is garbage, we won't care about or analyse
it.

> 
> Regards,
> Angelo