linux-kernel - Re: [PATCH] PCI: mediatek-gen3: Fix translation window

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Message-ID: <899c7275-ccca-43bb-b1ae-a3403dd18622@baylibre.com>
Date:   Wed, 11 Oct 2023 17:38:11 +0200
From:   Alexandre Mergnat <amergnat@...libre.com>
To:     Jianjun Wang <jianjun.wang@...iatek.com>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>,
        Rob Herring <robh@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>
Cc:     linux-pci@...r.kernel.org, linux-mediatek@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Ryder Lee <ryder.lee@...iatek.com>, jieyy.yang@...iatek.com,
        chuanjia.liu@...iatek.com, qizhong.cheng@...iatek.com,
        jian.yang@...iatek.com
Subject: Re: [PATCH] PCI: mediatek-gen3: Fix translation window

On 11/10/2023 14:26, Jianjun Wang wrote:
> The size of translation table should be a power of 2, using fls() cannot 
> get the proper value when the size is not a power of 2. For example, 
> fls(0x3e00000) - 1 = 25, hence the PCIe translation window size will be 
> set to 0x2000000 instead of the expected size 0x3e00000. Fix translation 
> window by splitting the MMIO space to multiple tables if its size is not 
> a power of 2.

Hi Jianjun,

I've no knowledge in PCIE, so maybe what my suggestion is stupid:

Is it mandatory to fit the translation table size with 0x3e00000 (in 
this example) ?
I'm asking because you can have an issue by reaching the maximum 
translation table number.

Is it possible to just use only one table with the power of 2 size above 
0x3e00000 => 0x4000000 ( fls(0x3e00000) = 26 = 0x4000000). The downside 
of this method is wasting allocation space. AFAIK I already see this 
kind of method for memory protection/allocation in embedded systems, so 
I'm wondering if this method is safer than using multiple table for only 
one size which isn't a power of 2.

-- 
Regards,
Alexandre