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Subject: Re: [PATCH] dma-debug: dynamic allocation of hash table
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Eric Dumazet <edumazet@google.com>, Christoph Hellwig <hch@lst.de>,
        Joerg Roedel <jroedel@suse.de>,
        Linux IOMMU <iommu@lists.linux-foundation.org>,
        Eric Dumazet <eric.dumazet@gmail.com>,
        linux-kernel <linux-kernel@vger.kernel.org>
References: <20200130191049.190569-1-edumazet@google.com>
 <e0a0ffa9-3721-4bac-1c8f-bcbd53d22ba1@arm.com>
 <CAMuHMdVSyD62nvRmN-v6CbJ2UyqH=d7xdVeCD8_X5us+mvCXUQ@mail.gmail.com>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <a305bd1f-8142-6557-4228-aae10c5114e1@arm.com>
Date:   Fri, 31 Jan 2020 11:33:48 +0000
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On 2020-01-31 9:06 am, Geert Uytterhoeven wrote:
> Hi Robin,
> 
> On Fri, Jan 31, 2020 at 12:46 AM Robin Murphy <robin.murphy@arm.com> wrote:
>> On 2020-01-30 7:10 pm, Eric Dumazet via iommu wrote:
>>> Increasing the size of dma_entry_hash size by 327680 bytes
>>> has reached some bootloaders limitations.
>>
>> [ That might warrant some further explanation - I don't quite follow how
>> this would relate to a bootloader specifically :/ ]
> 
> Increasing the size of a static array increases kernel size.
> Some (all? ;-) bootloaders have limitations on the maximum size of a
> kernel image they can boot (usually something critical gets overwritten
> when handling a too large image).  While boot loaders can be fixed and
> upgraded, this is usually much more cumbersome than updating the
> kernel.

Ah, OK - I'm all too familiar with bootloaders having image size limits, 
but I'm also used to implicitly-initialised statics being collected into 
a runtime-initialised .bss section, so I hadn't realised that there 
might still be platforms where that space is actually allocated in the 
image at link-time.

> Besides, a static array always consumes valuable unswapable memory,
> even when the feature would not be used (e.g. disabled by a command
> line option).

Indeed, and that alone might have been a reasonable rationale for the 
patch - I was merely querying the wording of the commit message, not its 
intent :)

Thanks,
Robin.