linux-kernel - Re: [PATCH] dma-debug: Kconfig for PREALLOC_DMA_DEBUG

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Date:   Mon, 3 Dec 2018 18:33:22 +0100
From:   Christoph Hellwig <hch@....de>
To:     John Garry <john.garry@...wei.com>
Cc:     Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>, Qian Cai <cai@....us>,
        netdev@...r.kernel.org, linuxarm@...wei.com,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        yisen.zhuang@...wei.com
Subject: Re: [PATCH] dma-debug: Kconfig for PREALLOC_DMA_DEBUG_ENTRIES

On Mon, Dec 03, 2018 at 11:56:11AM +0000, John Garry wrote:
> On 01/12/2018 16:36, Christoph Hellwig wrote:
>> On Fri, Nov 30, 2018 at 07:39:50PM +0000, Robin Murphy wrote:
>>> I was assuming the point was to also add something like
>>>
>>> 	default 131072 if HNS_ENET
>>>
>>> so that DMA debug doesn't require too much thought from the user. If they
>>> still have to notice the overflow message and empirically figure out a
>>> value that does work, rebuilding the kernel each time is far less
>>> convenient than simply adding "dma_debug_entries=..." to their kernel
>>> command line and rebooting, which they can do today. If they do already
>>> know up-front that the default will need overriding and what the
>>> appropriate value is, then the command line still seems seems just as
>>> convenient.
>>
>> I'm not so fond of random drivers changing the defaults.  My idea
>> was rather to have the config option so that the defconfig files for
>> the Hisilicon SOCs with this hardware could select a larger number
>> without making a total mess of the kernel configuration.
>>
>> If we really have to we could do different defaults, but I'd still
>> much rather do this on a arch/platform basis than specific drivers.
>
> As I understand, some drivers could even use much more than this (131072), 
> to such a point where I can't imagine that we would want to set an arch 
> default to support them. For this HNS_ENET case, it is arm64 specific so it 
> would be an arch defconfig.

But I'm not sure we could always do the right thing for everyone.

I think we might be better of trying to just dynamically allocate
entries when we run out of them instead of coming up with a perfect
number.