linux-kernel - Re: Bug in add_dma_entry()'s debugging code

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Date:   Tue, 28 Nov 2023 16:54:23 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Christoph Hellwig <hch@....de>,
        Hamza Mahfooz <someguy@...ective-light.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Andrew <travneff@...il.com>,
        Ferry Toth <ferry.toth@...inga.info>,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        iommu@...ts.linux.dev,
        Kernel development list <linux-kernel@...r.kernel.org>,
        USB mailing list <linux-usb@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>
Subject: Re: Bug in add_dma_entry()'s debugging code

On 28/11/2023 4:34 pm, Andy Shevchenko wrote:
> On Tue, Nov 28, 2023 at 6:31 PM Robin Murphy <robin.murphy@....com> wrote:
>> On 28/11/2023 3:18 pm, Alan Stern wrote:
>>> On Tue, Nov 28, 2023 at 02:37:02PM +0100, Christoph Hellwig wrote:
> 
> ...
> 
>>>> The logical confcusion from that would be that IFF dma-debug is enabled on
>>>> any platform we need to set ARCH_DMA_MINALIGN to the cache line size.
>>>
>>> (IF, not IFF.)  And tell distributions that CONFIG_DMA_API_DEBUG is not
>>> meant for production systems but rather for kernel testing, right?
>>
>> Yikes, I'd hope that distros are heeding the warning that the Kconfig
>> calls out already. It's perhaps somewhat understated, as I'd describe
>> the performance impact to large modern systems with high-bandwidth I/O
>> as somewhere between "severe" and "crippling".
> 
> Independently on the distros configurations the (false positive)
> message(s) will make difficult to debug the actual things, shouldn't
> we have our code robust in any case?

Sure, I have no objection to making dma-debug more robust and useful for 
its intended purpose, I was just commenting on the fact that any 
potential change in behaviour from this should be of less concern to 
distros than the significant change in behaviour that enabling it 
*already* poses (i.e. globally serialising DMA operations and doing 
inherently slow stuff in what are normally expected to be fast paths).

Thanks,
Robin.