linux-kernel - Re: [PATCH] dma-debug: disable DMA_API

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Date:	Sat, 6 Jun 2009 00:11:15 +0200
From:	Torsten Kaiser <just.for.lkml@...glemail.com>
To:	Joerg Roedel <joro@...tes.org>
Cc:	Joerg Roedel <joerg.roedel@....com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Linus Torvalds <torvalds@...ux-foundation.org>, mingo@...e.hu,
	lethal@...ux-sh.org, hancockrwd@...il.com, jens.axboe@...cle.com,
	bharrosh@...asas.com, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org
Subject: Re: [PATCH] dma-debug: disable DMA_API_DEBUG for now

On Fri, Jun 5, 2009 at 8:20 PM, Joerg Roedel <joro@...tes.org> wrote:
> On Fri, Jun 05, 2009 at 05:52:27PM +0200, Torsten Kaiser wrote:
>> Should there be a warning if more then one possible match were found?
>
> True. That would be better. But I tried to keep the code change as small
> as possible without disabling the feature completly.
>
>> * driver maps address 'a' with size 1
>> * driver maps same address 'a' with size 2
>> * driver wrongly unmaps the second allocation with size 1
>>   -> no warning, because the first allocation is returned
>
> Hmm, I am not sure if we can handle this situation correctly in the
> dma-debug code. There is no unique key to identify a mapping request
> which allows to assign an unmap request to it. Currently dma-debug uses
> device and dma-address. But that seems not to be sufficient. The
> best-fit algorithm is also a but fuzzy of course.

Maybe we just shouldn't try to handle it at all:

static void add_dma_entry(struct dma_debug_entry *entry)
{
        struct hash_bucket *bucket;
        unsigned long flags;

        bucket = get_hash_bucket(entry, &flags);
        if(hash_bucket_find(bucket, entry)) {
                printk(KERN_ERR "DMA-API: device mapped same address twice, "
                  "this use case cannot be handled currently -
disabling debugging\n");
                global_disable = true;
        }
        hash_bucket_add(bucket, entry);
        put_hash_bucket(bucket, &flags);
}

This would allow this feature to remain for most cased, but would also
prevent all false warnings.

Torsten
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