netdev - Re: [PATCH] sky2: Fix WARNING: at lib/dma-debug.c:902 check

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Message-Id: <20100122002053X.fujita.tomonori@lab.ntt.co.jp>
Date:	Fri, 22 Jan 2010 00:22:10 +0900
From:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
To:	jarkao2@...il.com
Cc:	alan@...rguk.ukuu.org.uk, davem@...emloft.net,
	shemminger@...ux-foundation.org, mbreuer@...jas.com,
	akpm@...ux-foundation.org, flyboy@...il.com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	mchan@...adcom.com, pcnet32@...izon.net, romieu@...zoreil.com,
	mcarlson@...adcom.com
Subject: Re: [PATCH] sky2: Fix WARNING: at lib/dma-debug.c:902 check_sync

On Wed, 20 Jan 2010 23:53:22 +0100
Jarek Poplawski <jarkao2@...il.com> wrote:

> On Wed, Jan 20, 2010 at 10:24:14PM +0000, Alan Cox wrote:
> > > > Seems like an underlying bug in the DMA api. Maybe it just can't
> > > > handle operations on partial mapping.
> > > > 
> > > > Other drivers with same problem:
> > > >   bnx2, cassini, pcnet32, r8169, rrunner, skge, sungem, tg3, 
> > > 
> > > It seems using the same length (even without pci_unmap_len()) is
> > > crucial here, but I hope maintainers (added to CC) will take care.
> > 
> > The API needs fixing - if you've got a large mapping and you want to sync
> > part of it then we need to support that. Now it might well be that the
> > implementation on some braindead platform has to sync the entire thing,
> > and some implementations entire pages or cache lines.
> > 
> > You can't fix this in the drivers, they requested a service and they
> > don't have enough information nor is it their job to know about all the
> > platform specific rules.
> 
> Yes, the need to repeat some other values if there is a dedicated
> structure/pointer could be misleading. Btw, it seems to be a trivial
> overlooking since there is dma_sync_single_range() ready to use.

Yeah, dma_sync_single_range() enables you to do a partial sync. But
you must be really careful with a partial sync (as DMA-API.txt says).
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