linux-kernel - Re: [PATCH] quiet down swiotlb warnings

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Date:	Fri, 1 Jun 2007 13:00:49 -0700
From:	Andrew Vasquez <andrew.vasquez@...gic.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Rik van Riel <riel@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] quiet down swiotlb warnings

On Fri, 01 Jun 2007, Andi Kleen wrote:

> On Fri, Jun 01, 2007 at 03:38:57PM -0400, Rik van Riel wrote:
> > Andi Kleen wrote:
> > 
> > >An pci_map_sg failing typically leads to an IO error and we've
> > >always printk'ed those. Otherwise people will wonder why they
> > >get EIO.
> > 
> > In some situations.  In this case the qla2xxx driver uses
> > the pci_map_sg() failure as a throttling mechanism and
> 
> First WTF does it need swiotlb anyways? QA hardware should
> be definitely DAC capable, shouldn't it?

yes, the card can support 64bit DMA transfers. but in this case the
'required' DMA mask returned from dma_get_required_mask() states that a
32bit mask would suffice.

Here's a snippet from the bugzilla report
(https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=219216):

        QLogic Fibre Channel HBA Driver
        PCI: Enabling device 0000:1f:00.0 (0140 -> 0143)
        ACPI: PCI Interrupt 0000:1f:00.0[A] -> GSI 16 (level, low) -> IRQ 16
        qla2xxx 0000:1f:00.0: Found an ISP2432, irq 16, iobase 0xffffc20000020000
        *** qla2x00_config_dma_addressing: required_mask set to 000000007fffffff.
        *** qla2x00_config_dma_addressing: required_mask has no high-dword bits set.
        *** qla2x00_config_dma_addressing: set consistent 64bit mask returned 0.
        *** qla2x00_config_dma_addressing: defaulting to 32bit mask/consistent-mask.
        qla2xxx 0000:1f:00.0: Configuring PCI space...

	Which tells me that a 32bit DMA mask is being set for dma_set_mask()
	and pci_set_consistent_dma_mask() since dma_get_required_mask() is
	returning back 7fffffff -- no upper-dword bits set...
	...

> > printing out all the warnings will actually slow down the
> > system.
> 
> Another reason is that there is a lot of code that
> still doesn't check the return values and when that
> happens you might get data corruption too.
> 
> > 
> > Andi, what do you propose as a solution?
> 
> A different interface; like I wrote in my earlier mail.
> 
> Another probabibility would be to have a blocking interface
> to swiotlb that won't fail. That would be the better solution
> long term, but i was told it is hard to fit into some current
> driver interfaces.
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