Date:	Fri, 26 Feb 2010 08:46:45 -0600
From:	Robert Hancock <hancockrwd@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	linux-usb@...r.kernel.org
Subject: Re: [RFC PATCH] fix problems with NETIF_F_HIGHDMA in networking 
	drivers

On Fri, Feb 26, 2010 at 3:36 AM, David Miller <davem@...emloft.net> wrote:
> From: Robert Hancock <hancockrwd@...il.com>
> Date: Mon, 22 Feb 2010 20:45:45 -0600
>
>> Many networking drivers have issues with the use of the NETIF_F_HIGHDMA flag.
>> This flag actually indicates whether or not the device/driver can handle
>> skbs located in high memory (as opposed to lowmem). However, many drivers
>> incorrectly treat this flag as indicating that 64-bit DMA is supported, which
>> has nothing to do with its actual function. It makes no sense to make setting
>> NETIF_F_HIGHDMA conditional on whether a 64-bit DMA mask has been set, as many
>> drivers do, since if highmem DMA is supported at all, it should work regardless
>> of whether 64-bit DMA is supported. Failing to set NETIF_F_HIGHDMA when it
>> should be can hurt performance on architectures which use highmem since it
>> results in needless data copying.
>>
>> This fixes up the networking drivers which currently use NETIF_F_HIGHDMA to
>> not do so conditionally on DMA mask settings.
>>
>> For the USB kaweth and usbnet drivers, this patch also uncomments and corrects
>> some code to set NETIF_F_HIGHDMA based on the USB host controller's DMA mask.
>> These drivers should be able to access highmem unless the host controller is
>> non-DMA-capable, which is indicated by the DMA mask being null.
>>
>> Signed-off-by: Robert Hancock <hancockrwd@...il.com>
>
> Well, if the device isn't using 64-bit DMA addressing and the platform
> uses direct (no-iommu) mapping of physical to DMA addresses , won't
> your change break things?  The device will get a >4GB DMA address or
> the DMA mapping layer will signal an error.
>
> That's really part of the what the issue is I think.
>
> So, this trigger the check in check_addr() in
> arch/x86/kernel/pci-nommu.c when such packets try to get mapped by the
> driver, right?
>
> That will make the DMA mapping call fail, and the packet will be
> dropped permanently.  And hey, on top of it, many of these drivers you
> remove the setting from don't even check the mapping call return
> values for errors.
>
> So even bigger breakage.  One example is drivers/net/8139cp.c,
> it just does dma_map_single() and uses the result.
>
> It really depends upon that NETIF_F_HIGHDMA setting for correct
> operation.
>
> And even if something like swiotlb is available, now we're going
> to do bounce buffering which is largely equivalent to what
> a lack of NETIF_F_HIGHDMA will do.  Except that once NETIF_F_HIGHDMA
> copies the packet to lowmem it will only do that once, whereas if
> the packet goes to multiple devices swiotlb might copy the packet
> to a bounce buffer multiple times.
>
> We definitely can't apply your patch as-is.

Hmm.. Yeah, there is a bit of a mess there. I'm thinking of the
particular example of i386 where you have 32-bit DMA devices with more
than 4GB of RAM. If you then allow the device to access highmem then
the DMA mapping API can get presented with addresses above 4GB and
AFAIK I don't think it can cope with that situation on that platform.

Problem is that the NETIF_F_HIGHDMA check is generally too restrictive
in that situation, and it's really conflating two things into one (the
genuine can't-access-highmem part, and the "oh by the way, if highmem
can be >4GB then we can't access that") . If you have 3GB of RAM on
i386 with one of these drivers, you'll have packets being bounced
through lowmem without any real reason. I'll have a look into things a
bit further..
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