Re: [PATCH 2.6.34] ehci-hcd: add option to enable 64-bit DMA support

On Wed, Feb 17, 2010 at 10:26 PM, Greg KH <[email protected]> wrote:
> On Wed, Feb 17, 2010 at 09:10:13PM -0600, Robert Hancock wrote:
>> Add a module parameter to allow the user to enable 64-bit DMA suppor=
t in EHCI,
>> which has been forcibly disabled since 2003 - see:
>>
>> http://www.mail-archive.com/linux-usb-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org/ms=
g17230.html
>>
>> At that time the comment was "it'd only matter on a few big Intel bo=
xes anyway",
>> however the situation is much different today when many new machines=
 have 4GB
>> or more of RAM and IOMMU/SWIOTLB are thus needlessly required for US=
B transfers.
>> For now, the support remains disabled by default and is controlled b=
y an
>> allow_64bit module parameter.
>>
>> Note that some USB device drivers may require updates to pass the DM=
A
>> capabilities up to their higher layers to avoid unnecessary IOMMU or=
 bounce-
>> buffer use (i.e. networking layer NETIF_F_HIGHDMA). Some of these ch=
ecks were
>> disabled by the patch listed above, and more may be required again t=
oday.
>> However, those previous checks were done incorrectly using dma_suppo=
rted,
>> which checks to see whether a device's DMA mask can be validly set t=
o a given
>> mask, not whether its previously set mask will accomodate the mask p=
assed in.
>>
>> Signed-off-by: Robert Hancock <[email protected]>
>
> What is the "advantage" that setting this option would allow people t=
o
> do that the code currently does not? =A0Is such an advantage measurab=
le at
> the slow rates that the EHCI driver controls?

I expect it would likely be quite system-dependent. However,
particularly with devices like high-speed USB storage on systems with
lots of RAM, especially 8GB or more (where more of the memory can't be
addressed with 32-bit than can), I suspect it would likely be
measurable in terms of CPU usage, throughput or both, especially if
there's no hardware IOMMU and software bounce-buffering is required.

>
> Is there any way to dynamically figure out if we can enable this or n=
ot?
> Adding module paramaters sucks, as they are hard to configure for mos=
t
> users, and they tend to be ignored.

Well, the option only has an effect if the controller indicates the
64-bit addressing feature flag in the first place. The only issue
would be with potential controllers that report the feature flag but
it doesn't work. Based on the libata experience with AHCI which has a
similar 64-bit feature flag, there are some controllers that didn't do
it properly, but not many (only ATI SB600 it appears, and only on
certain boards, as it seems it was a BIOS configuration issue).

Having the ability to turn it on manually for testing would likely be
the first step towards turning it on for all controllers that indicate
support, not the end goal in itself.

>
> And are you really ok with enabling this on a system-wide level, and =
not
> on a per-controller level? =A0Does that work properly on all systems?

As above, the only issue is if the controller reports the capability
but it doesn't function properly. If a user runs into the case where
one of their controllers doesn't work with it enabled, that likely
means that we need to add blacklisting for that device ID, etc. (or
blacklist 64-bit DMA for their platform in general).

>
> And if the system does not support it, and a user enables it, who is
> going to support their broken system? =A0:)

That would be me/us :-) We need those reports though, to know how to
proceed, and better to be from those who opted into testing for now..
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