Inefficient PCI DMA usage (was: [experimental patch] UHCI updates)

Inefficient PCI DMA usage (was: [experimental patch] UHCI updates)

[Johannes Erdfelt]

On Fri, Jan 19, 2001, Miles Lane <miles@megapathdsl.net> wrote:
> Johannes Erdfelt wrote:
> 
> > TODO
> > ----
> > - The PCI DMA architecture is horribly inefficient on x86 and ia64. The
> >   result is a page is allocated for each TD. This is evil. Perhaps a slab
> >   cache internally? Or modify the generic slab cache to handle PCI DMA
> >   pages instead?
> 
> This might be the kind of thing to run past Linus when the 2.5 tree 
> opens up.  Are these inefficiencies necessary evils due to workarounds 
> for whacky bugs in BIOSen or PCI chipsets or are they due to poor 
> design/implementation?

Looks like poor design/implementation. Or perhaps it was designed for
another reason than I want to use it for.

2.5 is probably where any core changes will happen, if any. But for now
I suspect I'll need to workaround it in my driver.

I should also check architectures other than x86 and ia64.

JE

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Re: Inefficient PCI DMA usage (was: [experimental patch] UHCI updates)

[Russell King]

Johannes Erdfelt writes:
> On Fri, Jan 19, 2001, Miles Lane <miles@megapathdsl.net> wrote:
> > Johannes Erdfelt wrote:
> > 
> > > TODO
> > > ----
> > > - The PCI DMA architecture is horribly inefficient on x86 and ia64. The
> > >   result is a page is allocated for each TD. This is evil. Perhaps a slab
> > >   cache internally? Or modify the generic slab cache to handle PCI DMA
> > >   pages instead?
> > 
> > This might be the kind of thing to run past Linus when the 2.5 tree 
> > opens up.  Are these inefficiencies necessary evils due to workarounds 
> > for whacky bugs in BIOSen or PCI chipsets or are they due to poor 
> > design/implementation?
> 
> Looks like poor design/implementation. Or perhaps it was designed for
> another reason than I want to use it for.

Why?  What are you trying to do?  Allocate one area per small structure?
Why not allocate one big area and allocate from that (like the tulip
drivers do for their TX and RX rings)?

I don't really know what you're trying to do/what the problem is because
there isn't enough context left in the original mail above, and I have
no idea whether the original mail appeared here or where I can read it.

> I should also check architectures other than x86 and ia64.

This is an absolute must.
   _____
  |_____| ------------------------------------------------- ---+---+-
  |   |         Russell King        rmk@arm.linux.org.uk      --- ---
  | | | | http://www.arm.linux.org.uk/personal/aboutme.html   /  /  |
  | +-+-+                                                     --- -+-
  /   |               THE developer of ARM Linux              |+| /|\
 /  | | |                                                     ---  |
    +-+-+ -------------------------------------------------  /\\\  |
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Re: Inefficient PCI DMA usage (was: [experimental patch] UHCI updates)

[Johannes Erdfelt]

On Sat, Jan 20, 2001, Russell King <rmk@arm.linux.org.uk> wrote:
> Johannes Erdfelt writes:
> > On Fri, Jan 19, 2001, Miles Lane <miles@megapathdsl.net> wrote:
> > > Johannes Erdfelt wrote:
> > > 
> > > > TODO
> > > > ----
> > > > - The PCI DMA architecture is horribly inefficient on x86 and ia64. The
> > > >   result is a page is allocated for each TD. This is evil. Perhaps a slab
> > > >   cache internally? Or modify the generic slab cache to handle PCI DMA
> > > >   pages instead?
> > > 
> > > This might be the kind of thing to run past Linus when the 2.5 tree 
> > > opens up.  Are these inefficiencies necessary evils due to workarounds 
> > > for whacky bugs in BIOSen or PCI chipsets or are they due to poor 
> > > design/implementation?
> > 
> > Looks like poor design/implementation. Or perhaps it was designed for
> > another reason than I want to use it for.
> 
> Why?  What are you trying to do?  Allocate one area per small structure?
> Why not allocate one big area and allocate from that (like the tulip
> drivers do for their TX and RX rings)?
> 
> I don't really know what you're trying to do/what the problem is because
> there isn't enough context left in the original mail above, and I have
> no idea whether the original mail appeared here or where I can read it.

I was hoping the context from the original TODO up there was sufficient
and it looked like it was enough.

TD's are around 32 bytes big (actually, they may be 48 or even 64 now, I
haven't checked recently). That's a waste of space for an entire page.

However, having every driver implement it's own slab cache seems a
complete waste of time when we already have the code to do so in
mm/slab.c. It would be nice if we could extend the generic slab code to
understand the PCI DMA API for us.

> > I should also check architectures other than x86 and ia64.
> 
> This is an absolute must.

Not really. The 2 interesting architectures are x86 and ia64 since
that's where you commonly see UHCI controllers. While you can add UHCI
controllers to most any other architecture which has PCI, you usually
see OHCI on those systems.

I was curious to see if any other architectures implemented it
differently and I was just expecting too much out of the API. You pretty
much confirmed my suspicions when you suggested doing what the tulip
driver does.

JE

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Re: [linux-usb-devel] Re: Inefficient PCI DMA usage (was: [experimentalpatch] UHCI updates)

[David Brownell]

The usb-ohci driver, widely used on non-x86 platforms, has hit
the same issue.  (Including on some ARM setups.)  An EHCI driver
(usb 2.0, 60 MByte/sec) under way has it too.

The alternative of having every device driver implement their
own simplified (?) kmem_cache code would just seem wrong; it'd
certainly be the cause of lots of bugs.  Simplest to just let
the kmem cache code reach into a different page pool, and make
drivers init their kmem caches appropriately.

I'd hope such a thing wouldn't need to wait for the 2.5 tree to
branch, since using 2.4 effectively on some non-Intel architectures
will require wider use of the pci_{alloc,free}_consistent() stuff.

- Dave



----- Original Message -----
From: Johannes Erdfelt <johannes@erdfelt.com>
To: Russell King <rmk@arm.linux.org.uk>
Cc: <linux-kernel@vger.kernel.org>; <linux-usb-devel@lists.sourceforge.net>
Sent: Saturday, January 20, 2001 9:34 AM
Subject: [linux-usb-devel] Re: Inefficient PCI DMA usage (was: [experimentalpatch] UHCI
updates)


> On Sat, Jan 20, 2001, Russell King <rmk@arm.linux.org.uk> wrote:
> > Johannes Erdfelt writes:
> > > On Fri, Jan 19, 2001, Miles Lane <miles@megapathdsl.net> wrote:
> > > > Johannes Erdfelt wrote:
> > > >
> > > > > TODO
> > > > > ----
> > > > > - The PCI DMA architecture is horribly inefficient on x86 and ia64. The
> > > > >   result is a page is allocated for each TD. This is evil. Perhaps a slab
> > > > >   cache internally? Or modify the generic slab cache to handle PCI DMA
> > > > >   pages instead?
> > > >
> > > > This might be the kind of thing to run past Linus when the 2.5 tree
> > > > opens up.  Are these inefficiencies necessary evils due to workarounds
> > > > for whacky bugs in BIOSen or PCI chipsets or are they due to poor
> > > > design/implementation?
> > >
> > > Looks like poor design/implementation. Or perhaps it was designed for
> > > another reason than I want to use it for.
> >
> > Why?  What are you trying to do?  Allocate one area per small structure?
> > Why not allocate one big area and allocate from that (like the tulip
> > drivers do for their TX and RX rings)?
> >
> > I don't really know what you're trying to do/what the problem is because
> > there isn't enough context left in the original mail above, and I have
> > no idea whether the original mail appeared here or where I can read it.
>
> I was hoping the context from the original TODO up there was sufficient
> and it looked like it was enough.
>
> TD's are around 32 bytes big (actually, they may be 48 or even 64 now, I
> haven't checked recently). That's a waste of space for an entire page.
>
> However, having every driver implement it's own slab cache seems a
> complete waste of time when we already have the code to do so in
> mm/slab.c. It would be nice if we could extend the generic slab code to
> understand the PCI DMA API for us.
>
> > > I should also check architectures other than x86 and ia64.
> >
> > This is an absolute must.
>
> Not really. The 2 interesting architectures are x86 and ia64 since
> that's where you commonly see UHCI controllers. While you can add UHCI
> controllers to most any other architecture which has PCI, you usually
> see OHCI on those systems.
>
> I was curious to see if any other architectures implemented it
> differently and I was just expecting too much out of the API. You pretty
> much confirmed my suspicions when you suggested doing what the tulip
> driver does.
>
> JE
>
>
> _______________________________________________
> linux-usb-devel@lists.sourceforge.net
> To unsubscribe, use the last form field at:
> http://lists.sourceforge.net/lists/listinfo/linux-usb-devel

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