Re: [PATCH][1/4] usb: dma bounce buffer support using HCD_BOUNCE flag

["Magnus Damm" <magnus.damm@xxxxxxxxx>]

On Jan 14, 2008 1:03 PM, David Brownell <david-b@xxxxxxxxxxx> wrote:
> For what it's worth, the SA1111 has a somewhat similar requirement
> coming from a DMA erratum that makes every other megabyte invalid
> in terms of DMA addressing.  So it does bounce buffering ... but
> it's done transparently to usbcore, in its DMA mapping operations.

Yeah, that sounds like a similar requirement. I suppose we're talking
CONFIG_DMABOUNCE and arch/arm/common/dmabounce.c?

My first working version did hook into the DMA mapping operations for
the sh architecture. It used declared coherent memory as buffers which
wasn't efficient but seemed to work pretty well.

> Could you talk a bit more about why that approach isn't appropriate
> here?  My first guess would be that it just costs too much, and
> this is used on systems without manny spare resources.  Including
> for example ones where the memcpy overheads would hurt battery
> lifetimes, even if there were spare CPU cycles.

The sm501 chip can be hooked up to several different architectures,
and it can be attached to the system in multiple ways. I have some pci
cards here that are equipped with sm501/sm502 using pci. I use them on
x86. Then I have some embedded sh boards from renesas where the sm501
is hooked up using a local bus (just a CS bank I think). The sm501 is
equipped with local memory in both cases, and the device is unable to
do dma to other parts of the system memory - at least that's true when
it's hooked up on the local bus. I think that applies for the pci
configuration as well, but I there may be some magic pci bus mastering
mode that i'm unaware of.

So using architecture specific DMA mapping operations is of course
always an option, but since the chip can be hooked up to all sorts of
architectures i think it makes more sense to make it architecture
independent. The only requirement for the architecture would be to
have dma_declare_coherent_memory() support, and that interface seems
to fit perfectly for our needs in this case.

I also suspect that other usb host controllers exist with similar
requirements, so having host controller and architecture independent
support may be a good idea for other devices as well.

> ISTR this specific issue (needing memory from a particular area) came
> up on some of the Philips/NXP chips and some other hardware too.  But:
>
>
> > --- 0001/drivers/usb/core/hcd.h
> > +++ work/drivers/usb/core/hcd.h       2008-01-13 19:54:10.000000000 +0900
> > @@ -165,6 +165,7 @@ struct hc_driver {
> >
> >       int     flags;
> >  #define      HCD_MEMORY      0x0001          /* HC regs use memory (else I/O) */
> > +#define      HCD_BOUNCE      0x0002          /* HC needs bounce buffers */
>
> I think this makes "bounce buffers" become a too-overloaded term.
>
> These are most certainly bounce buffers ... but the significance
> isn't that detail, but a different one:  that they're HC-local
> memory.  That is, normal bounce buffers couldn't do the job.
>
> If the normal bounce buffer logic is inappropriate, then I'd suggest
> coming up with a less generic term and symbol.  Maybe HCD_LOCAL_MEM
> would capture all the relevant details.

Sure, I totally agree with you. I wasn't sure if a flag was the
correct way to go either, for a while I was thinking of adding two
callbacks for map and unmap and let them handle things for us. I do
think a flag is cleaner though.

Any comments? Can I just change the name of the flag to HCD_LOCAL_MEM
and repost?

Thanks!

/ magnus

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