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SubjectRe: Proposed SDIO layer rework
On Fri, Sep 05, 2008 at 03:02:16PM +0100, Ben Dooks wrote:
> On Fri, Sep 05, 2008 at 03:47:21PM +0200, Christer Weinigel wrote:
> > Ben Dooks wrote:
> > >>Most of the CPU is probably spent doing PIO transfers to the SDIO
> > >>controller, if DMA starts working in the s3cmci driver, the CPU load
> > >>difference will be even larger.
> > >
> > >I'm not sure if I'll get the time to look at this before the new kernel
> > >is released... anyway DMA may not be much of a win for smaller transfers
> > >anyway, since the setup (the cache will need to be cleaned out or the
> > >transfer memory made unbuffered) and complete time will add another
> > >IRQ's worth of response time. This means small transfers are probably
> > >better off using PIO.
> >
> > Yes. For the DMA-capable S3C SPI driver I wrote, I added some
> > thresholds, so for smaller transfers than a certain number of bytes, I
> > skip DMA and just do a polled/interrupt transfer instead. For short
> > transfers at high clock rates it's not even worth getting an interrupt
> > per byte, it's better to just busy wait for each byte, since the
> > interrupt overhead is larger than the time between each byte.
> >
> > A SDIO CMD/response packet is 48 bits, so at 25 MHz that is only about 4
> > us and I think the interrupt overhead is more than that. So if we
> > really want to squeeze every last clock cycle out of the SDIO driver it
> > may be better to busy wait for the end of simple CMD52s instead of using
> > the an interrupt to complete the transfer.
> >
> > I'll clean up my s3cmci patches and send them to you, but I can't
> > promise when I'll be done, so it'll probably have to wait for the next
> > kernel release.
> Any chance of getting a list of what you've got in progress and at-least
> the byte/word patch sorted out before the next merge window?

I should have added that at least if I know what you are working on,
then we can come to some arrangement on who should do what for the
next kernel release. I would like to get the fixups for word/byte code
in, and anything else that does not require huge amounts of testing.

Ben (,

'a smiley only costs 4 bytes'

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