linux-kernel - Re: Proposed SDIO layer rework

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Date:	Fri, 5 Sep 2008 15:02:16 +0100
From:	Ben Dooks <ben-linux@...ff.org>
To:	Christer Weinigel <christer@...nigel.se>
Cc:	Ben Dooks <ben-linux@...ff.org>, linux-kernel@...r.kernel.org,
	Pierre Ossman <drzeus-list@...eus.cx>
Subject: Re: Proposed SDIO layer rework

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?

-- 
Ben

Q:      What's a light-year?
A:      One-third less calories than a regular year.

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