[PATCH] MMC/SD SPI-mode driver

[Aleksandr Rybalko]

On Fri, 5 Apr 2013 09:57:21 -0700
Adrian Chadd <adrian at freebsd.org> wrote:

> On 5 April 2013 07:57, Patrick Kelsey <kelsey at ieee.org> wrote:
> 
> > My understanding is that in the AR71xx (and possibly other related
> > Atheros SoCs), the boot flash can be read through memory mapped
> > access to a region starting at the reset address, as this is how
> > booting from a serial flash is accomplished.  In that scenario, the
> > SPI controller
> 
> Yes.
> 
> > will be translating each read access into an SPI bus transaction
> > that will produce the desired data from the flash.  Since the
> > hardware is performing the SPI signalling, you wouldn't have the
> > CPU overhead of bitbanging each SPI bit out.  Performance would
> > still be limited by the value set in the SPI clock divider register
> > (which controls the bit times on the wire) and the speed of the
> > clock that runs the SPI unit itself.
> 
> Yes.
> 
> The problem here is that we're clocking out 8 bits via bit banging,
> then we read the 8 bits from the Read register.
> 
> So luckily we don't have to bitbang the reads and writes, but still.
> 
> The SPI bus code looks pretty simplistic. There's no DELAY() entries
> in the SPI code either. The only thing we haven't changed is the SPI
> clock.
> 
> Now, what Ray at zrouter did was to experiment with turning SPI
> _flash_ reads into remap-and-memcpy. Look at
> ath79_spi_do_read_flash_data(). It's a cute hack to improve read
> performance, but you _absolutely need to know_  that it's a flash chip
> and that the bus is locked for the duration of the read. You need the
> SPI bus locked so you don't have someone come along during a transfer
> loop and try to fiddle with the SPI registers.

Yeah, found :)
here it is http://zrouter.org/hg/FreeBSD/head/rev/a92b653f5b46

> 
> The other thing that has shown up is that they're doing some transfers
> in more than 8 bits at a time? That may also help.
> 
> Thanks,
> 
> 
> Adrian
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WBW
-- 
Aleksandr Rybalko <ray at freebsd.org>