[U-Boot] [PATCH 3/7] eSPI: add eSPI controller support

[Mike Frysinger]

On Wednesday 26 August 2009 23:01:52 Hu Mingkai-B21284 wrote:
> From: Mike Frysinger [mailto:vapier at gentoo.org]
> > On Monday 16 March 2009 04:56:22 Hu Mingkai-B21284 wrote:
> > > But the eSPI controller integrate the chip select into the controller 
> > > itself, it use the transfer length to control the duration of the chip
> > > select signal.
> >
> > so you must tell the controller ahead of time how many bytes
> > are going to be transferred ?  this sounds like a retarded
> > hardware implementation to me.  how exactly do you support
> > arbitrary transfer lengths ?
>
> IIRC every transaction byte length control every transaction.

but what if you dont know the length of the transaction ahead of time ?  if 
you look at the spi flash framework, it'll write a few bytes (like the opcode 
to read the status register) and then it'll just keep reading 1 byte at a time 
(polling the status register).  there is no way of knowing the length ahead of 
time.  most of the SPI code in u-boot operates this way.

> > > The spi_flash_cmd_* function split the transfer into two steps: first
> > > the command transfer and then the data transfer. At the first step,
> > > the transfer length is set to the command length, and when transferred
> > > the command, the chip select will be negated, so the eSPI controller
> > > thinks the transfer is cancelled and doesn't transfer the data again.
> >
> > the transfer state is passed in via the flags parameter.
> > this way the SPI controller knows when a transfer starts
> > (SPI_XFER_BEGIN) and when it ends (SPI_XFER_END).
>
> As above, we can't use the start/end flags parameter.

then your controller wont be able to work with most (any?) of the code in u-
boot.  you need to reject every transfer that isnt done with both BEGIN and 
END flags set.
-mike
-------------- next part --------------
A non-text attachment was scrubbed...
Name: not available
Type: application/pgp-signature
Size: 836 bytes
Desc: This is a digitally signed message part.
Url : http://lists.denx.de/pipermail/u-boot/attachments/20090827/cbfe3ffe/attachment.pgp