Re: [PATCH 2/2] spi: bcm-mspi: Add support for Broadcom MSPI driver.

[Scott Branden]

On 15-05-14 11:28 AM, Florian Fainelli wrote:
> On 14/05/15 11:19, Scott Branden wrote:
> > Hi Mark,
> >
> > On 15-05-14 03:31 AM, Mark Brown wrote:
> > > On Wed, May 13, 2015 at 05:19:06PM -0700, Scott Branden wrote:
> > >
> > > > The purpose of this mspi interface is to connect to NOR flash.  There
> > > > are
> > > > other SPI interfaces on the devices used to connect to other SPI
> > > > devies.  We
> > > > don't have any need to support full duplex slaves on this port (NOR
> > > > have any
> > > > hardware wired this way - pun realized after typing this).
> > >
> > > Chip vendors often say this sort of thing and then get surprised by what
> > > their users choose to do, and even if it only ever gets used with flash
> > > all it would take is some new flash command which can use full duplex
> > > for something.  Please write the code so it at least tries to handle
> > > full duplex operation, if you can't test it fully that's not the end of
> > > the world.  It doesn't look like it should be particularly difficult.
> > Yes, there is always room for improvements in code.  In this case - it
> > really is not worth our time to add code we can't test.  We try to
> > deliver code that we can test and actually works.  Yes, if anyone needs
> > to use the mspi for full duplex operation code can be added in the
> > future - it is software.  This block has gone through many generations
> > of our SoCs and has only been used for this purpose - the bootROM boots
> > from this SPI only.  It is dedicated for this purpose.
> >
> > Also, there are other SPI blocks on the SoC that are used for connect
> > other SPI devices.  These SPI blocks are properly designed for
> > full-duplex operation.  This mspi block is designed for connecting to
> > NOR flash.
>
> This is an implementation detail and architectural choice that is
> specific to the Cygnus SoCs where a single MSPI is present, right?
>
> This same MSPI block is instantiated multiple times in BCM7xxx chips to
> interface to general purpose SPI slaves, ranging from Ethernet switches
> to bluetooth chips etc... those will definitively want full-duplex to be
> working.
>
> I would greatly appreciate if some thought to supporting full duplex
> transfers was given in this driver drop. If you cannot test that, then
> maybe just flag the driver has been half-duplex only for now...
>
> Thank you.
Hi Florian,
 Yes, we can flag that - but we have no understanding of how the block 
operates in this mode.  It would be great if your team could add the 
necessary full duplex support in a patch enhancement to the driver?

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