Re: [spi-devel-general] Re: [PATCH] spi: Added spi master driver for Freescale MPC83xx SPI controller

[David Brownell <david-b@pacbell.net>]

On Friday 07 April 2006 2:16 am, Vitaly Wool wrote:
> Hi,
> 
> > I guess I'm surprised you're not using txrx_buffers() and having
> > that whole thing be IRQ driven, so the per-word cost eliminates
> > the task scheduling.  You already paid for IRQ handling ... why
> > not have it store the rx byte into the buffer, and write the tx
> > byte froom the other buffer?  That'd be cheaper than what you're
> > doing now ... in both time and code.  Only wake up a task at
> > the end of a given spi_transfer().
> >   
> I might be completely wrong here, but I was asking myself this very 
> question, and it looks like that's the way to implement full duplex 
> transfers.

Well, not the _only_ way.  The polling-type txrx_word() calls are
also full duplex.  My point is more that it's bad/inefficient to
incur both IRQ _and_ task switch overheads per word, when it would
be a lot simpler to just have the IRQ handler do its normal job.

(And that's even true if you've turned hard IRQ handlers into threads
for PREEMPT_RT or whatever.  In that case the "IRQ overhead" is a
task switch, but you're still saving _additional_ task switches.)


> For txrx_buffers to be properly implemented, you need to take a lot of 
> things into account. 

No more than the usual sort of driver thing.  SPI is a lot simpler
than most hardware, it's just a fancy shift register.  There's not
a lot of configuration possible, and not much can go wrong.


> The main idea is not to lose the data in the  
> receive buffer due to overflow, and thus you need to set up 'Rx buffer 
> not free' int or whatever similar which will actually trigger after the 
> first word is sent.

I think of it more as "after first word is received", but they are
the same event ... you can't send a word without receiving one, or
receive one without sending one.  SPI is fundamentally full duplex.

Now, if you have a FIFO as well as a shift register, then yes the
synchronization can get tricky.  It should still be possible to
have the RX and TX buffers be identical though.


> So therefore implementing txrx_buffers within these  
> conditions doesn't make much sense IMHO, unless you meant having a 
> separate thread to read from the Rx buffer, which is woken up on, say, 
> half-full Rx buffer.

I'll disagree on any need for a separate thread.  Kumar's IRQ handler
was already reading the RX byte and storing it.  However, he stored it
in a scratch byte ... rather than putting it right into the RX buffer.
There's no point to incurring extra costs like that (or like the extra
context switch overhead).

- Dave

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