Subject: [PATCH 1/2] v1 ARM: sun4i: spi: Allow Rx transfers larger than FIFO size

From: [email-masked] (mrnuke)
Date: Wed, 19 Mar 2014 13:03:45 -0500

On Wednesday, March 19, 2014 05:55:07 PM Maxime Ripard wrote:
> Hi,
> 
> Thanks for working on this.
> 
> I have a few general comments first:
>   - Use a decent mailer for your patches. [...]
>   - The vX should be between the brackets, [...]
OOPS

>   - Those two patches should be merged together. [...]
Sure.

> > -	/* We don't support transfer larger than the FIFO */
> > -	if (tfr->len > SUN4I_FIFO_DEPTH)
> > +	if (tfr->len > SUN4I_MAX_XFER_SIZE)
> > +		return -EINVAL;
> 
> Why do you still need this test?
> 
SUN4I_MAX_XFER_SIZE is not the FIFO size. It is the max number we can write in 
SUN4I_BURST_CNT_REG. That's the maximum SPI burst size (16 MiB - 1) that the 
hardware supports. Anything larger, and we'd need to break up the transfer in 
several bursts. That's a different problem than the one I'm addressing.

> > -	sun4i_spi_drain_fifo(sspi, SUN4I_FIFO_DEPTH);
> > +	/* FIFO is drained during the interrupt handler */
> 
> Then don't mention it.
> 
Got it, boss.

> > +	/* Receive FIFO 3/4 full */
> > +	if (status & SUN4I_INT_CTL_RF_F34) {
> > +		sun4i_spi_drain_fifo(sspi, SUN4I_FIFO_DEPTH);
> > +		/* Only clear the interrupt _after_ draining the FIFO */
> > +		sun4i_spi_write(sspi, SUN4I_INT_STA_REG,
> > SUN4I_INT_CTL_RF_F34);
> 
> Not that it's important, but it really doesn't matter to do it before
> or after, the interrupts are disabled in the handler.
> 
I think this was one of those bits that keeps being set by hardware until the 
Rx FIFO is under 48 bytes (less than 3/4 full).  I'll have to look into this 
again.

Alex