SPI, _sz fields in struct spi_command

[Ian Lepore]

On Wed, 2013-02-20 at 07:15 -0700, Warner Losh wrote:
> On Feb 20, 2013, at 5:21 AM, Aleksandr Rybalko wrote:
> 
> > Hello ARM and MIPS hackers!
> > 
> > Sorry for cross-post, but we have supported SPI devices on both
> > platforms (and seems no others).
> > 
> > Anyone know any reasons to keep both TX and RX _sz fields with same
> > values?
> 
> I wrote them to be separate for a reason....  I think I'd thought there'd be cases when you'd want to throw away the results or transmit garbage...  I can't think of why right now...
> 
> > sys/dev/flash/at45d.c
> > static int
> > at45d_get_mfg_info(device_t dev, uint8_t *resp)
> > {
> > 	...
> > 	cmd.tx_cmd_sz = cmd.rx_cmd_sz = 5;
> > 	...
> > }
> > 
> > or sys/dev/flash/mx25l.c
> > static int
> > mx25l_read(device_t dev, off_t offset, caddr_t data, off_t count)
> > {
> > 	...
> > 	cmd.tx_cmd_sz = 5;
> > 	cmd.rx_cmd_sz = 5;
> > 	...
> > }
> > 
> > That always require second but unused buffer or writable tx buffer. And
> > not all controllers able to TX with RX same time. (at least rt305x
> > can't). So if nobody have any objections, I will update drivers (SPI
> > controllers and SPI attached devices) to set unused _sz field to zero.
> > IIRC, I will require help with AT91 controller update, at least with
> > testing.
> 
> The AT91, I think, required a minimum size, which is why the at45d driver set it I think..
> 
> I can't imagine an SPI controller that can't do both, because when you write something with SPI, you usually have to read back the status at the same time...
> 
> Warner

I think with at91 you really must do same-sized xfers in both directions
or you'll get underflow/overflow errors from the hardware.  It might be
possible to just ignore the error, but even then the only useful way the
xfer sizes could be different is one of them would be zero.  Different
non-zero sizes just don't make sense.

-- Ian