A spi transfer with zero length is not invalid. For example, such
transfer (len == 0 && delay_usecs != 0) can be used to achieve delay
before first CLK edge after chipselect assertion.
Signed-off-by: Atsushi Nemoto <[email protected]>
---
Though some discussion about zero length transfer were raised by this
patch last time, I think there were no explicit objection to this
patch itself.
diff --git a/drivers/spi/atmel_spi.c b/drivers/spi/atmel_spi.c
index 293b7ca..5dff5e0 100644
--- a/drivers/spi/atmel_spi.c
+++ b/drivers/spi/atmel_spi.c
@@ -606,7 +606,7 @@ static int atmel_spi_transfer(struct spi_device *spi, struct spi_message *msg)
return -ESHUTDOWN;
list_for_each_entry(xfer, &msg->transfers, transfer_list) {
- if (!(xfer->tx_buf || xfer->rx_buf)) {
+ if (!(xfer->tx_buf || xfer->rx_buf) && xfer->len) {
dev_dbg(&spi->dev, "missing rx or tx buf\n");
return -EINVAL;
}
Atsushi Nemoto <[email protected]> wrote:
> A spi transfer with zero length is not invalid. For example, such
> transfer (len == 0 && delay_usecs != 0) can be used to achieve delay
> before first CLK edge after chipselect assertion.
>
> Signed-off-by: Atsushi Nemoto <[email protected]>
> ---
> Though some discussion about zero length transfer were raised by this
> patch last time, I think there were no explicit objection to this
> patch itself.
Well, the patch doesn't seem to do any harm, but I can't see much of a
point to it either if zero-length transfers aren't going to be
allowed...
Also, if the length is zero, the driver will end up doing a memset with
length zero at some point. Is that allowed?
Haavard
On Sat, 29 Mar 2008 17:56:51 +0100, Haavard Skinnemoen <[email protected]> wrote:
> > Though some discussion about zero length transfer were raised by this
> > patch last time, I think there were no explicit objection to this
> > patch itself.
>
> Well, the patch doesn't seem to do any harm, but I can't see much of a
> point to it either if zero-length transfers aren't going to be
> allowed...
Well, some wired device might want long delay before first CLK edge.
I think most device do not have such constraint, but ...
Anyway, SPI framework does not reject it. So I think the controller
driver should accept it unless it is hard to support on that hardware.
> Also, if the length is zero, the driver will end up doing a memset with
> length zero at some point. Is that allowed?
I believe memset() with zero length should be allowed. It seems
ambiguous that dma_sync_single_for_device() with zero length is legal
or not. Actually, it seems OK on ARM and AVR32.
---
Atsushi Nemoto