dmaengine: at_hdmac: fix residue computation

As claimed by the programmer datasheet and confirmed by the IP designer,
the Block Transfer Size (BTSIZE) bitfield of the Channel x Control A
Register (CTRLAx) always refers to a number of Source Width (SRC_WIDTH)
transfers.

Both the SRC_WIDTH and BTSIZE bitfields can be extacted from the CTRLAx
register to compute the DMA residue. So the 'tx_width' field is useless
and can be removed from the struct at_desc.

Before this patch, atc_prep_slave_sg() was not consistent: BTSIZE was
correctly initialized according to the SRC_WIDTH but 'tx_width' was always
set to reg_width, which was incorrect for MEM_TO_DEV transfers. It led to
bad DMA residue when 'tx_width' != SRC_WIDTH.

Also the 'tx_width' field was mostly set only in the first and last
descriptors. Depending on the kind of DMA transfer, this field remained
uninitialized for intermediate descriptors. The accurate DMA residue was
computed only when the currently processed descriptor was the first or the
last of the chain. This algorithm was a little bit odd. An accurate DMA
residue can always be computed using the SRC_WIDTH and BTSIZE bitfields
in the CTRLAx register.

Finally, the test to check whether the currently processed descriptor is
the last of the chain was wrong: for cyclic transfer, last_desc->lli.dscr
is NOT equal to zero, since set_desc_eol() is never called, but logically
equal to first_desc->txd.phys. This bug has a side effect on the
drivers/tty/serial/atmel_serial.c driver, which uses cyclic DMA transfer
to receive data. Since the DMA residue was wrong each time the DMA
transfer reaches the second (and last) period of the transfer, no more
data were received by the USART driver till the cyclic DMA transfer loops
back to the first period.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Acked-by: Torsten Fleischer <torfl6749@gmail.com>
Tested-by: Jirí Prchal <jiri.prchal@aksignal.cz>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/drivers/dma/at_hdmac_regs.h b/drivers/dma/at_hdmac_regs.h
index bc8d5ebedd19..7f5a08230f76 100644
--- a/drivers/dma/at_hdmac_regs.h
+++ b/drivers/dma/at_hdmac_regs.h
@@ -112,6 +112,7 @@
 #define		ATC_SRC_WIDTH_BYTE	(0x0 << 24)
 #define		ATC_SRC_WIDTH_HALFWORD	(0x1 << 24)
 #define		ATC_SRC_WIDTH_WORD	(0x2 << 24)
+#define		ATC_REG_TO_SRC_WIDTH(r)	(((r) >> 24) & 0x3)
 #define	ATC_DST_WIDTH_MASK	(0x3 << 28)	/* Destination Single Transfer Size */
 #define		ATC_DST_WIDTH(x)	((x) << 28)
 #define		ATC_DST_WIDTH_BYTE	(0x0 << 28)
@@ -182,7 +183,6 @@ struct at_lli {
  * @txd: support for the async_tx api
  * @desc_node: node on the channed descriptors list
  * @len: descriptor byte count
- * @tx_width: transfer width
  * @total_len: total transaction byte count
  */
 struct at_desc {
@@ -194,7 +194,6 @@ struct at_desc {
 	struct dma_async_tx_descriptor	txd;
 	struct list_head		desc_node;
 	size_t				len;
-	u32				tx_width;
 	size_t				total_len;
 
 	/* Interleaved data */