On Thu, Jun 18, 2015 at 01:25:41PM +0200, Cyrille Pitchen wrote: > 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. > Applied, thanks -- ~Vinod -- To unsubscribe from this list: send the line "unsubscribe dmaengine" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
