• Cyrille Pitchen's avatar
    dmaengine: at_hdmac: fix residue computation · 93dce3a6
    Cyrille Pitchen authored
    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: default avatarCyrille Pitchen <cyrille.pitchen@atmel.com>
    Acked-by: default avatarTorsten Fleischer <torfl6749@gmail.com>
    Tested-by: default avatarJirí Prchal <jiri.prchal@aksignal.cz>
    Acked-by: default avatarNicolas Ferre <nicolas.ferre@atmel.com>
    Signed-off-by: default avatarVinod Koul <vinod.koul@intel.com>
    93dce3a6
at_hdmac.c 56.8 KB