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    SubjectRe: [PATCH 01/25] dmaengine: dw-edma: Drop dma_slave_config.direction field usage
    On Thu, Mar 24, 2022 at 04:48:12AM +0300, Serge Semin wrote:
    > The dma_slave_config.direction field usage in the DW eDMA driver has been
    > introduced in the commit bd96f1b2f43a ("dmaengine: dw-edma: support local
    > dma device transfer semantics"). Mainly the change introduced there was
    > correct (indeed DEV_TO_MEM means using RD-channel and MEM_TO_DEV -
    > WR-channel for the case of having eDMA accessed locally from
    > CPU/Application side), but providing an additional
    > MEM_TO_MEM/DEV_TO_DEV-based semantics was quite redundant if not to say
    > potentially harmful (when it comes to removing the denoted field). First
    > of all since the dma_slave_config.direction field has been marked as
    > obsolete (see [1] and the structure dc [2]) and will be discarded in
    > future, using it especially in a non-standard way is discouraged. Secondly
    > in accordance with the commit denoted above the default
    > dw_edma_device_transfer() semantics has been changed despite what it's
    > message said. So claiming that the method was left backward compatible was
    > wrong.
    >
    > Anyway let's fix the problems denoted above and simplify the
    > dw_edma_device_transfer() method by dropping the parsing of the
    > DMA-channel direction field. Instead of having that implicit
    > dma_slave_config.direction field semantic we can use the recently added
    > DW_EDMA_CHIP_LOCAL flag to distinguish between the local and remote DW
    > eDMA setups thus preserving both cases support. In addition to that an
    > ASCII-figure has been added to clarify the complication out.
    >
    > [1] Documentation/driver-api/dmaengine/provider.rst
    > [2] include/linux/dmaengine.h: dma_slave_config.direction
    >
    > Co-developed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
    > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
    > Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
    >
    > ---
    >
    > In accordance with agreement with Frank and Manivannan this patch is
    > supposed to be moved to the series:
    > Link: https://lore.kernel.org/dmaengine/20220310192457.3090-1-Frank.Li@nxp.com/
    > in place of the patch:
    > [PATCH v5 6/9] dmaengine: dw-edma: Don't rely on the deprecated "direction" member
    > Link: https://lore.kernel.org/dmaengine/20220310192457.3090-7-Frank.Li@nxp.com/
    > ---
    > drivers/dma/dw-edma/dw-edma-core.c | 49 +++++++++++++++++++++---------
    > 1 file changed, 34 insertions(+), 15 deletions(-)
    >
    > diff --git a/drivers/dma/dw-edma/dw-edma-core.c b/drivers/dma/dw-edma/dw-edma-core.c
    > index 5be8a5944714..e9e32ed74aa9 100644
    > --- a/drivers/dma/dw-edma/dw-edma-core.c
    > +++ b/drivers/dma/dw-edma/dw-edma-core.c
    > @@ -339,21 +339,40 @@ dw_edma_device_transfer(struct dw_edma_transfer *xfer)
    > if (!chan->configured)
    > return NULL;
    >
    > - switch (chan->config.direction) {
    > - case DMA_DEV_TO_MEM: /* local DMA */
    > - if (dir == DMA_DEV_TO_MEM && chan->dir == EDMA_DIR_READ)
    > - break;
    > - return NULL;
    > - case DMA_MEM_TO_DEV: /* local DMA */
    > - if (dir == DMA_MEM_TO_DEV && chan->dir == EDMA_DIR_WRITE)
    > - break;
    > - return NULL;
    > - default: /* remote DMA */
    > - if (dir == DMA_MEM_TO_DEV && chan->dir == EDMA_DIR_READ)
    > - break;
    > - if (dir == DMA_DEV_TO_MEM && chan->dir == EDMA_DIR_WRITE)
    > - break;
    > - return NULL;
    > + /*
    > + * Local Root Port/End-point Remote End-point
    > + * +-----------------------+ PCIe bus +----------------------+
    > + * | | +-+ | |
    > + * | DEV_TO_MEM Rx Ch <----+ +---+ Tx Ch DEV_TO_MEM |
    > + * | | | | | |
    > + * | MEM_TO_DEV Tx Ch +----+ +---> Rx Ch MEM_TO_DEV |
    > + * | | +-+ | |
    > + * +-----------------------+ +----------------------+
    > + *
    > + * 1. Normal logic:
    > + * If eDMA is embedded into the DW PCIe RP/EP and controlled from the
    > + * CPU/Application side, the Rx channel (EDMA_DIR_READ) will be used
    > + * for the device read operations (DEV_TO_MEM) and the Tx channel
    > + * (EDMA_DIR_WRITE) - for the write operations (MEM_TO_DEV).
    > + *
    > + * 2. Inverted logic:
    > + * If eDMA is embedded into a Remote PCIe EP and is controlled by the
    > + * MWr/MRd TLPs sent from the CPU's PCIe host controller, the Tx
    > + * channel (EDMA_DIR_WRITE) will be used for the device read operations
    > + * (DEV_TO_MEM) and the Rx channel (EDMA_DIR_READ) - for the write
    > + * operations (MEM_TO_DEV).
    > + *
    > + * It is the client driver responsibility to choose a proper channel
    > + * for the DMA transfers.
    > + */

    I think it'd be good to document this using some form in "enum dw_edma_dir"
    declaration.

    Thanks,
    Mani

    > + if (chan->dw->chip->flags & DW_EDMA_CHIP_LOCAL) {
    > + if ((chan->dir == EDMA_DIR_READ && dir != DMA_DEV_TO_MEM) ||
    > + (chan->dir == EDMA_DIR_WRITE && dir != DMA_MEM_TO_DEV))
    > + return NULL;
    > + } else {
    > + if ((chan->dir == EDMA_DIR_WRITE && dir != DMA_DEV_TO_MEM) ||
    > + (chan->dir == EDMA_DIR_READ && dir != DMA_MEM_TO_DEV))
    > + return NULL;
    > }
    >
    > if (xfer->type == EDMA_XFER_CYCLIC) {
    > --
    > 2.35.1
    >

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