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Date: Thu, 24 Mar 2022 19:00:04 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Serge Semin <Sergey.Semin@...kalelectronics.ru>
Cc: Gustavo Pimentel <gustavo.pimentel@...opsys.com>,
Vinod Koul <vkoul@...nel.org>,
Jingoo Han <jingoohan1@...il.com>,
Bjorn Helgaas <bhelgaas@...gle.com>,
Frank Li <Frank.Li@....com>,
Serge Semin <fancer.lancer@...il.com>,
Alexey Malahov <Alexey.Malahov@...kalelectronics.ru>,
Pavel Parkhomenko <Pavel.Parkhomenko@...kalelectronics.ru>,
Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
Rob Herring <robh@...nel.org>,
Krzysztof Wilczyński <kw@...ux.com>,
linux-pci@...r.kernel.org, dmaengine@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [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@...aro.org>
> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
> Signed-off-by: Serge Semin <Sergey.Semin@...kalelectronics.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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