Re: [PATCH v3] spi: spi-ep93xx: Use dma_data_direction for ep93xx_spi_dma_{finish,prepare}

[Nick Desaulniers]

Nathan,
Thanks for your patience and continued support working towards an
optimal solution to fix this warning. Some thoughts below.

On Fri, Oct 5, 2018 at 12:29 PM Nathan Chancellor
<natechancellor@xxxxxxxxx> wrote:
>
> Clang warns when one enumerated type is implicitly converted to another.
>
> drivers/spi/spi-ep93xx.c:342:62: warning: implicit conversion from
> enumeration type 'enum dma_transfer_direction' to different enumeration
> type 'enum dma_data_direction' [-Wenum-conversion]
>         nents = dma_map_sg(chan->device->dev, sgt->sgl, sgt->nents, dir);
>                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
> ./include/linux/dma-mapping.h:428:58: note: expanded from macro
> 'dma_map_sg'
> #define dma_map_sg(d, s, n, r) dma_map_sg_attrs(d, s, n, r, 0)
>                                ~~~~~~~~~~~~~~~~          ^
> drivers/spi/spi-ep93xx.c:348:57: warning: implicit conversion from
> enumeration type 'enum dma_transfer_direction' to different enumeration
> type 'enum dma_data_direction' [-Wenum-conversion]
>                 dma_unmap_sg(chan->device->dev, sgt->sgl, sgt->nents, dir);
>                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
> ./include/linux/dma-mapping.h:429:62: note: expanded from macro
> 'dma_unmap_sg'
> #define dma_unmap_sg(d, s, n, r) dma_unmap_sg_attrs(d, s, n, r, 0)
>                                  ~~~~~~~~~~~~~~~~~~          ^
> drivers/spi/spi-ep93xx.c:377:56: warning: implicit conversion from
> enumeration type 'enum dma_transfer_direction' to different enumeration
> type 'enum dma_data_direction' [-Wenum-conversion]
>         dma_unmap_sg(chan->device->dev, sgt->sgl, sgt->nents, dir);
>         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
> ./include/linux/dma-mapping.h:429:62: note: expanded from macro
> 'dma_unmap_sg'
> #define dma_unmap_sg(d, s, n, r) dma_unmap_sg_attrs(d, s, n, r, 0)
>                                  ~~~~~~~~~~~~~~~~~~          ^
> 3 warnings generated.
>
> dma_{,un}map_sg expect an enum of type dma_data_direction but this
> driver uses dma_transfer_direction for everything. Convert the driver to
> use dma_data_direction for these two functions.
>
> There are two places that strictly require an enum of type
> dma_transfer_direction: the direction member in struct dma_slave_config
> and the direction parameter in dmaengine_prep_slave_sg. To avoid using
> an explicit cast, add a simple function, ep93xx_dma_data_to_trans_dir,
> to safely map between the two types because they are not 1 to 1 in
> meaning.
>
> Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> ---
>
> v1 -> v2:
>
> * Fix escaped hash symbols for '#define' lines (\# -> #)
>
> v2 -> v3:
>
> * Instead of changing the dir parameter in the prepare and finish
>   functions, convert the driver to use dma_data_direction for everywhere
>   that makes sense. Add a helper function to convert between the two
>   types as there is still a couple of locations that need a transfer
>   enum (and no such function exists). Alternatively, the function could
>   just convert from transfer -> data and be used in dma_{,un}map_sg and
>   the rest of the driver be left alone. Hopefully I understood what Mika
>   wanted!
>
>  drivers/spi/spi-ep93xx.c | 37 ++++++++++++++++++++++++++-----------
>  1 file changed, 26 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/spi/spi-ep93xx.c b/drivers/spi/spi-ep93xx.c
> index f1526757aaf6..667da7964d63 100644
> --- a/drivers/spi/spi-ep93xx.c
> +++ b/drivers/spi/spi-ep93xx.c
> @@ -246,6 +246,19 @@ static int ep93xx_spi_read_write(struct spi_master *master)
>         return -EINPROGRESS;
>  }
>
> +static enum dma_transfer_direction
> +ep93xx_dma_data_to_trans_dir(enum dma_data_direction dir)
> +{
> +       switch (dir) {
> +       case DMA_TO_DEVICE:
> +               return DMA_MEM_TO_DEV;
> +       case DMA_FROM_DEVICE:
> +               return DMA_DEV_TO_MEM;
> +       default:
> +               return DMA_TRANS_NONE;
> +       }
> +}
> +
>  /**
>   * ep93xx_spi_dma_prepare() - prepares a DMA transfer
>   * @master: SPI master
> @@ -257,7 +270,7 @@ static int ep93xx_spi_read_write(struct spi_master *master)
>   */
>  static struct dma_async_tx_descriptor *
>  ep93xx_spi_dma_prepare(struct spi_master *master,
> -                      enum dma_transfer_direction dir)
> +                      enum dma_data_direction dir)
>  {
>         struct ep93xx_spi *espi = spi_master_get_devdata(master);
>         struct spi_transfer *xfer = master->cur_msg->state;
> @@ -277,9 +290,9 @@ ep93xx_spi_dma_prepare(struct spi_master *master,
>                 buswidth = DMA_SLAVE_BUSWIDTH_1_BYTE;
>
>         memset(&conf, 0, sizeof(conf));
> -       conf.direction = dir;
> +       conf.direction = ep93xx_dma_data_to_trans_dir(dir);
>
> -       if (dir == DMA_DEV_TO_MEM) {
> +       if (dir == DMA_FROM_DEVICE) {
>                 chan = espi->dma_rx;
>                 buf = xfer->rx_buf;
>                 sgt = &espi->rx_sgt;
> @@ -343,7 +356,9 @@ ep93xx_spi_dma_prepare(struct spi_master *master,
>         if (!nents)
>                 return ERR_PTR(-ENOMEM);
>
> -       txd = dmaengine_prep_slave_sg(chan, sgt->sgl, nents, dir, DMA_CTRL_ACK);
> +       txd = dmaengine_prep_slave_sg(chan, sgt->sgl, nents,
> +                                     ep93xx_dma_data_to_trans_dir(dir),

So we've already calculated this once already in this function, and
the value of `dir` hasn't changed since.  While the compiler might be
able to optimize out this common sub expression (CSE or GVN I think
are the compiler passes that could optimize this out), I think it
would be better to guarantee that this precalculated value is always
used by replacing the rematerialization of the value
(`ep93xx_dma_data_to_trans_dir(dir)`) with `conf.direction`.

> +                                     DMA_CTRL_ACK);
>         if (!txd) {
>                 dma_unmap_sg(chan->device->dev, sgt->sgl, sgt->nents, dir);
>                 return ERR_PTR(-ENOMEM);
> @@ -360,13 +375,13 @@ ep93xx_spi_dma_prepare(struct spi_master *master,
>   * unmapped.
>   */
>  static void ep93xx_spi_dma_finish(struct spi_master *master,
> -                                 enum dma_transfer_direction dir)
> +                                 enum dma_data_direction dir)
>  {
>         struct ep93xx_spi *espi = spi_master_get_devdata(master);
>         struct dma_chan *chan;
>         struct sg_table *sgt;
>
> -       if (dir == DMA_DEV_TO_MEM) {
> +       if (dir == DMA_FROM_DEVICE) {
>                 chan = espi->dma_rx;
>                 sgt = &espi->rx_sgt;
>         } else {
> @@ -381,8 +396,8 @@ static void ep93xx_spi_dma_callback(void *callback_param)
>  {
>         struct spi_master *master = callback_param;
>
> -       ep93xx_spi_dma_finish(master, DMA_MEM_TO_DEV);
> -       ep93xx_spi_dma_finish(master, DMA_DEV_TO_MEM);
> +       ep93xx_spi_dma_finish(master, DMA_TO_DEVICE);
> +       ep93xx_spi_dma_finish(master, DMA_FROM_DEVICE);
>
>         spi_finalize_current_transfer(master);
>  }
> @@ -392,15 +407,15 @@ static int ep93xx_spi_dma_transfer(struct spi_master *master)
>         struct ep93xx_spi *espi = spi_master_get_devdata(master);
>         struct dma_async_tx_descriptor *rxd, *txd;
>
> -       rxd = ep93xx_spi_dma_prepare(master, DMA_DEV_TO_MEM);
> +       rxd = ep93xx_spi_dma_prepare(master, DMA_FROM_DEVICE);
>         if (IS_ERR(rxd)) {
>                 dev_err(&master->dev, "DMA RX failed: %ld\n", PTR_ERR(rxd));
>                 return PTR_ERR(rxd);
>         }
>
> -       txd = ep93xx_spi_dma_prepare(master, DMA_MEM_TO_DEV);
> +       txd = ep93xx_spi_dma_prepare(master, DMA_TO_DEVICE);
>         if (IS_ERR(txd)) {
> -               ep93xx_spi_dma_finish(master, DMA_DEV_TO_MEM);
> +               ep93xx_spi_dma_finish(master, DMA_FROM_DEVICE);
>                 dev_err(&master->dev, "DMA TX failed: %ld\n", PTR_ERR(txd));
>                 return PTR_ERR(txd);
>         }
> --
> 2.19.0
>


-- 
Thanks,
~Nick Desaulniers