Re: [PATCH] mmc: core: Lower max_seg_size if too high for DMA

[Ulf Hansson <ulf.hansson@linaro.org>]

On 31 October 2018 at 16:57, Tony Lindgren <tony@atomide.com> wrote:
> With CONFIG_DMA_API_DEBUG_SG a device may produce the following warning:
>
> "DMA-API: mapping sg segment longer than device claims to support"
>
> We default to 64KiB if a DMA engine driver does not initialize dma_parms
> and call dma_set_max_seg_size(). This may be lower that what many MMC
> drivers do with mmc->max_seg_size = mmc->max_blk_size * mmc->max_blk_count.
>
> Let's do a sanity check for max_seg_size being higher than what DMA
> supports in mmc_add_host() and lower it as needed.
>
> Cc: Kishon Vijay Abraham I <kishon@ti.com>
> Cc: Peter Ujfalusi <peter.ujfalusi@ti.com>
> Cc: Russell King <rmk+kernel@armlinux.org.uk>
> Reported-by: Russell King <rmk+kernel@armlinux.org.uk>
> Signed-off-by: Tony Lindgren <tony@atomide.com>
> ---
>  drivers/mmc/core/host.c | 16 ++++++++++++++++
>  1 file changed, 16 insertions(+)
>
> diff --git a/drivers/mmc/core/host.c b/drivers/mmc/core/host.c
> --- a/drivers/mmc/core/host.c
> +++ b/drivers/mmc/core/host.c
> @@ -13,6 +13,7 @@
>   */
>
>  #include <linux/device.h>
> +#include <linux/dma-mapping.h>
>  #include <linux/err.h>
>  #include <linux/idr.h>
>  #include <linux/of.h>
> @@ -415,6 +416,19 @@ struct mmc_host *mmc_alloc_host(int extra, struct device *dev)
>
>  EXPORT_SYMBOL(mmc_alloc_host);
>
> +static void mmc_check_max_seg_size(struct mmc_host *host)
> +{
> +       unsigned int max_seg_size = dma_get_max_seg_size(mmc_dev(host));

Is dma_get_max_seg_size() really intended to be called for any struct
device (representing the mmc controller) like this?

My understanding is that the dma_get_max_seg_size() is supposed to be
called by using the DMA engine device, no?

> +
> +       if (host->max_seg_size <= max_seg_size)
> +               return;
> +
> +       dev_info(mmc_dev(host), "Lowering max_seg_size for DMA: %u vs %u\n",
> +                host->max_seg_size, max_seg_size);
> +
> +       host->max_seg_size = max_seg_size;
> +}
> +
>  /**
>   *     mmc_add_host - initialise host hardware
>   *     @host: mmc host
> @@ -430,6 +444,8 @@ int mmc_add_host(struct mmc_host *host)
>         WARN_ON((host->caps & MMC_CAP_SDIO_IRQ) &&
>                 !host->ops->enable_sdio_irq);
>
> +       mmc_check_max_seg_size(host);
> +
>         err = device_add(&host->class_dev);
>         if (err)
>                 return err;
> --
> 2.19.1

Kind regards
Uffe