Re: [PATCH v2 1/2] mmc: dw_mmc: implement option for configuring DMA threshold

[Shawn Lin <shawn.lin@rock-chips.com>]

在 2026/04/14 星期二 16:36, Kaustabh Chakraborty 写道:
> Some controllers, such as certain Exynos SDIO ones, are unable to
> perform DMA transfers of small amount of bytes properly. Following the
> device tree schema, implement the property to define the DMA transfer
> threshold (from a hard coded value of 16 bytes) so that lesser number of
> bytes can be transferred safely skipping DMA in such controllers. The
> value of 16 bytes stays as the default for controllers which do not
> define it. This value can be overridden by implementation-specific init
> sequences.
> 
> Signed-off-by: Kaustabh Chakraborty <kauschluss@disroot.org>
> ---
>   drivers/mmc/host/dw_mmc.c | 5 +++--
>   drivers/mmc/host/dw_mmc.h | 2 ++
>   2 files changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/mmc/host/dw_mmc.c b/drivers/mmc/host/dw_mmc.c
> index 20193ee7b73eb..9dd9fed4ccf49 100644
> --- a/drivers/mmc/host/dw_mmc.c
> +++ b/drivers/mmc/host/dw_mmc.c
> @@ -40,7 +40,6 @@
>   				 SDMMC_INT_RESP_ERR | SDMMC_INT_HLE)
>   #define DW_MCI_ERROR_FLAGS	(DW_MCI_DATA_ERROR_FLAGS | \
>   				 DW_MCI_CMD_ERROR_FLAGS)
> -#define DW_MCI_DMA_THRESHOLD	16
>   
>   #define DW_MCI_FREQ_MAX	200000000	/* unit: HZ */
>   #define DW_MCI_FREQ_MIN	100000		/* unit: HZ */
> @@ -821,7 +820,7 @@ static int dw_mci_pre_dma_transfer(struct dw_mci *host,
>   	 * non-word-aligned buffers or lengths. Also, we don't bother
>   	 * with all the DMA setup overhead for short transfers.
>   	 */
> -	if (data->blocks * data->blksz < DW_MCI_DMA_THRESHOLD)
> +	if (data->blocks * data->blksz < host->dma_threshold)
>   		return -EINVAL;
>   
>   	if (data->blksz & 3)
> @@ -3245,6 +3244,8 @@ int dw_mci_probe(struct dw_mci *host)
>   		goto err_clk_ciu;
>   	}
>   
> +	host->dma_threshold = 16;

I'd prefer to set it in dw_mci_alloc_host() instead of picking up
a random place to put it, for better code management.

> +
>   	if (host->rstc) {
>   		reset_control_assert(host->rstc);
>   		usleep_range(10, 50);
> diff --git a/drivers/mmc/host/dw_mmc.h b/drivers/mmc/host/dw_mmc.h
> index 42e58be74ce09..fc7601fba849f 100644
> --- a/drivers/mmc/host/dw_mmc.h
> +++ b/drivers/mmc/host/dw_mmc.h
> @@ -164,6 +164,8 @@ struct dw_mci {
>   	void __iomem		*fifo_reg;
>   	u32			data_addr_override;
>   	bool			wm_aligned;
> +	/* Configurable data byte threshold value for DMA transfer. */

No here, there is a long section of comment before struct dw_mci{ } that
describes each member of it, please add it there.

> +	u32			dma_threshold;
>   
>   	struct scatterlist	*sg;
>   	struct sg_mapping_iter	sg_miter;
>