Re: [PATCH 3/4] mmc: dw_mmc: Add desc_num field for clarity

[Ulf Hansson]

On Thu, 9 Apr 2026 at 09:48, Shawn Lin <shawn.lin@xxxxxxxxxxxxxx> wrote:
>
> The ring_size field in struct dw_mci is misleadingly named.
> Despite its name, it does not represent the size of the descriptor
> ring buffer in bytes, but rather the number of descriptors allocated
> within the fixed-size ring buffer.
>
> The actual ring buffer size is fixed at PAGE_SIZE (or DESC_RING_BUF_SZ,
> which equals PAGE_SIZE). Within this buffer, we allocate either
> struct idmac_desc or struct idmac_desc_64addr descriptors, and
> ring_size stores the count of these descriptors.
>
> This naming has caused confusion, as it's also used to set
> mmc->max_segs (the maximum number of scatter-gather segments),
> which logically corresponds to the number of descriptors, not a
> size in bytes.
>
> No functional change is introduced by this naming-only patch.
>
> Signed-off-by: Shawn Lin <shawn.lin@xxxxxxxxxxxxxx>
> ---
>
>  drivers/mmc/host/dw_mmc.c | 16 ++++++++--------
>  drivers/mmc/host/dw_mmc.h |  2 ++
>  2 files changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/mmc/host/dw_mmc.c b/drivers/mmc/host/dw_mmc.c
> index 20193ee..df6daa6 100644
> --- a/drivers/mmc/host/dw_mmc.c
> +++ b/drivers/mmc/host/dw_mmc.c
> @@ -491,12 +491,12 @@ static int dw_mci_idmac_init(struct dw_mci *host)
>
>         if (host->dma_64bit_address == 1) {
>                 struct idmac_desc_64addr *p;
> -               /* Number of descriptors in the ring buffer */
> -               host->ring_size =
> +
> +               host->desc_num =
>                         DESC_RING_BUF_SZ / sizeof(struct idmac_desc_64addr);
>
>                 /* Forward link the descriptor list */
> -               for (i = 0, p = host->sg_cpu; i < host->ring_size - 1;
> +               for (i = 0, p = host->sg_cpu; i < host->desc_num - 1;
>                                                                 i++, p++) {
>                         p->des6 = (host->sg_dma +
>                                         (sizeof(struct idmac_desc_64addr) *
> @@ -519,13 +519,13 @@ static int dw_mci_idmac_init(struct dw_mci *host)
>
>         } else {
>                 struct idmac_desc *p;
> -               /* Number of descriptors in the ring buffer */
> -               host->ring_size =
> +
> +               host->desc_num =
>                         DESC_RING_BUF_SZ / sizeof(struct idmac_desc);
>
>                 /* Forward link the descriptor list */
>                 for (i = 0, p = host->sg_cpu;
> -                    i < host->ring_size - 1;
> +                    i < host->desc_num - 1;
>                      i++, p++) {
>                         p->des3 = cpu_to_le32(host->sg_dma +
>                                         (sizeof(struct idmac_desc) * (i + 1)));
> @@ -2858,10 +2858,10 @@ static int dw_mci_init_host(struct dw_mci *host)
>
>         /* Useful defaults if platform data is unset. */
>         if (host->use_dma == TRANS_MODE_IDMAC) {
> -               mmc->max_segs = host->ring_size;
> +               mmc->max_segs = host->desc_num;
>                 mmc->max_blk_size = 65535;
>                 mmc->max_seg_size = 0x1000;
> -               mmc->max_req_size = mmc->max_seg_size * host->ring_size;
> +               mmc->max_req_size = mmc->max_seg_size * host->desc_num;
>                 mmc->max_blk_count = mmc->max_req_size / 512;
>         } else if (host->use_dma == TRANS_MODE_EDMAC) {
>                 mmc->max_segs = 64;
> diff --git a/drivers/mmc/host/dw_mmc.h b/drivers/mmc/host/dw_mmc.h
> index 14fb2b3..a05100c 100644
> --- a/drivers/mmc/host/dw_mmc.h
> +++ b/drivers/mmc/host/dw_mmc.h
> @@ -80,6 +80,7 @@ struct dw_mci_dma_slave {
>   * @cmd_status: Snapshot of SR taken upon completion of the current
>   *     command. Only valid when EVENT_CMD_COMPLETE is pending.
>   * @ring_size: Buffer size for idma descriptors.
> + * @desc_num: Number of idmac descriptors available.
>   * @dms: structure of slave-dma private data.
>   * @phy_regs: physical address of controller's register map
>   * @data_status: Snapshot of SR taken upon completion of the current
> @@ -185,6 +186,7 @@ struct dw_mci {
>         const struct dw_mci_dma_ops     *dma_ops;
>         /* For idmac */
>         unsigned int            ring_size;

ring_size is no longer used beyond this, please drop it.

> +       unsigned short          desc_num;
>
>         /* For edmac */
>         struct dw_mci_dma_slave *dms;
> --
> 2.7.4
>

Kind regards
Uffe