linux-kernel - Re: [PATCH mmc-next v3 3/3] mmc: sdhci-of-dwcmshc: solve 128MB DMA boundary limitation

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Message-ID: <01010164e91d7bda-aa616cbf-7bb2-4fe4-85e5-a18e16433fde-000000@us-west-2.amazonses.com>
Date:   Mon, 30 Jul 2018 02:56:20 +0000
From:   Matthew Leon <matthewleon@...ux.com>
To:     Jisheng Zhang <Jisheng.Zhang@...aptics.com>
Cc:     Adrian Hunter <adrian.hunter@...el.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH mmc-next v3 3/3] mmc: sdhci-of-dwcmshc: solve 128MB DMA
 boundary limitation

Hey Jisheng,

Shouldn't we be splitting until all DMA blocks are less than 128M boundary?
I am a noob, but I think we should be prepared for boundaries that when
split in two, will still be greater than 128M. Feel free to disagree but
please explain why I may be wrong. Thank-you.

Sincerely,
Matthew Leon

On Sun, Jul 29, 2018 at 10:46 PM, Jisheng Zhang <Jisheng.Zhang@...aptics.com
> wrote:

> When using DMA, if the DMA addr spans 128MB boundary, we have to split
> the DMA transfer into two so that each one doesn't exceed the boundary.
>
> Signed-off-by: Jisheng Zhang <Jisheng.Zhang@...aptics.com>
> ---
>  drivers/mmc/host/sdhci-of-dwcmshc.c | 43 +++++++++++++++++++++++++++++
>  1 file changed, 43 insertions(+)
>
> diff --git a/drivers/mmc/host/sdhci-of-dwcmshc.c
> b/drivers/mmc/host/sdhci-of-dwcmshc.c
> index 1b7cd144fb01..e890fc8f5284 100644
> --- a/drivers/mmc/host/sdhci-of-dwcmshc.c
> +++ b/drivers/mmc/host/sdhci-of-dwcmshc.c
> @@ -8,21 +8,52 @@
>   */
>
>  #include <linux/clk.h>
> +#include <linux/mm.h>
>  #include <linux/module.h>
>  #include <linux/of.h>
> +#include <linux/sizes.h>
>
>  #include "sdhci-pltfm.h"
>
> +#define BOUNDARY_OK(addr, len) \
> +       ((addr | (SZ_128M - 1)) == ((addr + len - 1) | (SZ_128M - 1)))
> +
>  struct dwcmshc_priv {
>         struct clk      *bus_clk;
>  };
>
> +/*
> + * if DMA addr spans 128MB boundary, we split the DMA transfer into two
> + * so that the DMA transfer doesn't exceed the boundary.
> + */
> +static unsigned int dwcmshc_adma_write_desc(struct sdhci_host *host,
> +                                           void *desc, dma_addr_t addr,
> +                                           int len, unsigned int cmd)
> +{
> +       int tmplen, offset;
> +
> +       if (likely(!len || BOUNDARY_OK(addr, len)))
> +               return _sdhci_adma_write_desc(host, desc, addr, len, cmd);
> +
> +       offset = addr & (SZ_128M - 1);
> +       tmplen = SZ_128M - offset;
> +       _sdhci_adma_write_desc(host, desc, addr, tmplen, cmd);
> +
> +       addr += tmplen;
> +       len -= tmplen;
> +       desc += host->desc_sz;
> +       _sdhci_adma_write_desc(host, desc, addr, len, cmd);
> +
> +       return host->desc_sz * 2;
> +}
> +
>  static const struct sdhci_ops sdhci_dwcmshc_ops = {
>         .set_clock              = sdhci_set_clock,
>         .set_bus_width          = sdhci_set_bus_width,
>         .set_uhs_signaling      = sdhci_set_uhs_signaling,
>         .get_max_clock          = sdhci_pltfm_clk_get_max_clock,
>         .reset                  = sdhci_reset,
> +       .adma_write_desc        = dwcmshc_adma_write_desc,
>  };
>
>  static const struct sdhci_pltfm_data sdhci_dwcmshc_pdata = {
> @@ -36,12 +67,24 @@ static int dwcmshc_probe(struct platform_device *pdev)
>         struct sdhci_host *host;
>         struct dwcmshc_priv *priv;
>         int err;
> +       u32 extra;
>
>         host = sdhci_pltfm_init(pdev, &sdhci_dwcmshc_pdata,
>                                 sizeof(struct dwcmshc_priv));
>         if (IS_ERR(host))
>                 return PTR_ERR(host);
>
> +       /*
> +        * The DMA descriptor table number is calculated as the maximum
> +        * number of segments times 2, to allow for an alignment
> +        * descriptor for each segment, plus 1 for a nop end descriptor,
> +        * plus extra number for cross 128M boundary handling.
> +        */
> +       extra = DIV_ROUND_UP(totalram_pages, SZ_128M / PAGE_SIZE);
> +       if (extra > SDHCI_MAX_SEGS)
> +               extra = SDHCI_MAX_SEGS;
> +       host->adma_table_num = SDHCI_MAX_SEGS * 2 + 1 + extra;
> +
>         pltfm_host = sdhci_priv(host);
>         priv = sdhci_pltfm_priv(pltfm_host);
>
> --
> 2.18.0
>
>

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