Re: [RFC PATCH v6 4/5] mmc: tmio: Use dma_max_mapping_size() instead of a workaround

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Shimoda-san,

On Thu, Jun 13, 2019 at 5:37 PM Yoshihiro Shimoda
<yoshihiro.shimoda.uh@renesas.com> wrote:
> Since the commit 133d624b1cee ("dma: Introduce dma_max_mapping_size()")
> provides a helper function to get the max mapping size, we can use
> the function instead of the workaround code for swiotlb.
>
> Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>

Thanks for your patch!

> --- a/drivers/mmc/host/tmio_mmc_core.c
> +++ b/drivers/mmc/host/tmio_mmc_core.c

> @@ -1189,19 +1190,9 @@ int tmio_mmc_host_probe(struct tmio_mmc_host *_host)
>         mmc->max_blk_size = TMIO_MAX_BLK_SIZE;
>         mmc->max_blk_count = pdata->max_blk_count ? :
>                 (PAGE_SIZE / mmc->max_blk_size) * mmc->max_segs;
> -       mmc->max_req_size = mmc->max_blk_size * mmc->max_blk_count;
> -       /*
> -        * Since swiotlb has memory size limitation, this will calculate
> -        * the maximum size locally (because we don't have any APIs for it now)
> -        * and check the current max_req_size. And then, this will update
> -        * the max_req_size if needed as a workaround.
> -        */
> -       if (swiotlb_max_segment()) {
> -               unsigned int max_size = (1 << IO_TLB_SHIFT) * IO_TLB_SEGSIZE;
> -
> -               if (mmc->max_req_size > max_size)
> -                       mmc->max_req_size = max_size;
> -       }
> +       mmc->max_req_size = min_t(unsigned int,
> +                                 mmc->max_blk_size * mmc->max_blk_count,
> +                                 dma_max_mapping_size(&pdev->dev));
>         mmc->max_seg_size = mmc->max_req_size;

I'm always triggered by the use of min_t() and other casts:
mmc->max_blk_size and mmc->max_blk_count are both unsigned int.
dma_max_mapping_size() returns size_t, which can be 64-bit.

 1) Can the multiplication overflow?
    Probably not, as per commit 2a55c1eac7882232 ("mmc: renesas_sdhi:
    prevent overflow for max_req_size"), but I thought I'd better ask.
 2) In theory, dma_max_mapping_size() can return a number that doesn't
    fit in 32-bit, and will be truncated (to e.g. 0), leading to max_req_size
    is zero?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds