linux-kernel - Re: [PATCH 1/2] mmc: mxcmmc: really configure the DMA on demand

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Date:   Wed, 11 Aug 2021 13:36:10 +0200
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Juergen Borleis <jbe@...gutronix.de>
Cc:     linux-mmc <linux-mmc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Fabio Estevam <festevam@...il.com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Doug Anderson <dianders@...omium.org>,
        Sascha Hauer <kernel@...gutronix.de>
Subject: Re: [PATCH 1/2] mmc: mxcmmc: really configure the DMA on demand

On Thu, 29 Jul 2021 at 09:29, Juergen Borleis <jbe@...gutronix.de> wrote:
>
> Calling mxcmci_use_dma(host) is intended for the next transfer only, not
> for generic detection if DMA is possible. Without this change, the DMA gets
> never configured and thus, never used.

Wow, that's an old bug you found there.

>
> Signed-off-by: Juergen Borleis <jbe@...gutronix.de>

It looks like we should add a fixes tag and add a stable tag:

Fixes: f53fbde48ef0 ("mmc: mxcmmc: use dmaengine API")

> ---
>  drivers/mmc/host/mxcmmc.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/mmc/host/mxcmmc.c b/drivers/mmc/host/mxcmmc.c
> index 2fe6fcd..611f827 100644
> --- a/drivers/mmc/host/mxcmmc.c
> +++ b/drivers/mmc/host/mxcmmc.c
> @@ -834,7 +834,8 @@ static void mxcmci_set_ios(struct mmc_host *mmc, struct mmc_ios *ios)
>         else
>                 burstlen = 4;
>
> -       if (mxcmci_use_dma(host) && burstlen != host->burstlen) {
> +       if (host->dma != NULL && burstlen != host->burstlen) {
> +               /* reconfigure DMA on changes only */
>                 host->burstlen = burstlen;
>                 ret = mxcmci_setup_dma(mmc);
>                 if (ret) {

A few lines below here, you should not clear host->do_dma, as it can't
be set at this point (thus clearing it just adds confusion).

Also I wonder if patch2 is really needed to fix the bug, or should be
considered as nice cleanup instead?

Kind regards
Uffe