linux-kernel - Re: [PATCH 2/2] dmaengine i.MX SDMA: protect channel0

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Date:	Tue, 7 Dec 2010 15:50:33 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Sascha Hauer <s.hauer@...gutronix.de>
Cc:	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] dmaengine i.MX SDMA: protect channel0

On Mon, Dec 6, 2010 at 2:09 AM, Sascha Hauer <s.hauer@...gutronix.de> wrote:
> Channel 0 of the SDMA engine is a shared resource used by the
> other channels, thus we have to protect it with a mutex.
>
> Signed-off-by: Sascha Hauer <s.hauer@...gutronix.de>
> ---

This one is not making sense to me...

>  drivers/dma/imx-sdma.c |   11 +++++++++++
>  1 files changed, 11 insertions(+), 0 deletions(-)
>
> diff --git a/drivers/dma/imx-sdma.c b/drivers/dma/imx-sdma.c
> index 01c4a5f..b1f5947 100644
> --- a/drivers/dma/imx-sdma.c
> +++ b/drivers/dma/imx-sdma.c
> @@ -355,6 +355,7 @@ struct sdma_engine {
>        struct dma_device               dma_device;
>        struct clk                      *clk;
>        struct sdma_script_start_addrs  *script_addrs;
> +       struct mutex                    channel0_mutex;
>  };
>
>  #define SDMA_H_CONFIG_DSPDMA   (1 << 12) /* indicates if the DSPDMA is used */
> @@ -437,6 +438,8 @@ static int sdma_load_script(struct sdma_engine *sdma, void *buf, int size,
>        if (!buf_virt)
>                return -ENOMEM;
>
> +       mutex_lock(&sdma->channel0_mutex);
> +
>        bd0->mode.command = C0_SETPM;
>        bd0->mode.status = BD_DONE | BD_INTR | BD_WRAP | BD_EXTD;
>        bd0->mode.count = size / 2;
> @@ -447,6 +450,8 @@ static int sdma_load_script(struct sdma_engine *sdma, void *buf, int size,
>
>        ret = sdma_run_channel(&sdma->channel[0]);
>
> +       mutex_unlock(&sdma->channel0_mutex);
> +

In sdma_load_script() what data structure are we protecting at single
threaded init time prior to registering channels?

>        dma_free_coherent(NULL, size, buf_virt, buf_phys);
>
>        return ret;

> @@ -684,6 +689,8 @@ static int sdma_load_context(struct sdma_channel *sdmac)
>        context->gReg[6] = sdmac->shp_addr;
>        context->gReg[7] = sdmac->watermark_level;
>
> +       mutex_lock(&sdma->channel0_mutex);
> +
>        bd0->mode.command = C0_SETDM;
>        bd0->mode.status = BD_DONE | BD_INTR | BD_WRAP | BD_EXTD;
>        bd0->mode.count = sizeof(*context) / 4;
> @@ -692,6 +699,8 @@ static int sdma_load_context(struct sdma_channel *sdmac)
>
>        ret = sdma_run_channel(&sdma->channel[0]);
>
> +       mutex_unlock(&sdma->channel0_mutex);
> +

This sdma_load_context() is called from various ->prep() contexts.
What guarantees are there that we can sleep in these contexts?  At
least ->prep_memcpy() might be called in a atomic context.  I have
been assuming the same for all prep routines across archs.

--
Dan
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