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Re: [PATCH v4 2/4] dmaengine: Add STM32 DMAMUX driver

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  Re: [PATCH v4 2/4] dmaengine: Add STM32 DMAMUX driver Peter Ujfalusi <peter.ujfalusi@ti.com> - 2017-09-21 13:30 +0200
    Re: [PATCH v4 2/4] dmaengine: Add STM32 DMAMUX driver Pierre Yves MORDRET <pierre-yves.mordret@st.com> - 2017-09-21 14:50 +0200

#1736557 — Re: [PATCH v4 2/4] dmaengine: Add STM32 DMAMUX driver


Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki. Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki

On 2017-09-07 14:52, Pierre-Yves MORDRET wrote:
> This patch implements the STM32 DMAMUX driver.
> 
> The DMAMUX request multiplexer allows routing a DMA request line between
> the peripherals and the DMA controllers of the product. The routing
> function is ensured by a programmable multi-channel DMA request line
> multiplexer. Each channel selects a unique DMA request line,
> unconditionally or synchronously with events from its DMAMUX
> synchronization inputs. The DMAMUX may also be used as a DMA request
> generator from programmable events on its input trigger signals
> 
> Signed-off-by: M'boumba Cedric Madianga <cedric.madianga@gmail.com>
> Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
> ---
>  Version history:
>     v4:
>         * Get rid of st,dmamux property and custom API between STM32
>           DMAMUX and DMA.
>           DMAMUX will read DMA masters from Device Tree from now on.
>           Merely one DMAMUX node is needed now.
>         * Only STM32 DMA are allowed to be connected onto DMAMUX
>         * channelID is computed locally within the driver and crafted in
>           dma_psec to be passed toward DMA master.
>           DMAMUX router sorts out which DMA master will serve the
>           request automatically.
>         * This version forbids the use of DMA in standalone and DMAMUX at
>           the same time : all clients need to be connected either on DMA
>           or DMAMUX ; no mix up

Great that you got it working w/o a custom API!
I have one comment, which actually valid for the ti-dma-crossbar driver
as well...

> +static void *stm32_dmamux_route_allocate(struct of_phandle_args *dma_spec,
> +					 struct of_dma *ofdma)
> +{
> +	struct platform_device *pdev = of_find_device_by_node(ofdma->of_node);
> +	struct stm32_dmamux_data *dmamux = platform_get_drvdata(pdev);
> +	struct stm32_dmamux *mux;
> +	u32 i, min, max, ret;
> +	unsigned long flags;
> +
> +	if (dma_spec->args_count != 3) {
> +		dev_err(&pdev->dev, "invalid number of dma mux args\n");
> +		return ERR_PTR(-EINVAL);
> +	}
> +
> +	if (dma_spec->args[0] > dmamux->dmamux_requests) {
> +		dev_err(&pdev->dev, "invalid mux request number: %d\n",
> +			dma_spec->args[0]);
> +		return ERR_PTR(-EINVAL);
> +	}
> +
> +	mux = kzalloc(sizeof(*mux), GFP_KERNEL);
> +	if (!mux)
> +		return ERR_PTR(-ENOMEM);
> +
> +	spin_lock_irqsave(&dmamux->lock, flags);
> +	mux->chan_id = find_first_zero_bit(dmamux->dma_inuse,
> +					   dmamux->dma_requests);

you pick the first available chan_id here under the lock.

> +	spin_unlock_irqrestore(&dmamux->lock, flags);
> +	if (mux->chan_id == dmamux->dma_requests) {
> +		dev_err(&pdev->dev, "Run out of free DMA requests\n");
> +		kfree(mux);
> +		return ERR_PTR(-ENOMEM);
> +	}
> +
> +	/* Look for DMA Master */
> +	for (i = 1, min = 0, max = dmamux->dma_reqs[i];
> +	     i <= dmamux->dma_reqs[0];
> +	     min += dmamux->dma_reqs[i], max += dmamux->dma_reqs[++i])
> +		if (mux->chan_id < max)
> +			break;
> +	mux->master = i - 1;
> +
> +	/* The of_node_put() will be done in of_dma_router_xlate function */
> +	dma_spec->np = of_parse_phandle(ofdma->of_node, "dma-masters", i - 1);
> +	if (!dma_spec->np) {
> +		dev_err(&pdev->dev, "can't get dma master\n");
> +		kfree(mux);
> +		return ERR_PTR(-EINVAL);
> +	}
> +
> +	/* Set dma request */
> +	spin_lock_irqsave(&dmamux->lock, flags);
> +	if (!IS_ERR(dmamux->clk)) {
> +		ret = clk_enable(dmamux->clk);
> +		if (ret < 0) {
> +			spin_unlock_irqrestore(&dmamux->lock, flags);
> +			kfree(mux);
> +			dev_err(&pdev->dev, "clk_prep_enable issue: %d\n", ret);
> +			return ERR_PTR(ret);
> +		}
> +	}
> +	spin_unlock_irqrestore(&dmamux->lock, flags);
> +
> +	set_bit(mux->chan_id, dmamux->dma_inuse);

But nothing stops other parallel threads to pick the same chan_id since
you have released the lock (released, got the lock to protect the set
dma request and released it again). imho the find_first_zero_bit() and
the set_bit() should be done within the same lock to avoid race conditions.

- Péter

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#1736638

On 09/21/2017 01:25 PM, Peter Ujfalusi wrote:
> 
> Texas Instruments Finland Oy, Porkkalankatu 22, 00180 Helsinki. Y-tunnus/Business ID: 0615521-4. Kotipaikka/Domicile: Helsinki
> 
> 
> Great that you got it working w/o a custom API!
> I have one comment, which actually valid for the ti-dma-crossbar driver
> as well...

Yes. That cleans up a little bit the sw architecture. But still this custom API
allowed both DMAMUX and DMA at the same time since using the same channel ID
allocator.
Ok this is another story to be addressed out of this thread ;)

> 
>> +static void *stm32_dmamux_route_allocate(struct of_phandle_args *dma_spec,
>> +					 struct of_dma *ofdma)
>> +
>> +	spin_lock_irqsave(&dmamux->lock, flags);
>> +	mux->chan_id = find_first_zero_bit(dmamux->dma_inuse,
>> +					   dmamux->dma_requests);
> 
> you pick the first available chan_id here under the lock.
> 
>> +	spin_unlock_irqrestore(&dmamux->lock, flags);
>> +	if (mux->chan_id == dmamux->dma_requests) {
>> ...
>> +	/* Set dma request */
>> +	spin_lock_irqsave(&dmamux->lock, flags);
>> +	if (!IS_ERR(dmamux->clk)) {
>> ...
>> +	spin_unlock_irqrestore(&dmamux->lock, flags);
>> +
>> +	set_bit(mux->chan_id, dmamux->dma_inuse);
> 
> But nothing stops other parallel threads to pick the same chan_id since
> you have released the lock (released, got the lock to protect the set
> dma request and released it again). imho the find_first_zero_bit() and
> the set_bit() should be done within the same lock to avoid race conditions.
> 
> - Péter
> 

Yep good catch : That's correct. Even if probability to happen is rather low, it
may happen.
Will solve that.

Py

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