linux-kernel - Re: [PATCH v17 2/4] dmaengine: tegra: Add tegra gpcdma driver

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Date:   Sun, 30 Jan 2022 13:05:36 +0300
From:   Dmitry Osipenko <digetx@...il.com>
To:     Akhil R <akhilrajeev@...dia.com>, devicetree@...r.kernel.org,
        dmaengine@...r.kernel.org, jonathanh@...dia.com,
        kyarlagadda@...dia.com, ldewangan@...dia.com,
        linux-kernel@...r.kernel.org, linux-tegra@...r.kernel.org,
        p.zabel@...gutronix.de, rgumasta@...dia.com, robh+dt@...nel.org,
        thierry.reding@...il.com, vkoul@...nel.org
Cc:     Pavan Kunapuli <pkunapuli@...dia.com>
Subject: Re: [PATCH v17 2/4] dmaengine: tegra: Add tegra gpcdma driver

29.01.2022 19:40, Akhil R пишет:
> +static int tegra_dma_device_pause(struct dma_chan *dc)
> +{
> +	struct tegra_dma_channel *tdc = to_tegra_dma_chan(dc);
> +	unsigned long wcount, flags;
> +	int ret = 0;
> +
> +	if (!tdc->tdma->chip_data->hw_support_pause)
> +		return 0;

It's wrong to return zero if pause unsupported, please see what
dmaengine_pause() returns.

> +
> +	spin_lock_irqsave(&tdc->vc.lock, flags);
> +	if (!tdc->dma_desc)
> +		goto out;
> +
> +	ret = tegra_dma_pause(tdc);
> +	if (ret) {
> +		dev_err(tdc2dev(tdc), "DMA pause timed out\n");
> +		goto out;
> +	}
> +
> +	wcount = tdc_read(tdc, TEGRA_GPCDMA_CHAN_XFER_COUNT);
> +	tdc->dma_desc->bytes_xfer +=
> +			tdc->dma_desc->bytes_req - (wcount * 4);

Why transfer is accumulated?

Why do you need to update xfer size at all on pause?

> +
> +out:
> +	spin_unlock_irqrestore(&tdc->vc.lock, flags);
> +
> +	return ret;
> +}

Still nothing prevents interrupt handler to fire during the pause.

What you actually need to do is to disable/enable interrupt. This will
prevent the interrupt racing and then pause/resume may look like this:

static int tegra_dma_device_resume(struct dma_chan *dc)
{
	struct tegra_dma_channel *tdc = to_tegra_dma_chan(dc);
	u32 val;

	if (!tdc->tdma->chip_data->hw_support_pause)
		return -ENOSYS;

	if (!tdc->dma_desc)
		return 0;

	val = tdc_read(tdc, TEGRA_GPCDMA_CHAN_CSRE);
	val &= ~TEGRA_GPCDMA_CHAN_CSRE_PAUSE;
	tdc_write(tdc, TEGRA_GPCDMA_CHAN_CSRE, val);
	
	enable_irq(tdc->irq);

	return 0;
}

static int tegra_dma_device_pause(struct dma_chan *dc)
{
	struct tegra_dma_channel *tdc = to_tegra_dma_chan(dc);
	u32 val;
	int ret;

	if (!tdc->tdma->chip_data->hw_support_pause)
		return -ENOSYS;

	disable_irq(tdc->irq);

	if (!tdc->dma_desc)
		return 0;

	val = tdc_read(tdc, TEGRA_GPCDMA_CHAN_CSRE);
	val |= TEGRA_GPCDMA_CHAN_CSRE_PAUSE;
	tdc_write(tdc, TEGRA_GPCDMA_CHAN_CSRE, val);

	/* Wait until busy bit is de-asserted */
	ret = readl_relaxed_poll_timeout_atomic(
			tdc->chan_base + TEGRA_GPCDMA_CHAN_STATUS,
			val, !(val & TEGRA_GPCDMA_STATUS_BUSY),
			TEGRA_GPCDMA_BURST_COMPLETE_TIME,
			TEGRA_GPCDMA_BURST_COMPLETION_TIMEOUT);
	if (ret) {
		dev_err(tdc2dev(tdc), "DMA pause timed out: %d\n", ret);
		tegra_dma_device_resume(dc);
	}

	return ret;
}