Re: [PATCH v7 2/4] dmaengine: tegra: Add tegra gpcdma driver

[Jon Hunter <jonathanh@nvidia.com>]

On 23/09/2021 08:51, Akhil R wrote:
> Adding GPC DMA controller driver for Tegra186 and Tegra194. The driver
> supports dma transfers between memory to memory, IO peripheral to memory
> and memory to IO peripheral.
> 
> Signed-off-by: Pavan Kunapuli <pkunapuli@nvidia.com>
> Signed-off-by: Rajesh Gumasta <rgumasta@nvidia.com>
> Signed-off-by: Akhil R <akhilrajeev@nvidia.com>
> ---
>   drivers/dma/Kconfig            |   12 +
>   drivers/dma/Makefile           |    1 +
>   drivers/dma/tegra186-gpc-dma.c | 1354 ++++++++++++++++++++++++++++++++++++++++
>   3 files changed, 1367 insertions(+)
>   create mode 100644 drivers/dma/tegra186-gpc-dma.c

...

> +static enum dma_status tegra_dma_tx_status(struct dma_chan *dc,
> +					   dma_cookie_t cookie,
> +					   struct dma_tx_state *txstate)
> +{
> +	struct tegra_dma_channel *tdc = to_tegra_dma_chan(dc);
> +	struct tegra_dma_desc *dma_desc = NULL;
> +	struct virt_dma_desc *vd;
> +	unsigned int residual;
> +	enum dma_status ret;
> +	unsigned long flags;
> +
> +	raw_spin_lock_irqsave(&tdc->lock, flags);
> +
> +	ret = dma_cookie_status(dc, cookie, txstate);
> +	if (ret == DMA_COMPLETE) {
> +		raw_spin_unlock_irqrestore(&tdc->lock, flags);
> +		return ret;
> +	}
> +
> +	vd = vchan_find_desc(&tdc->vc, cookie);
> +	if (vd)
> +		dma_desc = vd_to_tegra_dma_desc(vd);

This first case implies that the transfer has not started yet and so the 
residual is just dma_desc->bytes_requested.

> +	else if (tdc->dma_desc && tdc->dma_desc->vd.tx.cookie == cookie)
> +		dma_desc = tdc->dma_desc;
> +
> +	if (dma_desc) {
> +		residual =  dma_desc->bytes_requested -
> +					(dma_desc->bytes_transferred %
> +					dma_desc->bytes_requested);
> +		dma_set_residue(txstate, residual);
> +	} else {
> +		dev_err(tdc2dev(tdc), "cookie %d is not found\n", cookie);
> +	}
> +
> +	raw_spin_unlock_irqrestore(&tdc->lock, flags);
> +	return ret;
> +}
> +
> +static inline int get_bus_width(struct tegra_dma_channel *tdc,
> +				enum dma_slave_buswidth slave_bw)
> +{
> +	switch (slave_bw) {
> +	case DMA_SLAVE_BUSWIDTH_1_BYTE:
> +		return TEGRA_GPCDMA_MMIOSEQ_BUS_WIDTH_8;
> +	case DMA_SLAVE_BUSWIDTH_2_BYTES:
> +		return TEGRA_GPCDMA_MMIOSEQ_BUS_WIDTH_16;
> +	case DMA_SLAVE_BUSWIDTH_4_BYTES:
> +		return TEGRA_GPCDMA_MMIOSEQ_BUS_WIDTH_32;
> +	default:
> +		dev_err(tdc2dev(tdc), "given slave bus width is not supported\n");
> +		return -EINVAL;
> +	}
> +}
> +
> +static inline int get_burst_size_by_len(int len)
> +{
> +	int ret;
> +
> +	/* Get burst size based on the first set bit */
> +	switch (ffs(len)) {
> +	case 0:
> +	case 1:
> +	case 2:
> +		ret = TEGRA_GPCDMA_MMIOSEQ_BURST_1;
> +		break;
> +	case 3:
> +		ret = TEGRA_GPCDMA_MMIOSEQ_BURST_2;
> +		break;
> +	case 4:
> +		ret = TEGRA_GPCDMA_MMIOSEQ_BURST_4;
> +		break;
> +	case 5:
> +		ret = TEGRA_GPCDMA_MMIOSEQ_BURST_8;
> +		break;
> +	default:
> +		ret = TEGRA_GPCDMA_MMIOSEQ_BURST_16;
> +		break;
> +	}
> +
> +	return ret;
> +}
> +
> +static inline int get_burst_size(struct tegra_dma_channel *tdc,
> +				 u32 burst_size,
> +				 enum dma_slave_buswidth slave_bw,
> +				 int len)
> +{
> +	int burst_mmio_width, burst_byte, ret;
> +
> +	/*
> +	 * burst_size from client is in terms of the bus_width.
> +	 * convert that into words.
> +	 */
> +	burst_byte = burst_size * slave_bw;
> +	burst_mmio_width = ffs(burst_byte / 4);
> +
> +	/* Get burst size based on the first set bit */
> +	switch (burst_mmio_width) {
> +	case 0:
> +		ret = get_burst_size_by_len(len);
> +		break;
> +	case 1:
> +		ret = TEGRA_GPCDMA_MMIOSEQ_BURST_1;
> +		break;
> +	case 2:
> +		ret = TEGRA_GPCDMA_MMIOSEQ_BURST_2;
> +		break;
> +	case 3:
> +		ret = TEGRA_GPCDMA_MMIOSEQ_BURST_4;
> +		break;
> +	case 4:
> +		ret = TEGRA_GPCDMA_MMIOSEQ_BURST_8;
> +		break;
> +	default:
> +		ret = TEGRA_GPCDMA_MMIOSEQ_BURST_16;
> +		break;
> +	}
> +
> +	return ret;
> +}

Something seems a bit odd here in the sense that if burst_mmio_width == 
0, we could still end up with a burst of 16? I am not sure I understand 
this logic.

Jon

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