linux-kernel - Re: [PATCH v5] dma: Add Xilinx AXI Direct Memory Access Engine driver support

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Date:	Mon, 2 Mar 2015 12:41:34 -0600
From:	Josh Cartwright <joshc@...com>
To:	Kedareswara rao Appana <appana.durga.rao@...inx.com>
Cc:	dan.j.williams@...el.com, vinod.koul@...el.com,
	michal.simek@...inx.com, soren.brinkmann@...inx.com,
	svemula@...inx.com, linux-kernel@...r.kernel.org,
	Srikanth Thokala <sthokal@...inx.com>, anirudh@...inx.com,
	dmaengine@...r.kernel.org, appanad@...inx.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v5] dma: Add Xilinx AXI Direct Memory Access Engine
 driver support

Hello!

I looked through your driver and have some comments.

On Mon, Mar 02, 2015 at 11:25:11PM +0530, Kedareswara rao Appana wrote:
> This is the driver for the AXI Direct Memory Access (AXI DMA)
> core, which is a soft Xilinx IP core that provides high-
> bandwidth direct memory access between memory and AXI4-Stream
> type target peripherals.
> 
> Signed-off-by: Srikanth Thokala <sthokal@...inx.com>
> Signed-off-by: Kedareswara rao Appana <appanad@...inx.com>
[..]
> +++ b/drivers/dma/Kconfig
> @@ -425,6 +425,19 @@ config IMG_MDC_DMA
>  	help
>  	  Enable support for the IMG multi-threaded DMA controller (MDC).
>  
> +config XILINX_DMA
> +	tristate "Xilinx AXI DMA Engine"
> +	depends on (ARCH_ZYNQ || MICROBLAZE)

Why do you need this dependency?  I'm assuming this IP has usefulness
outside of microblaze and Zynq.

> +	select DMA_ENGINE
> +	help
> +	  Enable support for Xilinx AXI DMA Soft IP.
> +
> +	This engine provides high-bandwidth direct memory access
> +	between memory and AXI4-Stream type target peripherals.
> +	It has two stream interfaces/channels, Memory Mapped to
> +	Stream (MM2S) and Stream to Memory Mapped (S2MM) for the
> +	data transfers.

Odd indention here.  At least indent this paragraph like the sentence
above.

[..]
> +++ b/drivers/dma/xilinx/xilinx_dma.c
[..]
> +/**
> + * xilinx_dma_halt - Halt DMA channel
> + * @chan: Driver specific DMA channel
> + */
> +static void xilinx_dma_halt(struct xilinx_dma_chan *chan)
> +{
> +	int loop = XILINX_DMA_LOOP_COUNT;
> +
> +	dma_ctrl_clr(chan, XILINX_DMA_REG_CONTROL,
> +		     XILINX_DMA_CR_RUNSTOP_MASK);
> +
> +	/* Wait for the hardware to halt */
> +	do {
> +		if (dma_ctrl_read(chan, XILINX_DMA_REG_STATUS) &
> +			XILINX_DMA_SR_HALTED_MASK)
> +			break;
> +	} while (loop--);
> +
> +	if (!loop) {

Looks like a very subtle off-by-one error here.  (And elsewhere you use
this pattern).

  Josh
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