linux-kernel - Re: [PATCH] dmaengine: bcm2835: Add slave dma support

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Message-Id: <75BEAE20-81A9-4567-8AB2-1C1865A0B72D@sperl.org>
Date:	Wed, 15 Apr 2015 16:37:46 +0200
From:	Martin Sperl <martin@...rl.org>
To:	Noralf Trønnes <noralf@...nnes.org>
Cc:	dmaengine@...r.kernel.org, vinod.koul@...el.com,
	linux-kernel@...r.kernel.org, jonathan@...pberrypi.org,
	linux-rpi-kernel@...ts.infradead.org, dan.j.williams@...el.com
Subject: Re: [PATCH] dmaengine: bcm2835: Add slave dma support


> On 15.04.2015, at 11:56, Noralf Trønnes <noralf@...nnes.org> wrote:
> +#define MAX_LITE_TRANSFER	(SZ_64K - 1)
> +#define MAX_NORMAL_TRANSFER	SZ_1G
...
> +	if (c->ch >= 8) /* LITE channel */
> +		max_size = MAX_LITE_TRANSFER;
> +	else
> +		max_size = MAX_NORMAL_TRANSFER;
> +	period_len = min(period_len, max_size);
> +	d->frames = (buf_len - 1) / (period_len + 1);

I wonder if it is wise to split the transfers on 65535 bytes for the 
Lite DMA-channels - especially if you are transferring to word size
registers (like SPI_FIFO), you still push 16384 words into the register
and the last word of this transfer (word 16384) still is assumed 4 valid
bytes by the device and thus gets operated upon - even if the last byte
contains garbage from the DMA-transfer point of view.

So maybe it is better to separate on SZ_64K-4 or better still SZ_32K to
be on a power of 2 address boundary.--
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