linux-kernel - Re: [PATCH_V3 2/3] dma: jz4780: add driver for the Ingenic JZ4780 DMA controller

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Message-ID: <20150318111927.GJ32683@intel.com>
Date:	Wed, 18 Mar 2015 16:49:27 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Zubair Lutfullah Kakakhel <Zubair.Kakakhel@...tec.com>
Cc:	dan.j.williams@...el.com, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, alex@...x-smith.me.uk
Subject: Re: [PATCH_V3 2/3] dma: jz4780: add driver for the Ingenic JZ4780
 DMA controller

On Tue, Mar 17, 2015 at 06:07:36PM +0000, Zubair Lutfullah Kakakhel wrote:
> >> +static uint32_t jz4780_dma_transfer_size(unsigned long val, int *ord)
> >> +{
> >> +	*ord = ffs(val) - 1;
> >> +
> >> +	/* 8 byte transfer sizes unsupported so fall back on 4. */
> > okay falling back is not a good idea, same applies for default in
> > jz4780_dma_width(). The slave dma parameters are match with devices, so
> > programming something which you dont support, falling back or using defaults
> > is not a good idea
> How bad of an idea is it and what is a better way?
> I see similar stuff in imx-dma and dma-jz4740.
> 
> And my jz4740-mmc driver (I add jz4780 support and use it to test this dma driver mainly)
> falls back on defaults after it requests a larger transfer size.
Well emmc it might work, but consider you did fall back on audio playback,
you will get crapped out audio, or a SPI transfer.

That is why falling back causes issues, so right way would be return error
and have driver fixed for it

-- 
~Vinod
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