Re: Dreamcast DMA

[Paul M. <pau...@re...>]

On Sat, Jul 07, 2007 at 12:11:52PM +0100, Adrian McMenamin wrote:
> I think I have been over this before, but cannot find the emails on any
> search...
> 
> The Dreamcast afaics is a plain SH7750 machine and this is what the
> kernel configuration says:
> 
Not quite, it's an SH7091, but as far as the DMAC is concerned, they're
the same.

> CONFIG_NR_ONCHIP_DMA_CHANNELS:
> This allows you to specify the number of channels that the on-chip
> DMAC supports. This will be 4 for SH7750/SH7751 and 8 for the
> SH7750R/SH7751R.                                                                                                                                     
> 
> Yet if you actually set this to 4 the DC won't boot, but it will boot if
> set to 8.
> 
If it won't boot at 4, then it's a bug. Bumping up the channels to a
value not reflected by hardware is not the way you want to go.

> On top of this, I am trying to get the pvr2 driver to work properly now
> and it fails here....
> 
> #ifdef CONFIG_SH_DMA
> 	if (request_dma(pvr2dma, "pvr2") != 0) {
> 		printk(KERN_ERR "PVR2 failed to get DMA channel\n");
> 		free_irq(HW_EVENT_VSYNC, 0);
> 		return -EBUSY;
> 	}
> #endif
> 
> pvr2dma is static unsigned int pvr2dma = ONCHIP_NR_DMA_CHANNELS;
> 
> And ONCHIP_NR_DMA_CHANNELS is #define ONCHIP_NR_DMA_CHANNELS  4 for a
> Dreamcast
> 
That's likely due to the fact you're hitting a bogus channel as a result
of having bumped the on-chip channels to 8. I suppose leaving this
configurable was not what we wanted to do, but again, driver model
refactoring takes care of a lot of this.

Please post the oops when this is at 4, so we can debug that a bit
further.