linux-kernel - Re: [PATCH] Fix SH DMAC code to handle PVR2 cascade

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Date:	Wed, 3 Oct 2007 16:41:37 +0100 (BST)
From:	"Adrian McMenamin" <adrian@...golddream.dyndns.info>
To:	"Paul Mundt" <lethal@...ux-sh.org>,
	"linux-sh" <linuxsh-dev@...ts.sourceforge.net>,
	"LKML" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Fix SH DMAC code to handle PVR2 cascade

On Wed, October 3, 2007 7:18 am, Paul Mundt wrote:
> On Tue, Oct 02, 2007 at 10:09:27PM +0100, Adrian McMenamin wrote:
>> Fix SH DMAC code to correctly handle PVR2 cascade DMA.
>>
>> This updates http://lkml.org/lkml/2007/10/2/276
>>
>> (I decided it was better to have the true size of the transfer put in
>> via the API and refactor this here. And calc_xmit_shift(chan) should
>> return 5 but only returns 3 so I've not used it here)
>>
> It would be helpful to know why calc_xmit_shift() is broken here rather
> than just coding around it, as this will have implications for the other
> DMA channels on SH7091/SH7750.


>From include/asm-sh/cpu-sh4/dma.h

53 /*
54  * The DMA count is defined as the number of bytes to transfer.
55  */
56 static unsigned int ts_shift[] __maybe_unused = {
57         [XMIT_SZ_64BIT]         = 3,
58         [XMIT_SZ_8BIT]          = 0,
59         [XMIT_SZ_16BIT]         = 1,
60         [XMIT_SZ_32BIT]         = 2,
61         [XMIT_SZ_256BIT]        = 5,
62 };
63 #endif

ie ts_shift returns the number of bytes per transfer, but is then used as
a bit shift:

45 /*
46  * We determine the correct shift size based off of the CHCR transmit size
47  * for the given channel. Since we know that it will take:
48  *
49  *      info->count >> ts_shift[transmit_size]
50  *
51  * iterations to complete the transfer.
52  */
53 static inline unsigned int calc_xmit_shift(struct dma_channel *chan)
54 {
55         u32 chcr = ctrl_inl(CHCR[chan->chan]);
56
57         return ts_shift[(chcr & CHCR_TS_MASK)>>CHCR_TS_SHIFT];
58 }


(From arch/sh/drivers/dma/dma-sh.c)

I'm not anywhere where I can fix this at the moment, but i am sure it
could be patched quite trivally.




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