[PATCH] mxs/dma: Enlarge the CCW descriptor area to 4 pages

[Marek Vasut]

Dear Vinod Koul,

> On Wed, 2012-09-05 at 03:46 +0200, Marek Vasut wrote:
> > Dear Shawn Guo,
> > 
> > > On Tue, Sep 04, 2012 at 06:04:25AM +0200, Marek Vasut wrote:
> > > > In case of a large SPI flash, the amount of DMA descriptors
> > > > available to the DMA driver is not large enough anymore. For
> > > > example 8MB SPI flash now needs 129 descriptors to be transfered
> > > > in one long read. There are currently 53 descriptors available in
> > > > one PAGE_SIZE-big block. Enlarge the allocated descriptor area to
> > > > four PAGE_SIZE blocks to fulfill such requirements.
> > > > 
> > > > Signed-off-by: Marek Vasut <marex at denx.de>
> > > > Cc: Dan Williams <djbw at fb.com>
> > > > Cc: Fabio Estevam <fabio.estevam at freescale.com>
> > > > Cc: Shawn Guo <shawn.guo at linaro.org>
> > > 
> > > Acked-by: Shawn Guo <shawn.guo at linaro.org>
> > > 
> > > Vinod (copied) is the primary maintainer and collecting dma patches
> > > now.
> > 
> > Understood.
> > 
> > Let me ask about another thing. If there'll eventually be some device
> > that'd need even larger transfer (say ... hundreds of megs) we'd end up
> > with trouble again.
> > 
> > One way to fix this is to recycle descriptors that were already used
> > during the transfer, but can we really do it fast enough, so the DMA
> > would do it's job at one end of the descriptor chain and we'd be
> > building the other?
> 
> See the comment in dma_ctrl_flags:
> * @DMA_CTRL_ACK - if clear, the descriptor cannot be reused until the
> client *  acknowledges receipt, i.e. has has a chance to establish any
> dependency *  chains
> Setting this would mean that driver can reuse the descriptor once the
> transaction has completed.

Sure, but then building the DMA chain on-the-fly might be a little bit too 
crazy. Still, I think we're safe with using 4 pages for now, it's more than 
plenty.

> > The other (easier) way is to let the device that claims the particular
> > DMA channel allocate as much descriptors as it might ever need, which I
> > think is wrong.
> 
> Correct.
> 
> > What do you think ?
> > 
> > Best regards,
> > Marek Vasut


Best regards,
Marek Vasut