Re: [PATCH 2/2] dmaengine: usb-dmac: Add Renesas USB DMA Controller (USB-DMAC) driver

[Vinod Koul <vinod.koul-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

On Thu, Mar 05, 2015 at 12:35:49PM +0000, yoshihiro shimoda wrote:
> Hi Vinod,
> 
> Thank you for your review!
> 
> > On Mon, Feb 09, 2015 at 05:14:05PM +0900, Yoshihiro Shimoda wrote:
> > > +struct usb_dmac_chan {
> > > +	struct dma_chan chan;
> > > +	void __iomem *iomem;
> > > +	unsigned int index;
> > > +
> > > +	spinlock_t lock;
> > > +
> > > +	struct {
> > > +		struct list_head free;
> > > +		struct list_head pending;
> > > +		struct list_head active;
> > > +		struct list_head done;
> > > +		struct list_head wait;
> > > +		struct usb_dmac_desc *running;
> > > +		struct usb_dmac_desc *last_done;
> > > +
> > > +		struct list_head chunks_free;
> > > +
> > > +		struct list_head pages;
> > Thats too many lists, do we need so many? Shouldn't free and done be same
> > thing. Similarly whats meant by wait here?
> > Do you submit multiple descriptors to HW?
> 
> No, I don't submit multiple descriptors to HW.
> So, as you say in the end of this email, I am thinking that I should use virt-dma infrastructure.
That will be good and greatly simplify..

> > > +static enum dma_status usb_dmac_tx_status(struct dma_chan *chan,
> > > +					  dma_cookie_t cookie,
> > > +					  struct dma_tx_state *txstate)
> > > +{
> > > +	struct usb_dmac_chan *uchan = to_usb_dmac_chan(chan);
> > > +	enum dma_status status;
> > > +	unsigned long flags;
> > > +	unsigned int residue;
> > > +
> > > +	status = dma_cookie_status(chan, cookie, txstate);
> > > +	/* a client driver will get residue after DMA_COMPLETE */
> > > +	if (!txstate)
> > > +		return status;
> > > +
> > > +	spin_lock_irqsave(&uchan->lock, flags);
> > > +	if (status == DMA_COMPLETE)
> > > +		residue = usb_dmac_chan_get_residue_if_complete(uchan);
> > if it is completed then residue should be zero, so why are we computing this
> 
> This USB-DMAC has a function to detect a USB specific packet (called short-length-packet).
> If the USB-DMAC detects it, the USB-DMAC assumes the USB-DMAC completes the transfer.
> 
> For example:
>  - A client driver submits 2048 bytes as RX.
>  - When a USB controller received 512 + 488 bytes totally,
>    the USB-DMAC detected it and completed the transfer.
>   - However, a USB controller just knows the bytes of last packet (In this case, 488byte.)
>         So, the USB controller driver cannot know that it got how many bytes.
>     Therefore, this USB-DMAC driver is computing this bytes.
> 
> I'm not sure about the detail, but cppi41.c seems to compute the residue even if it is completed.
I suspected this to be case, so you are confirming my hunch, thanks :)

-- 
~Vinod

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