Re: [PATCH] [RFC] dmaengine: add fifo_size member

From: Vinod Koul <vkoul@kernel.org>
To: Jon Hunter <jonathanh@nvidia.com>
Cc: Sameer Pujar <spujar@nvidia.com>,
	Peter Ujfalusi <peter.ujfalusi@ti.com>,
	dan.j.williams@intel.com, tiwai@suse.com,
	dmaengine@vger.kernel.org, linux-kernel@vger.kernel.org,
	sharadg@nvidia.com, rlokhande@nvidia.com, dramesh@nvidia.com,
	mkumard@nvidia.com
Subject: Re: [PATCH] [RFC] dmaengine: add fifo_size member
Date: Tue, 20 Aug 2019 16:35:10 +0530	[thread overview]
Message-ID: <20190820110510.GQ12733@vkoul-mobl.Dlink> (raw)
In-Reply-To: <a93a472d-b8f7-973f-6068-607492421472@nvidia.com>

On 19-08-19, 16:56, Jon Hunter wrote:
> >>>>>>> On this, I am inclined to think that dma driver should not be involved.
> >>>>>>> The ADMAIF needs this configuration and we should take the path of
> >>>>>>> dma_router for this piece and add features like this to it
> >>>>>>
> >>>>>> Hi Vinod,
> >>>>>>
> >>>>>> The configuration is needed by both ADMA and ADMAIF. The size is
> >>>>>> configurable
> >>>>>> on ADMAIF side. ADMA needs to know this info and program accordingly.
> >>>>>
> >>>>> Well I would say client decides the settings for both DMA, DMAIF and
> >>>>> sets the peripheral accordingly as well, so client communicates the two
> >>>>> sets of info to two set of drivers
> >>>>
> >>>> That maybe, but I still don't see how the information is passed from the
> >>>> client in the first place. The current problem is that there is no means
> >>>> to pass both a max-burst size and fifo-size to the DMA driver from the
> >>>> client.
> >>>
> >>> So one thing not clear to me is why ADMA needs fifo-size, I thought it
> >>> was to program ADMAIF and if we have client programme the max-burst
> >>> size to ADMA and fifo-size to ADMAIF we wont need that. Can you please
> >>> confirm if my assumption is valid?
> >>
> >> Let me see if I can clarify ...
> >>
> >> 1. The FIFO we are discussing here resides in the ADMAIF module which is
> >>    a separate hardware block the ADMA (although the naming make this
> >>    unclear).
> >>
> >> 2. The size of FIFO in the ADMAIF is configurable and it this is
> >>    configured via the ADMAIF registers. This allows different channels
> >>    to use different FIFO sizes. Think of this as a shared memory that is
> >>    divided into n FIFOs shared between all channels.
> >>
> >> 3. The ADMA, not the ADMAIF, manages the flow to the FIFO and this is
> >>    because the ADMAIF only tells the ADMA when a word has been
> >>    read/written (depending on direction), the ADMAIF does not indicate
> >>    if the FIFO is full, empty, etc. Hence, the ADMA needs to know the
> >>    total FIFO size.
> >>
> >> So the ADMA needs to know the FIFO size so that it does not overrun the
> >> FIFO and we can also set a burst size (less than the total FIFO size)
> >> indicating how many words to transfer at a time. Hence, the two parameters.
> > 
> > Thanks, I confirm this is my understanding as well.
> > 
> > To compare to regular case for example SPI on DMA, SPI driver will
> > calculate fifo size & burst to be used and program dma (burst size) and
> > its own fifos accordingly
> > 
> > So, in your case why should the peripheral driver not calculate the fifo
> > size for both ADMA and ADMAIF and (if required it's own FIFO) and
> > program the two (ADMA and ADMAIF).
> > 
> > What is the limiting factor in this flow is not clear to me.
> 
> The FIFO size that is configured by the ADMAIF driver needs to be given
> to the ADMA driver so that it can program its registers accordingly. The
> difference here is that both the ADMA and ADMAIF need the FIFO size.

Can you please help describing what it is programming using the FIFO
size of ADMAIF?

Thanks
-- 
~Vinod