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Date:   Fri, 13 Apr 2018 09:13:33 +0530
From:   Vinod Koul <vinod.koul@intel.com>
To:     Baolin Wang <baolin.wang@linaro.org>
Cc:     Dan Williams <dan.j.williams@intel.com>,
        Eric Long <eric.long@spreadtrum.com>,
        Mark Brown <broonie@kernel.org>, dmaengine@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 4/5] dmaengine: sprd: Add Spreadtrum DMA configuration
Message-ID: <20180413034332.GI6014@localhost>
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On Thu, Apr 12, 2018 at 07:36:34PM +0800, Baolin Wang wrote:
> >>> >> +/*
> >>> >> + * struct sprd_dma_config - DMA configuration structure
> >>> >> + * @config: dma slave channel config
> >>> >> + * @fragment_len: specify one fragment transfer length
> >>> >> + * @block_len: specify one block transfer length
> >>> >> + * @transcation_len: specify one transcation transfer length
> >>> >> + * @wrap_ptr: wrap pointer address, once the transfer address reaches the
> >>> >> + * 'wrap_ptr', the next transfer address will jump to the 'wrap_to' address.
> >>> >> + * @wrap_to: wrap jump to address
> >>> >> + * @req_mode: specify the DMA request mode
> >>> >> + * @int_mode: specify the DMA interrupt type
> >>> >> + */
> >>> >> +struct sprd_dma_config {
> >>> >> +     struct dma_slave_config config;
> >>> >> +     u32 fragment_len;
> >>> >
> >>> > why not use _maxburst?
> >>>
> >>> Yes, I can use maxburst.
> >>>
> >>> >
> >>> >> +     u32 block_len;
> >>> >> +     u32 transcation_len;
> >>> >
> >>> > what does block and transaction len refer to here
> >>>
> >>>  Our DMA has 3 transfer mode: transaction transfer, block transfer and
> >>> fragment transfer. One transaction transfer can contain several blocks
> >>> transfer, and each block can be set proper block step. One block can
> >>> contain several fragments transfer with proper fragment step. It can
> >>> generate interrupts when one transaction transfer or block transfer or
> >>> fragment transfer is completed if user set the interrupt type. So here
> >>> we should set the length for transaction transfer, block transfer and
> >>> fragment transfer.
> >>
> >> what are the max size these types support?
> >
> > These types max size definition:
> >
> > #define SPRD_DMA_FRG_LEN_MASK GENMASK(16, 0)
> >
> > #define SPRD_DMA_BLK_LEN_MASK GENMASK(16, 0)
> >
> > #define SPRD_DMA_TRSC_LEN_MASK GENMASK(27, 0)
> >
> >>>
> >>> >
> >>> >> +     phys_addr_t wrap_ptr;
> >>> >> +     phys_addr_t wrap_to;
> >>> >
> >>> > this sound sg_list to me, why are we not using that here
> >>>
> >>> It is similar to sg list, but it is not one software action, we have
> >>> hardware registers to help to jump one specified address.
> >>>
> >>> >
> >>> >> +     enum sprd_dma_req_mode req_mode;
> >>> >
> >>> > Looking at definition of request mode we have frag, block, transaction list
> >>> > etc.. That should depend upon dma request. If you have been asked to
> >>> > transfer a list, you shall configure list mode. if it is a single
> >>> > transaction then it should be transaction mode!
> >>>
> >>> If I understand your points correctly, you mean we can specify the
> >>> request mode when requesting one slave channel by
> >>> 'dma_request_slave_channel()'. But we need change the request mode
> >>> dynamically following different transfer task for this channel, so I
> >>> am afraid we can not specify the request mode of this channel at
> >>> requesting time.
> >>
> >> Nope a channel has nothing to do with request type. You request and grab a
> >> channel. Then you prepare a descriptor for a dma transaction. Based on
> >> transaction requested you should intelligently break it down and create a
> >> descriptor which uses transaction/block/fragment so that DMA throughput is
> >> efficient. If prepare has sg list then you should use link list mode.
> >> Further if you support max length, say 16KB and request is for 20KB you may
> >> break it down for link list with two segments.
> >
> > OK. So I can add one more cell to specify the request mode for this channel.
> >
> > dmas = <&apdma 11 SPRD_DMA_BLK_REQ>
> >
> >>
> >> Each prep call has flags associated, that can help you configure interrupt
> >> behaviour.
> 
> After more thinking, I think this will be not useful for users, since
> users need configure one DMA channel through different 3 places,
> specify the request mode in dts, specify the interrupt type through
> prep call flags, and other configuration need to be configured through
> 'struct sprd_dma_config'. That is not one good design for users. What
> do you think? Thanks.

Agreed, users only care about grabbing a channel, setting a descriptor and
submitting that.

I think you need to go back and think about this a bit, please do go thru
dmaengine documentation and see other driver examples.

We don't typically expose these to users, they give us a transfer and we set
that up in hardware for efficient. Its DMA so people expect us to use fastest
mechanism available.

I would say setup as Link list for sg transfers and use one of them modes
(again think efficiency) for single transfers.

Also use all the parameters in dma_slave_config and also setup capabilities if
not done.

-- 
~Vinod