linux-kernel - Re: [PATCH v5 1/3] dmaengine: Add support for APM X-Gene SoC DMA engine driver

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Date:	Wed, 11 Feb 2015 13:38:07 +0530
From:	Rameshwar Sahu <rsahu@....com>
To:	Vinod Koul <vinod.koul@...el.com>
Cc:	dan.j.williams@...el.com, dmaengine@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	ddutile@...hat.com, jcm@...hat.com, patches@....com,
	Loc Ho <lho@....com>
Subject: Re: [PATCH v5 1/3] dmaengine: Add support for APM X-Gene SoC DMA
 engine driver

Hi Vinod,


On Fri, Feb 6, 2015 at 1:41 AM, Vinod Koul <vinod.koul@...el.com> wrote:
> On Thu, Feb 05, 2015 at 05:29:06PM +0530, Rameshwar Sahu wrote:
>> Hi Vinod,
>>
>> Thanks for reviewing this patch.
>>
>> Please see inline......
> Please STOP top posting
>
>> >
>> >> +}
>> >> +
>> >> +static void xgene_dma_issue_pending(struct dma_chan *channel)
>> >> +{
>> >> +     /* Nothing to do */
>> >> +}
>> > What do you mean by nothing to do here
>> > See Documentation/dmaengine/client.txt Section 4 & 5
>> This docs is only applicable on slave DMA operations, we don't support
>> slave DMA, it's only master.
>> Our hw engine is designed in the way that there is no scope of
>> flushing pending transaction explicitly by sw.
>> We have circular ring descriptor dedicated to engine. In submit
>> callback  we are queuing descriptor and informing to engine, so after
>> this it's internal to hw to execute descriptor one by one.
> But the API expectations on this are _same_
>
> No the API expects you to maintain a SW queue, then push to your ring buffer
> when you get issue_pending. Issue pending is the start of data transfer, you
> client will expect accordingly.

Okay, I will maintain a sw queue, and will push sw descriptor to hw in
this callback.
>
>> >> +     /* Run until we are out of length */
>> >> +     do {
>> >> +             /* Create the largest transaction possible */
>> >> +             copy = min_t(size_t, len, DMA_MAX_64BDSC_BYTE_CNT);
>> >> +
>> >> +             /* Prepare DMA descriptor */
>> >> +             xgene_dma_prep_cpy_desc(chan, slot, dst, src, copy);
>> >> +
>> > This is wrong. The descriptor is supposed to be already prepared and now it
>> > has to be submitted to queue
>>
>> Due to the race in tx_submit call from the client, need to serialize
>> the submission of H/W DMA descriptors.
>>  So we are making  shadow copy in prepare DMA  routine and  preparing
>> actual descriptor during tx_submit call.
> Thats an abuse of API and I dont see a reason why this race should happen in
> the first place.
>
> So you get a prep call, you prepare a desc in SW. Then submit pushes it to a
> queue. Finally in issue pending you push them to HW. Simple..?

I agree, I will do it and post another version soon.
>
> --
> ~Vinod
> --
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