linux-kernel - Re: [PATCH v2 4/4] dw_dmac: return proper residue value

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Date:	Thu, 24 Jan 2013 13:44:01 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:	Vinod Koul <vinod.koul@...el.com>, linux-kernel@...r.kernel.org,
	spear-devel <spear-devel@...t.st.com>
Subject: Re: [PATCH v2 4/4] dw_dmac: return proper residue value

On 24 January 2013 13:31, Andy Shevchenko
<andriy.shevchenko@...ux.intel.com> wrote:
> On Thu, 2013-01-24 at 10:37 +0530, Viresh Kumar wrote:

>> Is there a point updating residue here? I don't have a very good knowledge of
>> nollp transfers but this is what i know...
>>
>> The above "if" will pass if we are still doing transfers and fail if
>> all transfers are done.
>> After the end of each LLI we receive an interrupt, where we queue next
>> LLI. Better
>> would be to initialize dwc->residue at dwc_dostart() with total
>> length, start decrementing
>> it with desc->len for every lli interrupt we get
>
> It's mostly okay, but we have to handle few cases:
> - we have only first (master) descriptor
> - we have a chain of the descriptors: master + children
> - we have finished last transfer
>
> From my point of view we can't fully get rid of dwc_update_residue(),
> but modify it a bit (drop away for loop).

I feel the residue mechanism for all three cases can be handled by the code
i asked you to try. Lets see how it goes, after we see first level of
draft from you.

>> and if call for
>> getting residue comes in
>> middle of transfer, simple return residue - dwc_get_sent(desc) without
>> updating residue
>> field...
>
> Where? In the tx_status to call something which returns dwc->residue -
> dwc_get_sent() ?

Yes. My point was not to update dwc->residue with whatever data we get out of
dwc_get_sent().

>> > +       spin_lock_irqsave(&dwc->lock, flags);
>> > +
>>
>> why do you need locking here?
>
> What about the case when one CPU is getting an interrupt and runs
> scan_descriptors when the other, for example, in the middle of
> tx_status? So, I'm afraid the dma_set_residue(txstate, dwc->residue) is
> not atomic and we might end up with random numbers here.

I am not against the lock, but want both of us to know why it is there :)
So, i don't think that we need to protect dma_set_residue() at all. That's
not our job.

What we need to make sure is value read from dwc->residue is consistent.
And so, i believe we need a lock to protect read and write to dwc->residue.
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