Re: [PATCH V2 6/6] spi/spi-pl022: Request/free DMA channels as and when required.

From: Jassi Brar <jassisinghbrar@gmail.com>
To: Russell King - ARM Linux <linux@arm.linux.org.uk>
Cc: "pratyush.anand@st.com" <pratyush.anand@st.com>,
	"rajeev-dlh.kumar@st.com" <rajeev-dlh.kumar@st.com>,
	"bhavna.yadav@st.com" <bhavna.yadav@st.com>,
	"bhupesh.sharma@st.com" <bhupesh.sharma@st.com>,
	"shiraz.hashim@st.com" <shiraz.hashim@st.com>,
	"Koul, Vinod" <vinod.koul@intel.com>,
	"linus.walleij@linaro.org" <linus.walleij@linaro.org>,
	"grant.likely@secretlab.ca" <grant.likely@secretlab.ca>,
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	"spi-devel-general@lists.sourceforge.net"
	<spi-devel-general@lists.sourceforge.net>,
	"Williams, Dan J" <dan.j.williams@intel.com>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infrad
Subject: Re: [PATCH V2 6/6] spi/spi-pl022: Request/free DMA channels as and when required.
Date: Wed, 10 Aug 2011 16:54:36 +0530	[thread overview]
Message-ID: <CABb+yY3=q5Nq1wtM6Av2DrgUfjZR_Ao40QhBhv_ik9naAd5wRg@mail.gmail.com> (raw)
In-Reply-To: <20110810104025.GK1831@n2100.arm.linux.org.uk>

On Wed, Aug 10, 2011 at 4:10 PM, Russell King - ARM Linux
<linux@arm.linux.org.uk> wrote:
> On Wed, Aug 10, 2011 at 04:01:13PM +0530, Jassi Brar wrote:
>> On Wed, Aug 10, 2011 at 3:31 PM, Koul, Vinod <vinod.koul@intel.com> wrote:
>> > On Wed, 2011-08-10 at 14:59 +0530, viresh kumar wrote:
>> >> On 08/10/2011 02:30 PM, Russell King - ARM Linux wrote:
>> >> >> > They must be allocated when they are required and must be freed after we are
>> >> >> > done with transfers. So that they can be used by other users.
>> >> > Which DMA engine driver requires this?
>> >> >
>> >>
>> >> dw_dmac.c
>> >>
>> >> > Normally, when we have DMA engine drivers with multiple request signals,
>> >> > the slave peripheral side publishes several virtual channels which are
>> >> > claimed by the peripheral drivers.  This (amongst other things) allows
>> >> > the peripheral drivers to hold claim to one of the virtual channels
>> >> > all the time that it's required.
>> >>
>> >> If users of DMA expect DMA engine drivers to work this way, then we should
>> >> have this mentioned clearly in DMA slave documentation.
>> >>
>> >> @Dan/Vinod: What do you say?
>> > I would agree on both counts :)
>> >
>> > In some cases it does make sense to hold the channel for the lifetime
>> > like uart or where the channel has been tied to an interface by SoC
>> > designer.
>> > But in some cases like dw_dmac it seems you can assign channels
>> > dynamically to each usage, and runtime allocation ensures we have best
>> > utilization.
>> > So i would argue that there is no "one size fits all" here, if you can
>> > manage channels dynamically and utilize more efficiently then go ahead,
>> > but if you cant (h/w and usage constraint) then you should not be forced
>> > to do so.
>>
>> The idea is to have channel allocation as purely a s/w thing - no
>> actual commitment
>> of h/w resources. So we can afford to have channel allocated for the
>> whole lifetime
>> of a client.
>>
>> Some dmac drivers are written 'improperly', keeping in mind the
>> platforms that have fixed
>> ReqSig->Peri map and no more clients than actual req-sigs are active
>> simultaneously.
>> But such dmac drivers will fail if a new platform decides to hijack req-signals.
>>
>> So, imho, it is absolutely a good thing for every dmac driver to be
>> designed for re-routable
>> ReqSig->Peri map... which would force their design to allocate
>> virtual/software channels to clients
>> without commit much(any?) h/w resources.
>
> We discussed channel allocation at Linaro.  However, I am now _really_
> disappointed.
Honestly, I don't see how this deviates from my proposal and why you think
a dmac driver designed for fixed ReqSig->Peri map is future-proof (my that
very assertion made you really disappointed)!

All the PL080 platforms that I have worked, had a fixed map...and I am sure you
wouldn't have been happy looking at a dmac driver based upon that assumption.
Similarly for any other dmac driver developer, it makes sense to be safe by
assuming the dmac could have flexible map on some platform. And this precisely
why I said
{
 So, imho, it is absolutely a good thing for every dmac driver to be
 designed for re-routable ReqSig->Peri map... which would force their
design to allocate
 virtual/software channels to clients without commit much(any?) h/w resources.
}

> I discussed this with Linus on the bus back from Cambridge in the evening,
> and it appears that the story you gave me was inaccurate - Linus had not
> agreed to your proposal and saw more or less the same problems with it
> which I've been on at you about via your other email alias/lkml.  So that's
> essentially invalidated everything we discussed there as part of my thinking
> was "if Linus is happy with it, then...".
IIRC, Linus W mainly opined to involve device pointer during channel selection.
Other than that I thought there was only ambiguity about implementation details.

Linus W, was there anything you said wouldn't work with the scheme ?
Please tell now on the record.

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