RE: [PATCH v2 1/3] mtd: rawnand: cadence: support deferred prob when DMA is not ready

[Rabara, Niravkumar L]

Hi Miquel,

> -----Original Message-----
> From: Miquel Raynal <miquel.raynal@xxxxxxxxxxx>
> Sent: Thursday, 30 January, 2025 11:20 PM
> To: Rabara, Niravkumar L <niravkumar.l.rabara@xxxxxxxxx>
> Cc: Richard Weinberger <richard@xxxxxx>; Vignesh Raghavendra
> <vigneshr@xxxxxx>; linux@xxxxxxxxxxx; Shen Lichuan
> <shenlichuan@xxxxxxxx>; Jinjie Ruan <ruanjinjie@xxxxxxxxxx>; u.kleine-
> koenig@xxxxxxxxxxxx; linux-mtd@xxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; stable@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH v2 1/3] mtd: rawnand: cadence: support deferred prob
> when DMA is not ready
> 
> Hello,
> 
> >> > Driver works without external DMA interface i.e. has_dma=0.
> >> > However current driver does not have a mechanism to configure it
> >> > from device tree.
> >>
> >> What? Why are you requesting a DMA channel from a dmaengine in this
> case?
> >>
> >> Please make the distinction between the OS implementation (the
> >> driver) and the DT binding which describe the HW and only the HW.
> >>
> >
> > Let me clarify from bindings(hw) and driver prospective.
> >
> > Bindings :-
> > Cadence NAND controller HW has MMIO registers, so called slave DMA
> > interface for page programming or page read.
> >         reg = <0x10b80000 0x10000>,
> >               <0x10840000 0x10000>;
> >         reg-names = "reg", "sdma"; // sdma =  Slave DMA data port
> > register set
> >
> > It appears that dt bindings has captured sdma interface correctly.
> 
> Slave DMA is very confusing because in Linux we make the distinction
> between:
> 1- external DMA (generic DMA controller) driven
>    through the dmaengine API, through which we interact using the so
>    called slave API
> 2- peripheral DMA (DMA controller embedded in the NAND IP) when there is
>    no "external/generic" engine. In this case we control DMA transfers
>    using the registers of the NAND controller (or a nearby range, in
>    this case), the same driver handles both the NAND and the DMA part.
> 
> You used the wording Slave DMA (#1), but it feels like you are talking about
> the other (#2). Can you please confirm in which case we are?
>

My apologies for the confusion.
Slave DMA terminology used in cadence nand controller bindings and
driver is indeed confusing.  

To answer your question it is, 
1 - External DMA (Generic DMA controller).

Nand controller IP do not have embedded DMA controller (2 - peripheral DMA). 

FYR, how external DMA is used. 
https://elixir.bootlin.com/linux/v6.13.1/source/drivers/mtd/nand/raw/cadence-nand-controller.c#L1962

 
> > Linux Driver:-
> > Driver can read these sdma registers directly or it can use the DMA.
> > Existing driver code has hardcoded has_dma with an assumption that an
> > external DMA is always used and relies on DMA API for data transfer.
> 
> I am sorry but DMA API does not mean much. There are 3 APIs:
> - dma-mapping, for the buffers and the coherency
> - dmaengine, used in case #1 only, to drive the external DMA controllers
> - dma-buf to share buffers between areas in the kernel (out of scope)
> 
> > Thant is why it requires to use DMA channel from dmaengine.
> 
> If I understand it right, no :-)
> 
> Either you have an external DMA controller (#2) or an internal one (#1) but in
> this second case there is no DMA channel request nor any engine-related
> API. Of course you need to use the dma-mapping API for the buffers.
> 
> > In my previous reply, I tried to describe this driver scenario but maybe I
> mixed up.
> > has_dma=0, i.e. accessing sdma register without using dmaengine is
> > also working.
> 
> But do you have an external DMA engine in the end? Or is it specific to the
> NAND controller?
> 

Yes I am using external DMA engine. 

Thanks,
Nirav