Subject: Re: [PATCH 2/3] mtd: rawnand: sunxi: Add DMA support for sun8i

From: Miquel Raynal <[email-masked]>
Date: Mon, 15 Apr 2019 08:58:59 +0200

Hi Boris,

Boris Brezillon <[email-masked]> wrote on Sun, 14 Apr
2019 11:05:49 +0200:

> On Thu,  4 Apr 2019 18:21:10 +0200
> Miquel Raynal <[email-masked]> wrote:
> 
> > Allwinner NAND controllers can make use of DMA to enhance the I/O
> > throughput thanks to ECC pipelining. DMA handling with sun8i NAND IP
> > is a bit different than with the older SoCs, hence the introduction of
> > a new compatible to handle:
> > * the differences between register offsets,
> > * the burst length change from 4 to minimum 8,
> > * drive SRAM accesses through the AHB bus instead of the MBUS.  
> 
> Hm, now that you know MBUS accesses are working fine (IIRC, that's what
> you used for the SPL DMA-based implementation), why not directly use
> MBUS accesses on A33? I mean, it's likely faster than going through
> the DMA engine (which is shared by several IPs), and AFAIR, the MBUS
> setup is pretty simple.

Because all the driver is already in shape to use the external DMA
engine and it was very easy and quick (have a look at the diff of the
v3) to use it again.

However, the choice I am describing here is not DMA vs. MBUS (or MDMA),
it is MBUS vs. AHB, it is just about the bus that will access the SRAM
(this is what we have understood with Maxime from the datasheets and
the tests we have done). For this choice, we tested with both buses: no
throughput change so we think that it is not a bottleneck anyway.

Thanks,
Miquèl

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