linux-kernel - Re: [PATCH] mtd: nand: raw: atmel: add module param to avoid using dma

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Message-ID: <20180402223539.6dff3f38@bbrezillon>
Date:   Mon, 2 Apr 2018 22:35:39 +0200
From:   Boris Brezillon <boris.brezillon@...tlin.com>
To:     Peter Rosin <peda@...ntia.se>
Cc:     Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Josh Wu <rainyfeeling@...look.com>,
        Cyrille Pitchen <cyrille.pitchen@...ev4u.fr>,
        linux-kernel@...r.kernel.org,
        Nicolas Ferre <nicolas.ferre@...rochip.com>,
        Marek Vasut <marek.vasut@...il.com>,
        linux-mtd@...ts.infradead.org, Richard Weinberger <richard@....at>,
        Brian Norris <computersforpeace@...il.com>,
        David Woodhouse <dwmw2@...radead.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] mtd: nand: raw: atmel: add module param to avoid using
 dma

On Mon, 2 Apr 2018 22:23:17 +0200
Peter Rosin <peda@...ntia.se> wrote:


> > I don't use devmem2. Is 'readback' information accurate or is it
> > always what's been written? Because when you write 0x33 to 0xFFFFECBC,
> > 0x33 is read back, but just after that, when you read it again it's 0.  
> 
> Looking at the devmem2 source, it seems very likely that the compiler
> optimizes out the read and thus outputs what has been written.

Yep, had a look too, and it's missing a volatile specifier to prevent
that sort of optimizations.

> 
> >> BTW, how do I
> >> know which master is in use for the LCD controller? 8 or 9? Both?  
> > 
> > It's configurable on a per-layer basis through the SIF bit in
> > LCDC_<layer>CFG0. The driver tries to dispatch the load on those 2 AHB
> > masters [1].  
> 
> Ok, I only have one plane (in this case, i.e. no cursor, no overlays etc),
> would that mean that only one master is used?

Yep, it's always using the first one (master 8 on a sama5d3).

-- 
Boris Brezillon, Bootlin (formerly Free Electrons)
Embedded Linux and Kernel engineering
https://bootlin.com