Re: [PATCH 2/8] mtd: cfi_flash: allow 0x0 mapping

From: Sascha Hauer <s.hauer@pengutronix.de>
To: Ahmad Fatoum <a.fatoum@pengutronix.de>
Cc: barebox@lists.infradead.org,
	Rouven Czerwinski <r.czerwinski@pengutronix.de>
Subject: Re: [PATCH 2/8] mtd: cfi_flash: allow 0x0 mapping
Date: Thu, 12 Nov 2020 11:06:31 +0100	[thread overview]
Message-ID: <20201112100631.GG29830@pengutronix.de> (raw)
In-Reply-To: <0e23114b-624f-c789-531f-1d65fd8cbf92@pengutronix.de>

On Tue, Nov 10, 2020 at 10:43:30AM +0100, Ahmad Fatoum wrote:
> On 11/10/20 10:36 AM, Ahmad Fatoum wrote:
> > Hi,
> > 
> > On 11/10/20 10:15 AM, Sascha Hauer wrote:
> >> On Tue, Nov 10, 2020 at 08:48:11AM +0100, Sascha Hauer wrote:
> >>> On Tue, Nov 10, 2020 at 07:33:53AM +0100, Rouven Czerwinski wrote:
> >>>> On Mon, 2020-11-09 at 14:52 +0100, Lucas Stach wrote:
> >>>>> Am Montag, den 09.11.2020, 14:44 +0100 schrieb Rouven Czerwinski:
> >>>>>> Annotate the different read and write functions with
> >>>>>> zero_page_{access/faulting}. This allows the cfi_flash driver to be used
> >>>>>> on the QEMU virt machine with an enabled MMU.
> >>>>>
> >>>>> I don't like this zero-page access allow in a driver at all. If you
> >>>>> have some free address space somewhere, you could also solve this issue
> >>>>> by remapping the IO resource to somewhere else in the address space,
> >>>>> deviating from the 1:1 mapping. The Tegra PCIe host driver does this,
> >>>>> if you need some inspiration.
> >>>>
> >>>> I totally agree, remapping the IO region sounds like a much better
> >>>> choice.
> >>>
> >>> But where should it be mapped to? Just 4k upwards and hope that the area
> >>> just above the cfi flash isn't occupied by something else?
> >>
> >> Ok, here's the plan:
> >>
> >> In a board specific initcall map the memory to wherever it's convenient,
> >> modify the address of the CFI flash in the device node and add a big
> >> comment that there's nothing to see here.
> > 
> > The device tree specification defines a "virtual-reg property [that] specifies
> > an effective address that maps to the first physical address specified in
> > the reg property of the device node.  This property enables boot programs to
> > provide client programs with virtual-to-physical mappings that have been set up".
> > 
> > So, how about a tad more generic approach:
> > 
> > - barebox device tree extends cfi-flash node with virtual-reg of appropriate
> >   location
> > - Define a of_iomap() function that
> 
> Or rather a  more generic name, like dev_ioremap?.(In that case
> !CONFIG_OF is the same as MMU off).

I wouldn't use ioremap as name when the function has a different
semantics, but other than that: Sounds good. Make it so!

Sascha

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